Selective wave soldering defects and prevention

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 ºC), 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.