Best Paste for Gold Finished Surfaces

We use water soluble leaded past and have discovered that the solder joint finish is markedly duller when used on ENiAg finished surface PCBs as compared to HASL. Do you know what causes this and is there a better paste to use for gold finished surfaces?

Steve Hughes

Expert Panel Responses

More than likely, this is related to the dissolution of Gold into the solder joint. Gold is easily dissolved by molten Tin-based solders during the reflow process and the added Gold in the solder joint can result in a grainy appearance on the surface of the solder. One thing that can be investigated here is the thickness of the Gold layer on the ENIG board. This thickness should optimally be in the range of 5-15 microinches. Anything too thin and you may have a solderability issue due to the porous Gold surface. Anything too thick and you will end up with too much in Gold in the solder joint and may face grainy solder joints and also potentially a condition called gold embrittlement. This issue cannot be affected by trying another solder paste formulation unless the mechanism for the dull finish has more to do with solder paste activity than with the Gold dissolution.

Brian Smith
General Manager - Electronic Assembly Americas
DEK International

Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.

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