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.
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
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