Monday, December 4, 2017

The basic operation and process of SMT placement machin

The basic operation and process of SMT placement machine

As a high-tech products, safe and correct operation of the machine and the people are very important to safely operate the placement machine is the most basic operator should have the most accurate judgments, should follow the following basic safety rules and processes,


1, the machine operator should accept the correct method of operation training.
2, check the machine, replace the parts or repair and internal adjustment should be off the power (the maintenance of the machine must be pressed in the emergency button or power off the case.
3, make sure to "read the coordinates" and adjust the machine when YPU (programming part) is in your hands at any time to stop the action.
4, so that the "interlock" security equipment to keep effective at any time to stop the machine, the machine's safety testing can not skip, shorted, or prone to personal or machine safety incidents.
5, the production allows only one operator to operate a machine.
6, during operation, so that the body parts such as hand and first in the machine outside the scope of movement.
7, the machine must be properly grounded <true ground, not connected to the zero line.
8, do not use a gas or very dirty environment in the use of the machine.

note:
1) did not receive training is strictly prohibited on the machine operation.
2) the operation of equipment to be safe as the first, the machine operator should operate in strict accordance with the operating procedures of the machine, otherwise it may cause damage to the machine or harm personal safety.
3) the machine operator should be careful, careful.

SMT Mounter Workflow:
Function is mainly through the PCB board fixed -> components to absorb -> displacement -> positioning -> components placed and other operations.
1, to be mounted on the PCB board into the work area and fixed in the predetermined location,
2, the material is installed in the feeder, and according to the program set the location, installed to the patch head to learn the location.
3, the patch head will move to the position of the suction element, open the vacuum, the nozzle up to absorb the components, and then through the vacuum sensor to detect whether the device was sucked.
4, the component identification, read the components of the component characteristics and pick up the components compared to the evaluation of the evaluation does not meet, then the components will be thrown into the waste box.If the evaluation of the evaluation of the components after the center position and angle Calculation.
5, according to the program set, through the patch of the Z-axis to adjust the rotation angle of the component, through the patch head to move to the program set a good location, making the component center and PCB board placement point coincidence.
6, the placement machine nozzle will drop to the program set a good height, close the vacuum, the components fall, to complete a component placement operation.
7, all the components after the completion of the placement, the nozzle placed homing, the PCB board to the set position to complete the entire PCB board placement operation.
SMT soldering machine work flow summary: into the board and mark recognition -> automatic learning -> nozzle selection -> feeder selection -> component pick -> component testing to evaluate -> placement -> nozzle homing - > Out board

The similarities and differences of solder wire and silver-free solder wir

The similarities and differences of solder wire and silver-free solder wire 

(1) the melting point is different: due to the different metal alloy, so the melting point is also different. Silver-free lead-free solder wire melting point: 217 degrees, and lead-free solder wire melting point: 227 degrees difference of about 10 degrees.


(2) different metal composition: silver-free lead-free solder wire is made of tin silver copper alloy, and lead-free solder wire is a tin and copper alloy composition, excluding silver metal composition.


(3) Costs are different: silver-free lead-free solder wire is more expensive than lead-free solder wire because of the high price of silver metal. This is the biggest advantage of lead-free solder wire. The higher the cost, the more expensive the cost

 The similarities and differences of solder wire and silver-free solder wire

(4) the overall performance of different: due to the role of silver metal, lead-free solder wire than lead-free solder wire firmness better, stronger, more solid solder joints.Second, the conductive properties in the solder joint is also very important , Silver is a very good conductive metal, so silver-free lead-free solder wire than lead-free solder wire conductivity, thermal conductivity and so on are better. These properties are lead-free solder wire can not go beyond.


(5) solder color gloss different: lead-free solder wire gloss is showing a certain degree of brightness, look better look. This is one of the advantages of lead-free solder wire, but the solder color does not mean the welding Point of its own welding performance, while the silver-free lead-free solder wire solder joints affect the appearance of the solder color brightness of the color.As the silver metal is matte metal, not bright metal, so silver-free solder wire solder Point gloss is matte color and brightless.

The key process of SMT reflow welding rework technology

The key process of SMT reflow welding rework technology 

Reflow before the appropriate preheating PCB board; re-flow after the rapid cooling of the solder joints. Two of the most critical processes for the successful repair of SMT are also the two most likely to overlook the problem:
Since these two basic processes are often overlooked by the repair technician, in fact, sometimes the repair is worse than the situation before the repair. Although some "rework"defects can sometimes be found by the post-process inspector, but in most cases always do not see, but in the future circuit test will soon be exposed.Preheat - Prerequisites for successful rework.



It is true that PCB processing at high temperatures (315-426 ° C) for a long time poses a lot of potential problems. Heat damage, such as pad and lead warping, substrate delamination, raw white spots or blistering, discoloration. Board Alice and Burning are usually caused by the inspector's attention. However, it is precisely because it will not "burn bad plate" does not mean that "plate is not damaged." High temperature on the PCB "invisible" damage even more serious than the above list of problems. For decades, numerous tests have repeatedly demonstrated that PCBs and their components can "pass" rework after the test and test, the decay rate higher than the normal PCB board. This kind of substrate warpage and its circuit element attenuation and other "stealth" problem comes from different materials of different expansion coefficient. Obviously, these problems will not be self-exposed, even in the beginning of the circuit test was not found, but still lurking in the PCB components.

Although the "repair" looks good, but as people often say a word: "surgery is successful, but the patient unfortunately died."Due to the large thermal stress, the PCB assembly at room temperature (21 ° C) has a temperature difference of about 349 ° C for the circuit board and its components when it is exposed to a hot iron with a heat source of about 370 ° C, a welding tool or a hot head. Variety,Produce "popcorn" phenomenon."Popcorn" phenomenon refers to the existence of an integrated circuit or SMD in the device inside the moisture in the repair process of rapid heat, so that moisture expansion,the phenomenon of micro-burst or rupture. 

Therefore, the semiconductor industry and circuit board manufacturing industry requires production staff in the reflow before, as far as possible to shorten the warm-up time,quickly rose to reflow temperature. In fact, the preheating stage of the PCB assembly reflow process has been included. Regardless of PCB assembly plant is the use of wave soldering, infrared vapor phase or convection reflow, each method are generally preheated or heat treatment, the temperature is generally 140-160 ℃. In the implementation of reflow before the use of a simple short-term preheating PCB can solve many problems when the repair. This has been in the reflow process has been a few years of successful history.the benefits of preheating the PCB assembly before reflowing are multifaceted.Since the preheating of the plate reduces the reflow temperature, both wave soldering, IR / vapor welding and convection reflow can be performed at about 260 ° C.The benefits of preheating are multifaceted and integrated. First, preheating or "heat-treating" components prior to commencing reflow helps to activate the flux, remove the oxides and surface films of the surface to be welded, and the volatiles of the flux itself. Accordingly, this cleaning of the flux prior to reflow will enhance the wetting effect. Preheating is to heat the entire assembly below the melting point of the solder and the temperature of the reflow. This can greatly reduce the risk of thermal shock to the substrate and its components. Otherwise rapid heating will increase the temperature gradient within the assembly and produce thermal shock. The large temperature gradient generated inside the assembly will form thermomechanical stresses that cause the material with low thermal expansion to be brittle, causing rupture and damage. SMT chip resistors and capacitors are particularly susceptible to thermal shock.

In addition, if the entire assembly is preheated, the reflow temperature and the reflow time can be reduced. If there is no preheating, the only way to only further increase the reflow temperature, or extend the reflow time, no matter which way is not appropriate, should be avoided.Reduced rework makes the board more reliable.As a benchmark for welding temperature, the use of different welding methods, welding temperature is not the same, for example: Most of the wave soldering temperature of about 240-260 ℃, steam welding temperature of about 215 ℃, reflow temperature of about 230 ℃. To be fair, the rework temperature is not higher than the reflow temperature. Although the temperature is close, it is never possible to achieve the same temperature. This is because all the rework processes require only one local component to be heated, and the reflow requires the entire PCB assembly to be heated, both for wave soldering and vapor phase reflow.

Another factor limiting the reduction in reflow temperatures in rework is the industry standard, which means that the temperature of the components around the repair point must never exceed 170 ° C. Therefore, the reflow temperature in the reflow should be with the PCB component itself and to reflow the size of the components to adapt to the size of the PCB board is essentially due to the local rework, so the repair process limits the PCB board maintenance temperature. The temperature range of the localized rework is higher than the temperature in the production process to counteract the endotherm of the entire board assembly.

In this case, there is no good reason to say that the repair temperature of the entire board can not be higher than the reflow temperature in the production process, thus approaching the target temperature recommended by the semiconductor manufacturer.Three methods of preheating PCB components before repair or repair:Today, preheating PCB components are divided into three categories: oven, hot plate and hot air tank. It is effective to use an oven to preheat the substrate before reworking and reflow the components. Moreover, the use of baking is a favorable way to preheat the oven in baking out some of the internal circuits in the internal moisture and prevent the popcorn phenomenon. The so-called popcorn phenomenon refers to the repair of the SMD device in the humidity above the normal device humidity suddenly in the rapid warming will occur when the micro-crack. PCB in the preheating oven baking time is longer,Generally up to 8 hours or so.

One of the drawbacks of the preheating oven is that it is different from the hot plate and the hot air tank, and it is not possible to preheat by a technician and to rework it at the same time. Moreover, it is impossible to quickly cool the solder joint in the oven.Hot plate is the most ineffective way to preheat PCB board. Because the PCB components to repair is not all single-sided,Today is the world of hybrid technology, one side is all flat or flat PCB components is indeed rare. PCB on both sides of the substrate are generally installed components. It is not possible for these uneven surfaces to be preheated with hot plates.The second drawback of the hot plate is that once the solder reflows, the hot plate will continue to release heat to the PCB assembly. This is because, even after unplugging the power supply, there will still be stored residual heat in the hot plate to continue to conduct the PCB to prevent the solder joint cooling rate. This obstruction of solder joints can cause unnecessary lead precipitation to form a lead bath, reducing the solder joint strength and deteriorating.

The advantage of using a hot air tank to preheat is that the hot air tank does not take into account the shape (and bottom structure) of the PCB assembly, and the hot air can enter the corners of the PCB assembly directly and quickly. So that the entire PCB assembly heating evenly,And shorten the heating time.Secondary cooling of solder joint in PCB assembly

As mentioned earlier, the challenge of SMT to PCBA (PCB components) is that the repair process should mimic the production process. Facts have proved:
First, preheating the PCB assembly before reflow is necessary for the successful production of the PCBA; secondly, it is also important to quickly cool the components immediately after reflow. And these two simple craft has been ignored by people. However, in the through-hole technology and micro-welding of sensitive components, preheating and secondary cooling is even more important.

Common reflow equipment such as chain furnace, PCB components through the re-flow zone immediately after entering the cooling zone. With the PCB assembly into the cooling zone, in order to achieve rapid cooling,ventilation of PCB components is very important, the general repair and production equipment itself is integrated.The slow cooling of the PCB assembly after recirculation causes the unwanted lead-filled liquid pool in the liquid solder to reduce the solder joint strength. However, the use of rapid cooling can prevent the precipitation of lead, so that the grain structure is more tight, more solid solder joints.In addition, the faster cooling of the solder joint reduces the quality of the PCB assembly due to accidental movement or vibration during reflow. For production and rework, it is another advantage of the secondary cooling of the PCB assembly by reducing the possible misalignment and tombstone of small SMD.

Summary:
There are many benefits of secondary cooling PCB components when properly warmed up and reflowed, and these two simple procedures need to be included in the technician's rework. In fact, when preheating the PCB, the technician can do other preparatory work at the same time, such as solder paste and flux on the PCB.Of course, the need to solve the problem of new refurbished PCB component process, because it has not passed the circuit test, which is a real time to save. Obviously, there is no need to repair the PCB in the repair and save the cost.

SMT 0201 assembly from difficulty to conventional placement process

SMT 0201 assembly from difficulty to conventional placement process


AT one time the 0201 placement has been placed difficlity in the SMT industry, many OEM circuit board assemblers need to incorporate even smaller components and technologies into their products due to size, weight, and power consumption The contract manufacturer (CM-contract manufacturer) must also have new technology to keep the assembly process up to date and provide a complete range of services for the customer, and for the machine manufacturer, the challenge is to develop a more time in a dynamic technological change Resistant to outdated assembly equipment.

0201 placement challenge
The placement of the 0201 component is more challenging than its previous component intervention, mainly due to the fact that the 0201 package is about one-third of the corresponding 0402 size.the original acceptable machine placement accuracy has immediately become a limiting factor in the introduction of the 0201. In addition, the traditional industrial taping specifications allow too much movement for reliable 0201 mounts, and process control levels must also be increased So that the 0201 placement becomes a production reality.Although these obstacles are very large, but they are far from being insurmountable.Of course, they need all the determination, because the 0201 placement of the necessary technology to obtain a lot of money and the highest management of the study Development (R & D) promise.

Reliable 0201 placement key
SMT equipment manufacturers have been able to make all the circuit assembly machine to 100% speed compatible with the ability of 0201, the minimum draw reliability of 99.90%, the target of the reliability of 99.95%, and the lowest placement reliability of 99.99%. At the outset, each aspect of the design has been evaluated for its ability to complete a complete 0201 program, as well as a single element of the closely related machine component parameters that proves to be critical to achieving success.

These parameters include:
Component Feeder Workbench.
The R & D program concludes that the ability to precisely locate the carriage table
And making minimal adjustments to compensate for the inaccuracy of the tape - is a key factor in achieving the reliability of the component above 99.95%. To achieve this, the feeder table must be machined to ensure a single feed Repeatable positioning of the device and the use of a two-track linearly movable guide in conjunction with a high-resolution semi-closed loop servo system which allows for a small adjustment - based on the accuracy of the absorption accuracy determined by the visual system. Close to the center.

Component feeder.
The feeder must be made to achieve extremely tight tolerances to ensure that the suction position is maintained repeatable, regardless of element height and a large number of possible changes in the position of the component. The mechanism used to position and position the feeder in position must be durable and precise In order to maintain user friendliness, the materials used to make the feeder must be of high strength and light weight to allow for ergonomic operation while ensuring the precise and repeatable delivery of the carrier tape.

The drive sprocket drives the sprocket to play a key role in the capacity of the machine positioning element. The shape, taper and length of the drive sprocket wheel significantly affect the ability of the feeder to orient the tape. Other factors are also investigated Such as the diameter of the drive sprocket and the number of contacts with the sprocket, etc. The change in the basic sprocket design improves the positioning accuracy, the earlier design increases by 20% in the X direction, 50% in the Y direction, The

Absorb the head.
After the feeder element is properly fed, the next step is to draw the element on the vacuum nozzle and take it onto the circuit board. The vacuum nozzle needs to be adapted to absorb the impact during the draw and mount element, The slight change in the height of the solder paste and the risk of rupture of the element. For these reasons, the nozzle must be able to move within its fixture. Material selection, material hardness, machining tolerances and thermal characteristics must be understood to construct a reliable The nozzle must be free to move within its holder without sacrificing accuracy.

Suction nozzle assembly.
The nozzleshaft is also a key design element - by overriding the entire nozzle and shaft assembly, eliminating the overdrive phenomenon due to the inertia of the top and bottom of the mounting head If the nozzle and the shaft are not in a straight line, they produce a little jitter (whip) - or overpressure. Overvoltage causes a change in positioning accuracy, which depends on the speed of movement, the weight of the nozzle and the weight of the component. By eliminating overpressure, To reduce the number of negative factors associated with component picking and placement.

Nozzle design.
The change in nozzle design is a very important factor in allowing the inclusion of the 0201 element. In order to draw 0.6x0.3 mm of the element, the nozzle must have an outer diameter of no more than 0.40mm. This forms a long and thin nozzle , Bending is fragile but also must be kept in order to maintain the high reliability of the drawing. The change from the linear axis to the tapered design increases the nozzle strength and allows the nozzle to resist bending.

Base structure.
All the machines are vibrating during operation. The base frame design is the first step in reducing the speed and motion effects of vibration and harmonic resonance. By using cast iron base frames and art-level structure techniques, vibration and harmonic resonance can be achieved in the machine To a manageable level, so that negative effects can be met.

Up to standard
Through all six key factors, reliable 0201 placement barriers have been eliminated.Therefore, R & D's focus has shifted to newer, smaller components, and 0201 is no longer considered a leading edge component packaging technology.
For 0201 component placement, the current process window is about 0.075mm X and 0.075mm Y at 3 Sigma. In order to achieve 6 Sigma mount reliability, the tolerance of X and Y must be reduced to 0.050mm. High-speed placement equipment has a rating of 0.066mm, the actual standard deviation of about 0.035 ~ 0.045mm. With 0201 components become more widely used and manufacturing process tight, can achieve improved accuracy.

3D SPI principle and detection metho

3D SPI principle and detection method

SPI (SolderPaste Inspection) refers to the solder paste detection system, the main function is to detect the quality of solder paste printing, including volume, area, height, XY offset, shape, bridge, etc. How quickly and accurately detect extremely small solder paste , The general use of PMP (Chinese translation for the phase modulation profile measurement technology) and Laser (Chinese translation for the laser triangulation technology) detection principle.





Laser triangulation technique
The detection light source is laser, the laser beam in different height plane distortion, the detection head in a certain direction of continuous movement, the camera according to set the time interval to take pictures, so as to obtain a set of laser distortion data, and then calculate the test results (As shown below)
Advantages: faster detection
Disadvantages: 1) laser resolution is low, generally only 10 - 20um level.
2) Single sampling, low repeatability accuracy.
3) Sampling in the movement, the external shock and transmission vibration on the detection of a greater impact.
4) laser monochromatic light on the PCB board color is weak.
Market conditions: laser technology has gradually withdrawn from the SPI industry. At present, South Korea Parmi is still using laser technology (dual laser technology)

PMP phase modulation contour measurement technology
1) Using a white light source, the solder paste is measured by the phase change of the structural grating
2) Using the grayscale change of the structural grating, a high accuracy is obtained
3) the use of phase changes, each solder paste for 8 times to ensure that the detection of high repeatability accuracy
4) PMP technology is divided into FOV walk-off and Scan scanning two detection methods

4.1 FOV stop
When the test is carried out, the motion is not sampled and does not move during sampling, and the effect of vibration on the detection is minimized.
Advantages: 1) PMP principle detection resolution is high, 0.37um.2) stable multi-sampling, detection repeatability is extremely high .3) on the PCB color is not critical.
Disadvantages: the speed is relatively slow.
Market: to stabilize the detection results identified by the industry as the best SPI solution to South Korea KohYoung as the representative of foreign brands.

4.2Scan Scanning
The phase change of the structural grating is formed by the continuous motion of the detecting head, while sampling is carried out while moving.
Advantages: 1) PMP principle detection resolution is high, 0.37um.2) on the PCB color is not critical .3) multiple sampling, detection repeatability than laser-type equipment .4) detection speed faster than FOV stop.
Disadvantages: the impact of external shocks, detection repeatability is low.
Market: China Taiwan TRI as the representative of the Chinese Taiwan brand, represented by the foreign brand of Cyber.

SMT land structure technology & analysis

SMT land structure technology & analysis

The basic constituent elements of the surface mount assembly used to form the land pattern of the circuit board, that is, the combination of pads for the particular component type design. There is no worse than the design of the pad structure It is difficult and sometimes impossible to achieve the desired weld point when a pad structure is not designed correctly. The pad has two words: Land and Pad, which can often be used interchangeably; , In function, Land is a two-dimensional surface feature for surface mount components, and Pad is a three-dimensional feature for pluggable components. As a general rule, Land does not include plated through holes (PTH, plated through (via) is a plated through hole (PTH) that connects different circuit layers. The blind via connects the outermost layer with one or more inner layers and the buried bypass holes Only connect the inner layer.

 SMT land structure technology & analysis



As noted above, the pad Land typically does not include electroplated through-holes (PTH). The PTH in a pad Land will take a significant amount of solder during soldering and, in many cases, solder pads that are insufficiently solderable. However, in some cases, component wiring density forcing to change to this rule, the most noteworthy is the chip size of the package (CSP, chip scale package). 1.0mm (0.0394 ") spacing below, it is difficult to a The wires are routed through the "maze" of the pads. The blind vias and micro-channels are produced in the pads, allowing direct wiring to the other layer, since these bypass holes are small and blind, so they do not Will suck too much solder, the results of the solder the amount of tin is very small or no effect.

There are many industrial literatures available from IPC (Association Connecting Electronics Industries), EIA (Electronic Industry Alliance) and JEDEC (Solid State Technology Association), which should be used when designing the pad structure.The main document is the IPC-SM-782 "Surface Mount Design and Padding Structure Standard", which provides information on the pad structure used for surface mount components.When the J-STD-001 "Requirements for Welding Electrical and Electronic Assemblies" and IPC-A-610 "Acceptability of Electronic Assemblies" are used as solder joint process standards, the pad structure should conform to IPC-SM-782.If the pad deviates significantly from the IPC-SM-782, it will be difficult to achieve solder joints that conform to J-STD-001 and IPC-A-610.Component knowledge (ie, component structure and mechanical dimensions) is a fundamental requirement for the design of the pad structure. The IPC-SM-782 uses two components widely: EIA-PDP-100 "Registration of electronic components with standard mechanical form" And JEDEC 95 publication "Registration of Solid and Related Products and Standard Outline." It is indisputable that the most important of these documents is the JEDEC 95 publication because it deals with the most complex components. It provides all registration of solid elements And the standard profile of the mechanical map.

JEDEC publication JESD30 (also available for free download from JEDEC website) Defines the abbreviation of the component based on the characteristics of the package, material, terminal location, package type, pin form and number of terminals. Features, material, position, form and quantity The identifier is optional.

Package Features: A single or multiple letter prefix that confirms features such as pitch and contour.
Packaging material: a single letter prefix, confirm the main packaging material.

Terminal position: A single letter prefix that confirms the terminal position relative to the package profile.
Package Type: A two-letter mark that specifies the outline type of the package.
Pin New: A single letter suffix to confirm the pin form.

Number of terminals: a one, two or three digit suffix, indicating the number of terminals.
Surface Mount A simple list of encapsulation feature identifiers includes:
 E to expand the pitch (> 1.27 mm)
F spacing (<0.5 mm); limited to QFP components
 S contraction pitch (<0.65 mm); all components except QFP.
· T thin (1.0 mm body thickness)
Surface Mount A simple list of terminal position identifiers includes:
• Dual pins on either side of a square or rectangular package.
• The Quad pin is on the four sides of a square or rectangular package.
Surface Mount A simple list of package type identifiers includes:
· CC chip carrier (chip carrier) package structure
· FP flat pack (flat pack) package structure
· GA grid array (grid array) package structure

Small outline package structure
Surface Mount A simple list of pin form identifiers includes:
B is a straight handle or a spherical pin structure; this is a non-compliant pin form
F is a straight pin structure; this is a non-compliant pin form
 G A wing-shaped pin structure; this is a conforming pin form
A "J" -shaped pin structure; this is a conforming pin form
N is a pinless structure; this is a non-compliant pin form
An "S" -shaped pin structure; this is a conforming pin form

For example, the abbreviation F-PQFP-G208 describes 0.5 mm (F) plastic (P) square (Q) plane package (FP), winged pin (G), number of terminals 208.
Detailed analysis of the component and plate surface characteristics (ie, pad structure, reference points, etc.) is necessary. The IPC-SM-782 explains how to perform this analysis. Many components (especially dense pitch components) are strictly metric units Do not design an inch pad structure for metric components. The cumulative structural error does not match and can not be used for dense spacing elements. Remember, 0.65mm is equal to 0.0256 "and 0.5mm is equal to 0.0197".

The latest SMT | THT Mixed Welding Technology

The latest SMT | THT Mixed Welding Technology

In conventional electronic assembly processes, wave soldering techniques are commonly used for welding of PCB-mounted board assemblies (PTH). But the wave soldering has many shortcomings: can not be distributed in the welding surface of high-density, fine pitch patch components, bridge, leakage welding more; need to spray flux; printed circuit board by the greater thermal impact warping deformation. As the current circuit assembly density is getting higher and higher, the welding surface will inevitably be distributed with high density, fine pitch patch components, the traditional wave soldering process has been powerless, generally only the first soldering surface patch components reflow soldering , And then hand-repair welding the remaining plug-in solder joints, but there is poor solder joint quality problems. In order to meet these challenges, several new types of mixed welding technology are emerging, such as selective welding, through-hole reflow and use of shielded molds, etc., can protect the surface mount components to achieve through-hole component welding, significantly reduce the production process And cycle time. This article will be introduced one by one.
 SMT Technology



Several kinds of mixed welding technology introduction
1.1 Selective welding
The process characteristics of selective welding can be understood by comparison with wave soldering. The most obvious difference between the two is that the lower part of the PCB is completely immersed in the liquid solder, and in the selective welding, only part of the specific area and solder wave contact. Since the PCB itself is an undesirable heat transfer medium, it does not heat and melt the solder joints of adjacent components and PCB areas. The flux must also be pre-applied before welding. Compared with the wave soldering, the flux is only applied to the part of the PCB to be welded, rather than the entire PCB. Selective soldering is not suitable for solder patch components.
Selective welding process has two different processes: drag welding process, dip welding process.
(1) drag welding process. The selective trapping process is done on a single small tip solder wave, as shown in Figure 1. The trailing process is suitable for very tight space on the PCB for welding. For example: individual solder joints or pins, single row of leads can be trailer welding process. The PCB moves at different speeds and angles on the solder wave of the tip to achieve optimum welding quality. To ensure the stability of the welding process, the inner diameter of the tip is less than 6 mm. After the flow of the solder solution is determined, the welds are installed and optimized in different directions for different welding needs. Robot can be from different directions, that is, 0o ~ 12o different angles close to the solder wave, so the user can weld a variety of components on the electronic components, for most devices, the proposed tilt angle of 10o.

Compared with the dip welding process, the soldering process of the solder solution and the PCB board movement, making the welding heat conversion efficiency is better than the dip welding process. However, the heat required to form the weld connection is transmitted by the solder wave, but the solder wave of the single weld is of low quality and only the temperature of the solder wave is relatively high in order to achieve the requirements of the trailing process. Example: Soldering temperature is 275 ℃ ~ 300 ℃, dragging speed of 10 mm / s ~ 25 mm / s is usually acceptable. In the welding area for nitrogen, to prevent the solder wave oxidation, solder wave to eliminate the oxidation, making the trapping process to avoid the emergence of bridge defects, the advantages of increased drag welding process stability and reliability.
The machine has the characteristics of high precision and high flexibility. The module structure design system can be customized according to the special production requirements of customers, and can be upgraded to meet the needs of future production development. The movement radius of the manipulator can cover the flux nozzles, preheat, and solder nozzles, so that the same device can perform different welding processes. Machine-specific synchronization process can greatly shorten the veneer process cycle. The ability of the robot to make this choice welding has the characteristics of high precision and high quality welding. (X, Y, Z, U, q) of the manipulator allows the PCB to be able to use any of the & lt; RTI ID = 0.0 & gt; (x & lt; / RTI & gt; Optimize the angle and orientation of the contact tin surface for optimum welding quality. Robot plywood device installed on the tin wave height stylus, made of titanium alloy, under the control of the program can be measured regularly tin height, by adjusting the speed of tin pump to control the height of tin to ensure the stability of the process.
Despite the above advantages, the single-port solder wave trawling process is also inadequate: the welding time is the longest time in the three processes of flux spraying, preheating and welding. And because the solder joint is a one of the drag welding, with the number of solder joints increases, the welding time will be a substantial increase in the welding efficiency can not be compared with the traditional wave soldering process. But the situation is changing, multi-nozzle design can maximize production. For example, double-welded nozzles can be used to double production. Flux can also be designed as a dual nozzle.
(2) dip welding process. Immersion Selective welding system has a number of solder mouth, and PCB to be solder joints are one to one design, although the flexibility is less than the robot type, but the output is equivalent to the traditional wave soldering equipment, equipment cost relative to the lower hand. According to the size of the PCB, you can carry the board or multi-board parallel transmission, all the solder joints will be completed in parallel with the same time to complete the flux spraying, preheating, and welding. However, due to the distribution of solder joints on different PCBs, so the need for the production of different PCB solder mouth. The size of the tip is as large as possible to ensure that the welding process is stable and does not affect the surrounding adjacent devices on the PCB. This is important and difficult for the design engineer because the stability of the process may depend on it.

The use of immersion selective welding process, welding 0.7 mm ~ 10 mm solder joints, short pins and small size pad welding process is more stable, the possibility of bridging is also small, adjacent solder joints, the distance between the device and the tip Should be greater than 5 mm. Select the dip welding process, you can use the following parameters:
① solder temperature 27 5 ℃ ~ 300 ℃
② immersion speed 20 mm / s ~ 25 mm / s
③ immersion time 1 s ~ 3 s
④ dip speed 2 mm / s
⑤ Shock pump rate according to the number of nozzles

1.2 through-hole reflow
In short, through-hole reflow (THR) is the use of reflow soldering technology to assemble through-hole components and special-shaped components. As the product more and more attention to miniaturization, increase functionality and improve component density, many single and double panels are based on surface mount components (SMC). However, due to inherent strength, reliability and applicability, in some cases, through-hole devices are still better than SMC, especially in the PCB edge of the connector.

The use of through-hole devices on circuit boards based on surface mount components is disadvantageous in that the cost of individual soldering points is high, as additional processing steps are involved, including wave soldering, manual soldering or other selective soldering methods. The key to this type of assembly is the ability to provide synchronous reflow for through-holes and surface mount components in a single integrated process. Figure 2 for the through-hole reflow process.

The through-hole reflow process enables reflow soldering of through-hole devices and SMC devices simultaneously in a single step. The steps required to manufacture the process depend on the special components used in the assembly. For example, the computer board has a large number of SMC (which accounts for most of the components used) and a limited number of through-hole devices: connectors, discrete components, switches and jack devices. At present, SMC is fixed on the PCB using solder paste screen printing and reflow soldering. A similar process can be used to complete the interconnection of through-holes and shaped devices. 

In many cases, the use of the THR process eliminates the need for subsequent wave soldering operations.
1.2.1 Paste coating process
Compared with the general surface mount technology, through-hole reflow process using the amount of solder paste than the average SMT more, about 30 times its. The current through-hole reflow process mainly uses two kinds of solder paste coating technology, including solder paste printing and automatic point solder paste.
(1) solder paste printing. For THR processes, screen printing is the preferred method of depositing solder pastes on PCBs. The thickness of the stencil is a key factor, which will affect the amount of solder paste that is printed on the PCB. Can be used ladder stencil, in which the thicker area for the through-hole devices and set up. This steel mesh design can meet the requirements of different amount of solder paste.

(2) automatic point solder paste. The automatic spot solder paste successfully deposits the correct solder paste for the through-hole and the profiled assembly, which provides the flexibility and the ability for a large number of solder paste deposits that can not be achieved by screen printing. It is recommended to use nozzles that are slightly larger than the PTH diameter when the solder paste is plated at the Plated Through Hole (PTH). In this way, when the solder paste is forced, the solder paste is forced against the hole wall of the PTH and the material is slightly extruded from the bottom of the PTH, and the assembly is inserted in the opposite direction from the solder paste. If a nozzle smaller than the PTH diameter is used, the solder paste will be discharged from the hole and cause severe solder paste loss.
Through-hole reflow soldering in many ways can replace the wave soldering to achieve the welding of the insert components, especially in the processing of welding surface distribution of high-density patch components (or fine pitch SMD) plug-in solder joints, then The traditional wave soldering has been powerless, and the other through-hole reflow can greatly improve the quality of welding, which is sufficient to make up for the expensive equipment. The emergence of through-hole reflow, for the rich means of welding, to improve the circuit board assembly density (in the welding surface distribution of high-density patch components), to enhance the quality of welding, reduce the process, are of great help.

1.2.2 assembly process of through-hole reflow element
The special components used in computers, automation equipment and communication equipment Because of their high height, peculiar shape and heavy weight, it is required that the automatic patch device has the ability to handle a wide range of components,
 
(2) Adjustable clamps - Some components may require special clip picking and assembly; 

(3) Special plate support and clamping Specialized suction nozzle - with sufficient vacuum suction; System; 
(4) high assembly pressure;
(5) for the special-shaped components of high-precision assembly, the machine has a full image processing capacity.
One of the reasons for the industry's renewed interest in through-hole technology is that some brands of automatic placement devices, such as Advantech AX72 and Ploaris, Universal Instruments, have a strong ability to mount shaped and through-hole components. Components can be used tube, taped, waffle plate and other packaging, feeder directly installed in the placement machine. Automatic placement with the advantages of precision, reliability and high speed, and can be automatically installed components are also more and more. Manual placement is the next level of placement option, with some through-hole connectors due to the positioning pin design that helps to align. For high-pin components, these are becoming more and more important. Manual placement is that the installation cost is low and there is no set time, the disadvantage is low speed, and the accuracy is not stable.

1.2.3 Through-hole reflow soldering process
Reflow must be able to provide sufficient heat (temperature) for the entire assembly and all pin locations. Many special / through-hole devices are higher and have larger heat capacity than other surface mount components assembled on the assembly. For THR applications, the general use of infrared radiation reflow, to avoid hot air convection temperature uneven impact on welding. Separate top and bottom heating controls also help to reduce the temperature difference on the PCB assembly. For computer motherboards with a high stack of 25-pin DSUB connectors, the component body temperature is unacceptably high. The solution to this problem is to increase the bottom temperature and lower the top temperature. The time on the liquidus should be long enough to allow the flux to volatilize from PTH, which may be longer than the standard temperature curve. Sectional analysis may be important to confirm the correctness of the reflow profile. In addition, the peak temperature and thermal gradients on the components must be carefully measured and strictly controlled. (2) the distribution of temperature on the monitoring board, the temperature difference between the size of the components; (3) consider the thermal compatibility of the component body; (4) the heating rate , Liquid phase above time, reflux peak temperature, cooling rate.
The appropriate stable heating rate is required because, during this process, the viscosity of the solder paste is maintained due to the decrease in the viscosity of the solder paste and the viscosity of the solder paste is increased by the volatilization of the flux. It is important that the top of the component pins leave solder paste.
1.3 use shielded mold wave soldering technology
As a result of the traditional wave soldering technology can not cope with the welding surface fine pitch, high-density patch components of the welding, so a new method came into being: the use of shielding mold (Figure 3) masking patch components to achieve the welding surface of the plug Wave soldering.

1.3.1 Advantages of using shielded die wave soldering technology
1) to achieve double-sided mixed PCB wave soldering production, can significantly improve the double-sided mixed PCB production efficiency, to avoid the existence of manual welding quality consistency of the problem.
2) to reduce the paste solder paste preparation time, improve production efficiency and reduce production costs.

3) Production is equivalent to traditional wave soldering.

1.3.2 shield mold material
1) production mold must be anti-static, common materials: aluminum alloy, synthetic stone (domestic / imported), fiberboard. When using synthetic stone to avoid wave soldering sensor is not sensitive, it is recommended not to use black synthetic stone.
2) Make mold base material thickness. According to the thickness of the reverse plate components, select 5 mm ~ 8 mm thickness of the substrate making mold.

1.3.3 Mold process size requirements
1) Dimension of the mold: The length and width of the mold are equal to the length and width of the PCB, respectively, plus 60 mm of the width of the carrier side
And the mold width must be ≦ 350 mm, the specific process size shown in Figure 4. When the PCB width is less than 140 mm, consider placing two PCBs in a mold at the same time.
2) Edge of the process edge of 8 mm, the other side close to the edge of the installation of 10 mm wide, 10 mm high electric wood to increase the strength of the mold to reduce the deformation of the mold.
3) Each reinforced stapler must be screwed and must be between 150 mm and less.
4) After the mold is finished, it is necessary to install the buckle (fixed PCB on the mold) within 100 mm and within 100 mm. Note the following: ① Do not touch the parts for one week; ② Do not affect the DIP plug PCB can be solid in the mold.

5) the four corners of the mold to open a chamfer of R5.
6) PCBA on the mold in the tin furnace, some parts by the impact of tin waves will produce floating high, so some easy to float parts of the method used to solve the problem. At present the main way: ① metal iron block pieces; ② mold on the installation of pressure buckle pieces; ③ production of anti-floating high-pressure pieces of governance.

2 concluding remarks
As the current circuit board more and more complex, the traditional wave soldering technology has been greatly limited, especially for the welding surface distribution of high-density patch components (or fine pitch SMD) plug-in solder joints, if the manual welding There is a problem that the solder joint has poor quality consistency. The best solution to this challenge is to use the new hybrid welding technology described above, which protects the surface mount components from welding of through-hole components. Choose which welding technology depends on the characteristics of the product may be, if the product batch is small, variety, you can consider the selective drag welding process technology, no need to create a special mold, but the equipment investment. If the product type is single, large batch, and want to be compatible with the traditional wave soldering process, you can consider the use of shielded mold wave soldering technology can be achieved

Selective wave soldering defects and prevention


Defects and prevention, the current impact of the main problem of welding process is lead to lead-free welding conversion and miniaturization trend. Micro-welding refers to a printed circuit board with more SMD components. Processing of solder joint welding technology includes more reflow applications. The assembled through-hole components should be automatically welded to ensure optimum quality. The connection of the components and the electronic board depends on the number of solder joints, but for most products, selective welding is the best way to replace the pallet type wave soldering or manual welding.
Welding technology is now very skilled, but still has a typical flaw. Only locally available lead-free solders have a high melting point and require a higher operating temperature, which increases the risk of certain defects, including the following:
Stripping of solder joints Tail  Tin bridge  Solder ball  Copper pad dissolves

High temperature to the flux to bring no small challenge. Too little flux may cause welding defects such as Weld, and too much will cause electron migration due to residual flux. This paper will discuss these typical flaws and describe how to optimize process parameters to prevent defects.
Solder stripping
Pad stripping, solder joint peeling and solder tear tearing are due to differences in thermal expansion coefficients between PCB substrates such as epoxy / glass FR-4 laminates and copper copper copper on PCBs. In the contact with the solder process, the circuit board Z direction of the thermal expansion will be relatively large. This expansion causes the pad to become conical. This is because the thermal expansion coefficient of epoxy resin is much larger than copper through hole and line. The circuit board continues to expand even if the solder joint has been chosen by selecting the solder wave or immersed in the solder in the nozzle, since most of the heat of curing has reached the adjacent sheet.
After the circuit board leaves the solder, the heat transfer on the solder assembly to the connection is stopped and the connection is cooled to room temperature. At this stage, the heat of curing spreads into the solder zone (see Figure 1), increasing the temperature of all parts on and around the solder joints. When all the curing energy is completely released, the solder joint temperature gradually drops to room temperature. Then the solder joints begin to cure, the circuit board cools and restores its original planar shape. This movement causes considerable pressure on the surface of the solder joint, which is still not strong at this stage. Therefore, this pressure may cause the pad to float. In addition, if the adhesion between the pad and the circuit board than the solder and electronic board adhesion, it will lead to solder surface rupture, which is known as the welding foot tear.
Solder stripping is specifically described in IPC-A-610D5.2.10. Accepts the release of the bottom of the solder from the top of the main surface (weld surface) of the plated through-hole connector. In general, it is difficult to eliminate these shortcomings. But it can be improved by selecting a suitable Z-axis expansion coefficient such as a suitable circuit board material, reducing the pad size of the plated via hole or printing solder resist on the pad.

At this stage, the heat of curing spreads into the solder zone (see Figure 1) increasing the temperature of all parts on and near the solder joints.
Tailing
The next flaw problem is mainly to optimize the multi-peak soldering process by reducing trailing. The wire is a solder residue only outside the nozzle area, and its profile is related to the edge of the nozzle. These residues are generally caused by solder collapse. These lines contain different shapes of solder particles, such as different sizes of oxidized solder webs and solder balls, but these particles are mostly very small. Use the right amount of flux to completely cover the nozzle area to remove the tailing of the multimodal nozzle. In the absence of flux, the solder on the solder can also cause tailing.

 Effect of parameters on trailing. Only the welding temperature (the lower the better) and the amount of flux (the more the better) has a significant impact on the tail.
Perform a large-scale Taguchi experiment (L16 vs. 9 different parameters) to find the parameters that affect the trailing. Experiments show that only two parameters, solder temperature and flux, have a significant effect on tailing. So when there is a smear, you should try to adjust the amount of flux. In addition, applying more flux at the outer edge of the nozzle helps to reduce the solder temperature. With SnPB, the solder temperature can be reduced to 260 ° C. The post-drop temperature is sufficient to complete the through-hole filling.

Tail; solder residue only around the nozzle area
Tin bridge
Tin bridges exhibit different phenomena in selective welding (drag welding) and multi-wave soldering (dual in-line package) processes. In the process of drag welding, stable solder flow is essential. The solder flows out of the assembly in the opposite direction. When the solder begins to flow to the back (along the board), the tin bridge appears. The hot nitrogen scraper forces the solder to reverse flow and eliminates the tin bridge.
If the solder begins to flow along the pin, as shown in Figure 4, the PIN pin will leave the nozzle away from the solder. At this time, the solder will cool and cure to form a tin bridge. Horizontal welding can reduce the risk of unstable solder flow.

Unstable solder flow. Lead-free solder has a tendency to deviate from the nozzle and flow along the pin.
In a dual in-line package process, tin bridges can be avoided if designed properly. The pin is shorter, the pad is smaller, the gap between the PIN feet can reduce the risk of forming the tin bridge. Taguchi experiments show the effects of machine parameters. 10 mg / cm 2; or more flux and lower solder temperature are the best combination to prevent the formation of tin bridges. In addition, experiments show that the preheating temperature has little effect on selective soldering when the thermal mass of the circuit board is not high. Dual in-line package time is short, the lowest slowdown in solder output.

Solder ball
The solder balls are mainly due to high temperature and become more viscous solder resist. In addition, fluxes are more likely to produce solder balls than other materials. In a dual in-line package process, solder balls typically appear between the individual pins, such as Figure 6, where the solder balls appear around the tin bridge.

 In the weld zone, the tin bridge between the four pins is surrounded by a large number of solder balls.
Copper pad dissolves
The higher the welding temperature, so that copper is dissolved into the risk of solder. As the lead-free solder tin content more, at high temperatures, the circuit board on the rapid dissolution of copper speed. In the choice of solder peaks, the flow of solder to form solder joints, this process is much more important than the double in-line packaging process. The more frequent the contact (robot speed 1mm / sec or slower), the higher the welding temperature (> 300 ÂșC), the greater the risk. In addition to the appropriate parameters for the machine, thick copper layer is also very important, but also because of this, should be checked every two months, a copper content of solder to ensure that copper content can not exceed 1%; otherwise the reliability of solder joints will be large discount.

The assembly process, the temperature reached 320 ℃, the copper pad on all the copper are dissolved into solder.
To sum up
Select the appropriate parameters, selective welding can be successfully used. Due to the need for high temperatures, other welding processes into lead-free soldering are a considerable challenge. The biggest advantage of the selective soldering process is its flexibility to optimize each component. If necessary, it is possible to apply more flux for each infiltrated element, for longer periods of time, and to damage other components around it because of the high temperature.

A Practical Guide to Achieving Lead-Free Electronics Assembly

A Practical Guide to Achieving Lead-Free Electronics Assembly


To successfully achieve lead-free electronics assembly, each participant in the manufacturing process, from purchasing to engineering to maintenance to Quality/Inspection, must have a solid understanding of the changes required of them. This pertains to considerations regarding design, components, PWBs, solder alloys, fluxe s, printing, reflow, wave soldering, rework, cleaning, equipment wear & tear and inspection.


With the WEEE and RoHS Directive in Europe (in its most recent revision) potentially outlawing lead from electronics produced and imported in the EU as early as 2006 and foreign competition driving the implementation of lead-free electronics assembly around the world, additional questions regarding how manufacturers can successfully transition to lead-free assembly continue to arise.

A great deal of consortia work and empirical data exists on lead-free soldering. What has been lacking, however, are studies directly related to real-world applications and advice on such topics as procurement, design, processes, maintenance, inspection, etc. This paper shall address each step of the manufacturing cycle and discuss the means to overcome the many challenges of lead-free assembly.


Purchasing’s main challenge is to requisition components and PWBs suitable for lead-free assembly and to balance the needs for parts with the myriad of lead finish, PWB surface finish and solder alloys currently available.

Although vendors are offering some components with lead-free lead finishes such tin, Pd/Ni, Au/Ni, and Pd/Au/Ni, purchasing will be far more restricted in terms of part availability than in the past. When attempting to purchase lead-free components, one may run into several obstacles: only a single-source for a part, a part that is not quite suitable, a change in lead-times, significantly more expensive, or no source at all. To overcome these obstacles, Purchasing needs to work in close conjunction with Engineering/Design

and vendors to ensure that the lead-free parts needed are available and compatible with the manufacturing process.

As with components, there is some availability of lead-free PWB surface finishes . OSPs, Au/Ni, Immersion Sn, Ag, and Lead-Free HAL finish PWBs have been on the market for some time now. Again, Purchasing needs to work in close conjunction with Engineering/Design and vendors to ensure that the lead-free parts needed are available compatible with the manufacturing process.

Materials Management

The many component and PWB coatings, as well as several possible solder alloys results in a huge matrix of potential material intermix, and clearly can complicate materials management. More than ever, purchasing will need to be attuned to which parts go with which product. Once again, Purchasing must work in close conjunction with Engineering to ensure that ordering is streamlined and that the appropriate parts are available for particular jobs.

Engineering

The switch to lead-free assembly affects virtually all aspects of the Engineering function. Engineering personnel will have to pay close attention to design, components, PWBs, solder alloys, fluxes, and the printing, reflow, wave soldering, rework and cleaning processes and equipment.

Design

Established PCB-design rules may need to change during the transition to lead-free soldering. Currently, industry guidelines govern component lead-pad and land size, track width and spacing, via and through-hole dimensions, and similar factors to ensure manufacturability and reliability. However, the physical characteristics of any solder include subtle factors, such as its ductility and elasticity. In addition, the local heating of component leads and their pads causes some thermal expansion during operation, which tin -lead solder accommodates and matches.

In determining design solutions, Design should try to remain with as many standard parts as possible. This will reduce the unpredictability encountered with atypical parts. In addition, if the assembly is designed to have a long life, factor in the reduced moisture resistance of parts. Furthermore, Design must factor in the higher temperatures required for connectors .

Material Considerations

The first critical duty is to ensure that the parts to be used will be compatible and reliable for their particular application. Compatibility relates to components , PWBs , solder alloy and flux. Reliability relates to component concerns, which includes such factors as Moisture Sensitivity Level (MSL) Rating, wetting and tin whiskering.

Component Reliability Concerns

The higher melting temperatures of the lead-free solders that are coming into use mandate components that can withstand the increased temperature stresses of the soldering process. Life -test data for many components at these higher temperatures is less comprehensive than it is for tin/lead processes. To maximize reliability, Engineering should start looking now at all critical components, design rules, fabrication processes, component engineering, and reliability records.

A critical factor in the transition to lead-free assembly is the MSL rating of components. To date, industry testing has demonstrated that there is no generic solution for maintaining an IC’s MSL with a higher reflow profile. However, it has been demonstrated that degradation of MSL may increase with increasing profile dwell above 200°C and that MSL typically degrades by one level for every 5 to 10°C increase of peak reflow temperature. Therefore, all ICs must be reclassified for lead-free applications and the impact to MSL. This could result in an increased need to pre -bake parts and more stringent storage methods.

As discussed above, several lead-free component lead finishes are available. It should be noted that these different materials have different wetting characteristics and that Engineering should consider wetting when specifying components. Engineering also needs to balance the fact that increased reflow temperatures can improve wetting, but worsen reliability. In addition, Design should be aware of reduced solderability on second-side reflow and through-hole processes.

Another hot topic of discussion is tin whiskering, which continues to be an oft -misunderstood and debated subject. Proponents of matte tin argue that whiskering is a result of the plating process, and not necessarily inherent to pure tin. They demonstrate that whiskering can also occur with Sn/Bi, etc. Others, however, suggest that a dopant is needed to aebate and studies regarding this topic, work closely with component vendors and participate in studies to determine the most suitable lead finish for their applications.

PWBs

Several PWB lead-free surface finish options exist. Many of these, such as OSPs and Au/Ni, have been available for years. Engineering should determine the finish of choice based upon wetting, storage, planarity and cost issues. In addition, it must be ensured that board materials can withstand reflow temperatures without warpage or other damage. For many cases, FR-4 will remain acceptable, but other applications may require a modification.

Solder Alloy and Flux

Unfortunately, despite a great deal of research, comprehensive and comparative data on lead-free alloys is lacking. The list of solder alloy requirements is lengthy and involved. In general, technical requirements include being non-“hazardous”, mechanically reliable, thermal fatigue resistant, good wetting, relatively low melt ing temperature and compatible with a variety of lead-bearing and lead-free surface coatings. In addition, one must consider logistical issues such as alloy cost, availability and patent issues. While most of the world has settled on the tin -silver-family of alloys, a good deal of debate still exists as to which exact composition is ideal, and of course others will choose alloys from outside of this family. As with all other technical issues, although there has been much consortia work on alloy selection, the alloy of choice will come down to the specific requirements of each unique assembly. Your choice of alloy is dependent upon your application and should be proven out to your standards.

As with alloys, what is a suitable flux (paste, liquid flux and cored wire) for one manufacturer may not be for another. Select flux chemistries suitable for lead-free processing and your particular application. One should consider a flux’s activation temperature, activity level, compatibility with chosen alloy and reliability properties such as SIR, electromigration.

Process Considerations

Once it is confirmed that the parts and materials to be used in lead-free assembly are available, suitable and reliable, it is time to get the processes optimized in order to achieve maximum throughput and reliability. To do so, Engineering must refocus attention to paste handling, printing, reflow, wave soldering, rework & repair and cleaning.

Paste Handling

Shelf-lives with lead-free pastes may be reduced as compared to tin/lead, and storage conditions may be slightly more stringent. However, in general, the same rules as with tin/lead apply. For example, prevent/minimize paste’s exposure to heat and humidity, allo w paste to come to room temp erature before using and do not mix old and new paste in the same jar. If one follows proper paste handling procedures now and has good results from these, there should be very few issues when transitioning to lead-free paste use.

Printing

In general, no major changes to the printing process should be necessary. That is, lead-free pastes should exhibit similar features on the stencil and the same equipment set points should transition well. One can expect similar performance in terms of stencil life, aperture release, print definition, high-speed print capabilities, print repeatability, etc. However, this depends on the paste manufacturer and if they have density issues resolved. If one experiences a significant difference in printing a lead-free solder paste versus the equivalent tin/lead paste, it may be related to the metal loading or flux chemistry of the paste in

use. In this case, Engineering should work with the paste vendor, or try competitive pastes, in order to resolve these issues.

As tin/lead solder alloys tend to have better wetting than most lead-free alloys, some stencil design modifications may be needed to maximize spread of paste and counteract inferior wetting. Engineers should run tests with lead-free alloys on their current stencils to confirm adequate spread and wetting. If wetting is not sufficient and cannot be rectified by other means, stencil design modifications may be in order.


Reflow

This is the SMT process area that will be most affected by a switch to lead-free processing. Most lead-free alloys require higher reflow temperatures than the 210-220°C peak temperature of tin/lead; anywhere from 235-260°C is common. This higher reflow temperature dictates that one should minimize ? T and maximize wetting through the reflow profile (including cooling), and could possibly mandate reflow equipment changes .

Profile - Depending upon the oven utilized and the density of the assembly being processed, the Ramp -to-Spike process is generally recommended for lead-free assembly. This profile offers superior wetting and less thermal exposure than the traditional Ramp -Soak-Spike profile .

Depending upon the alloy selected, wave soldering will require a pot temperature of 260-275°C. This increase of temperature and the change in solder alloy will require some additional process changes.

Flux- May require a change in liquid fluxes to compensate for the poor wetting of some alloys and high thermal stresses of the wave process. If changing fluxes, particular attention should be paid to both to operating window it offers and the material’s reliability characteristics.

Equipment- Most modern wave solder machines can provide the necessary heat (preheat and wave) for lead-free soldering. However, as shown in figures 3 and 41, the high-tin lead-free alloys rapidly dissolve the materials often used in wave solder equipment. Stainless steel pots, nozzles, impellers and other parts will need to be replaced with cast iron and other materials available from wave soldering equipment manufacturers or be covered with an appropriate paint that should protect the parts for 2-3 years. In addition, a nitrogen blanket may be required, depending upon the alloy and flux selected.

Rework and Repair

Materials - Operators must be re-trained for lead-free rework, as the lead-free solders do not flow as well as tin/lead. This could also require stronger cored wire fluxes to be used. As with any change of flux chemistry, if changing wire solders, particular attention should be paid to both to operating window it offers and the material’s reliability characteristics. Some wires often assumed to be safe to leave uncleaned are actually classified as rosin fully -activated and could cause field failures .

All rework should use the same lead-free solder alloy as originally used on the solder joint; different lead-free solder formulations should not be mixed on the same joint. If more than one alloy is in use in the production process (i.e., Sn/Ag/Cu for SMT and Sn/Cu for wave soldering), operators should be trained to use the correct wire for each part . For this reason alone, it is advisable to use a single solder alloy for all assembly operations.

Equipment- It is necessary to ensure that the desoldering and soldering stations are suitable for lead-free processing, i.e. can reach the necessary temperatures for lead-free soldering. It should be noted that lead-free soldering can wear out tips at a much higher rate than tin/lead.

Cleaning

In general, studies have demonstrated that post-process flux residues from lead-free applications are still cleanable. Water soluble chemistries may be cleaned in water, no-clean and RMA chemistries with a saponifier or cleaning solvent. However, it has been found that an increase in pressure, cleaning times and/or cleaner concentrations often is necessary. The efficiency of the cleaning equipment, strength of the cleaner, melting point of the alloy being used and thermal stability and propensity of the flux to “char” all affect the cleanability of an assembly.

urrent test fixture settings could possibly damage lead-free solder joints. In addition, the higher reflow temperatures may result in charring and make probing through “pin probeable” flux residues more difficult. This could warrant changing flux chemistries or even residue removal in some cases.

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