Letter 5092

Cleaning/reducing nickel surface prior to soldering

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Can someone tell us how to clean nickel with a gold flash on it prior to soldering? The problem appears to be excessive nickel oxide or nickel hyroxide on the surface.

We routinely reflow lead-tin solder contacts to electroless nickel overplated with a thin electroless gold flash (around 4 microinches). The solder is deposited in a paste form with acid flux in the paste. The solder is eutectic, and the reflow has to be done at low temperature (less than 230 C).

The solder usually adheres well, but in some cases the bond is not strong. Solder will adhere to most of the nickel area, but peels off other parts. When we analyze the defective areas we find that there is residual nickel oxide and 9 to 11% phosphorus. In some cases we also see boron.

The gold flash usually shows nickel oxide on its surface, either from incomplete rinsing or pinholes.

Lars Carlson
- San Diego CA

If I understand your problem correctly, first of all you should not see any piholes exposing nickel or its oxide on a gold surface. Exposed nickel probably oxidizes during/after hot soldering process. You may just need a thicker gold flash. If the gold is peeling off from the nickel surface, and you could see it on the peeled solder, the acid dip before gold plating is inadequate.

Mandar Sunthankar
ionEdge Corp - Ft. Collins, CO

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