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Long term effect of lead free solder on plating

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Slaine

#40023

Long term effect of lead free solder on plating | 27 February, 2006

What are the long term issues with high tin content solders on parts plated with silver over copper? Is it possible that the silver will be absorbed by the solder then over a long period of time the tin will react with the copper causing it to part from the substrate giving long term reliability issues. (thinking of either coper or brass sustrates.) therefore the plating that must be used should be silver nickle copper. Tin plating has been ruled out for the part im thinking of as the tin mets at 232 degrees and creates other problems. At the edge of my knowladge on this so forgive me if im way off the mark.

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#40024

Long term effect of lead free solder on plating | 27 February, 2006

Silver on your copper or brass part is a solderability protection. The silver diffuses into the solder and you solder to the base metal, either the copper or brass in your case. Soldering to copper is well known. The issues are soldering to brass and silver diffusion in the solder in either base metal.

SOLDERING TO BRASS: Zinc from the brass diffuses into the tin causing solderability problems. So, you need a barrier plate between the brass and the tin. Use a flash of copper or nickel-plating between the brass and tin. Nickel has a longer shelf life, but copper is easier to control so we suggest you use copper.

SILVER DIFFUSION: Silver creates an even weaker intermetallic compound (IMC) with tin than does gold. We�re not sure of the critical Ag concentration. We have heard the 4 percent gold number that is bandied about for silver. Remember the 4 percent assumes a uniform distribution, but typically there is a much larger concentration near the interface. We know that 2 percent silver is safe.

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Slaine

#40025

Long term effect of lead free solder on plating | 27 February, 2006

on sections ive looked at with 7 microns of silver and 4microns of copper, the silver had been totally absorbed into the solder in most places and a large amount in some places all the copper as well, so i think the answer is that to provide long term reliability i need a silcer over nikle plating finish.

thanks for your input dave.

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