Preventing Common Mistakes With The SMT Process During Soldering

There are more designs than ever using small components utilizing surface mouth pads. While they are critical in many modern-day electronics, if the solders are not done correctly, it can lead to defects which can cause a high rate of failure. While some might seem minor during production, they can lead to increased costs to rework and eliminate these issues. It is important to minimize defects in the production of surface mount technologies (SMT assemblies). 

Basic Defects Found in SMT Assemblies

The most common defects are listed below:

  • Bridging Between Pads
  • De-Wetting Solders
  • Poor Wetting with Lead-Free Solder
  • Component Shifting and Tombstoning

Bridging Between Pads

Found commonly in low viscosity solders, it causes a short between pads adjacent to one another. It can also occur when temperatures are outside the proper soldering range. This can result in poor wetting which can create shorts between components. The way to solve the problem is to place a solder mask around each of the pads. This creates enough space for the excess solder to properly wick. 

De-Wetting Solders

This problem occurs during the selection of solder paste. When defects occur, it is important to look into this when going down the checklist of things to investigate. This problem doesn’t happen when using lead-free, water-soluble solder paste. It can also happen with halide-based pastes when soldering onto HASL finishes. When the surface is heavily oxidized or if the solder paste has expired it can also happen. 

One way to deal with this is to use a highly activated solder paste, with active flux, when applying the solder during assembly. This creates a durable bond. Cleaning surfaces and removing all traces of oxidation can help as well.

Poor Wetting With Lead-Free Solder

For specific compliance issues, lead-free tin-silver-copper solders are required. However, it can cause problems with poor wetting. This is one reason why surface finishes are used when conductors are exposed. It is also important to make sure the temperature is in the appropriate range to ensure proper bonding.

Component Shifting And Tombstoning

While tombstoning may sound rather cryptic, is it also an issue related to wettability in relation to SMT assembly equipment. During an ideal soldering process, the molten solder will wet all pads in surface mount assemblies. This issue can occur when there is an extreme temperature mismatch between components. Another way tombstoning can occur is when the solder is not uniformly applied to the surfaces appropriately. 

For all of your SMT assembly needs, reach out to Star Engineering. With a wealth of knowledge, our experts can address any issues arising with assemblies. Why not reach out to us today and find out how we can help you minimize mistakes and maximize profitability?

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