Oct 18, 2018 
High melting temperature of lead-free solder requires higher operating temperatures, which increases the risk of soldering quality defects, the following problem will happened: Solder peeling ,solder smearing,solder bridging, solder balls, dissolved copper pad.
Normally we will take the following handling solution:
1. Solder peeling
PCB solder peeling is due to the thermal expansion difference between the PCB base material Resen /glass FR-4 material and the copper hole or copper lead.
After the PCB leaving the solder, the connecting parts temperature cool to room temperature. At this stage, the curing heat diffusion to solder area, prompting the joints as well as all the parts near the temperature increases.
When all of the curing energy fully released, solder temperature gradually to room temperature. Then solder begins to solidify, the circuit board was cooled, and to restore its original planar shape.
This movement has brought to the surface of the solder joint considerable pressure, but at this stage is still strong joints. Therefore, this pressure may cause the pads float away.
2.SolderTrailing
Wired refers only outside the region of the nozzle solder residue, its contour edge related with the nozzle.
These residues are generally due to solder melting caused the collapse.
These wired contain solder particles of different shapes, different sizes, such as network and tin oxide thin solder balls, but most of these particles are very small.
Use the right amount of flux to the nozzle area can be cleared completely covered tailing multimodal nozzle next.
Under no circumstances flux, solder resist on the solder can also cause tailing.
Experiments show that only the amount of solder temperature and flux of these two parameters have a significant impact on the smearing.
So when there is tailing case, you should try to adjust the amount of flux. In addition, the outer edge of the nozzle apply more solder flux helps reduce temperature.
Use SnPB, solder temperature can be reduced to 260 ?. After lowering the temperature enough to complete the through-hole filling.
3.Solder bridges
If the solder began to flow along the pin, as shown, PIN place left foot will slip away from the solder nozzle.
At this time, the solder cools, solidification, forming bridging. Horizontal welding can reduce the risk of instability in the solder flow.
Unstable solder flow. Lead-free solder nozzle deviates along pin flow trends
In a dual in-line package process, if properly designed to avoid bridging.
Pin shorter, smaller pad spacing between PIN feet wide can reduce the risk of some form of bridging.
Taguchi experiment shows the influence of machine parameters. 10 mg / cm2 or more, the amount of flux and solder temperature low to prevent the formation of bridging the best combination.
4.Solder balls
Solder ball is mainly due to high temperatures and become more viscous solder resist produced. In addition, more flux than other substances produced solder balls. In a dual in-line package soldering process, solder balls usually appear between individual PIN foot
5.Copper pad dissolved
Higher soldering temperatures, the risk of dissolution of copper into the solder. Because lead-free solder containing more tin, at high temperatures, the dissolution rate of copper circuit board has accelerated dramatically.
On the selective soldering peak flow of solder joints formed contact more frequently (robot speed 1mm / sec or slower), the higher soldering temperature (> 300 oC), the greater the risk.
In addition to setting the appropriate parameters for the machine, thick copper layer is also very important, because of this, solder copper content should be checked once every two months to ensure that no more than 1% copper content; otherwise, the reliability of large joints discount.
Select the appropriate parameter, selective soldering can be successfully used. If necessary, it can be coated as a problem more elements each infiltration flux, soaked longer without other elements around because the temperature is too high and damage.
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