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Mixing solder alloys

It is not recommended that alloys are mixed as combinations of metals will be created that could potentially affect long term reliability. Research has shown that some combinations can severely shorten the life of a product. The combination of tin/lead with bismuth solders has been found to create solder joints which are particularly unreliable and so this should be avoided.

Backward compatibility

Manufacturers should be aware that there are a few issues with the use of new “lead-free” components and continued use of tin/lead solders.

The wetting properties of tin plated termination coatings will be different to tin/lead coatings. As a result, reflow profiles and wave soldering conditions may need to be modified. Hand soldering should not present difficulties although wetting times may be slightly longer.

Lead-free ball-grid arrays can be successfully attached to PCBs with tin/lead solder paste but unless the reflow temperature is increased, the balls will not fully melt and this can affect long term reliability.

Defects

All of the defects that can occur with tin/lead also occur with lead-free solders but many will be more likely to occur including:

• Poor wetting
• PCB warping
• Cracks in plated through holes (Figure A)
• Conductive anodic filaments
• Tomb-stoning (Figure B)

 

The higher temperature can also cause delamination of multilayer PCBs and damage components.

Trials will identify whether any of these is likely to occur and determine what corrective action will be required.

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