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Tin Plating discoloring

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I have tin plated parts (lead free)that are discoloring between 250-275 degree C. Plating is 120u Tin over 50u nickel. This is a stamped contact and is formed to accept a blade. When heated to temperature the part is only discoloring on the outside surfaces. Discoloring is a yellow/brownish color. We are looking for possible answers or contributions for this. Any ideas?

Thank You

Bob Lento
Autosplice - San Diego, California, USA

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I also have tin plated stamped metal parts (not leadframes) dis-coloring in the temp. range of 250 - 260 C. Parts are from multiple suppliers and plating processes - tin over copper on phospher bronze, and tin over nickel on 301 stainless. Is there an alternate lead-free, solderable plating which will not dis-color in the high-temp reflow process?

Lynn Rischar
SMT assembly - Dallas, Texas

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Tin migrates into many metals to form intermetallic compounds. These compounds are not usually the silver colour normally associated with tin. The tin metal also melts at 232C and exceeding the melting point speeds up this migration process. I suspect that you are forming the intermetallics and depleting the surface of free tin, thereby causing a discoloration.

Trevor Crichton R&D practical scientist The Pheasantries - Chesham, U.K.

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The discoloration is cause by tin oxide.
The color of tin oxide will change from slight yellow to brown with the increase of SnO film thickness.
Because the thickness is only about 10nm, it is difficult to detect by edx.

Geng
- Shanghai, China