Copper Dissolution issue

What can we do to avoid copper dissolution in wave soldering process? The PCB board thickness is 93 mil and the composition of SAC305.

I.L.

Expert Panel Responses

According to studies at Celestica and other organizations, you've got approximately 25 to 30 seconds of exposure. Wave soldering should contact the solder for only 2-5 seconds. The bigger issue is rework or solder fountain, where the exposure times are much longer.

Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions

Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.

Copper dissolution in wave is affected by several sources, first is the copper plate density on the board. This seems to have a large effect on dissolution rates. Alloy type, there are several new alloys out that have lower copper dissolution rates. Dwell time and pot temperature also have a direct relationship to this issue. Search out a lower dissolution alloy, keep you dwell time to a minimal by balancing preheat and pot temperature. Also look at type of nozzles and turbulence at point of soldering. Try to keep turbulence to a minimal this should help significantly.

Karl Seelig

Deck Street Consultants

In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

If SAC305 solder is used in wave operation the high tin content will tend to leach copper from boards and terminations. This alloy will leach or dissolve metals such as copper more rapidly than 63/37; in fact several times faster in lab tests was demonstrated. Several process changes may help reduce the dissolution such as reducing solder temperature by 5-10�C or reducing contact time at the wave. Increasing conveyor speed a little is also an option. However these may impact hole-fill especially in boards as thick as 0.093 inches. Another option is the use of tin-copper solder with additives. There are alloys 2 in the market at this time which have shown to reduce copper dissolution. Dissolution of copper can be close to the dissolution of copper with 63/37 using these solders. SnCu solders with additives if properly optimized in the wave solder will give good hole-fill but reduced dissolution. Reduced dissolution will also have the benefit of reducing solder analysis frequency and solder pot maintenance. SnCu solders with certain additives will also give brighter solder joints than SAC305. The additives create patented alloys which can outperform SAC solders in the area of shrinkage effects. SAC305 tends to give joints with hot tears in some cases; this is also reduced with these patented solders. SnCu without additives is however not recommended.

Peter Biocca
Senior Market Development Engineer
Kester

Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.

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