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topic 537

Soldering gold plated copper


(1996)

When applying solder over gold plated Cu, and reflowing in a convection oven, the solder turns grainy, porous, and does not appear very healthy when observed under a 40X microscope. The Au is applied over Ni. Ni is 80-120 microinches, Au is 90-110 microinches and the solder is 63/37 water soluble. Does this pose a reliability risk? How can I improve? What tests can be done to check for reliability or function?

Saleem A Ganiy



(1996)

Saleem,

Your problem is that the gold is soluble in the molten solder, this forms an intermetallic and makes the solder look dull and grainy, the joint that is formed will also be brittle.
Less gold is actually better when solder is to be applied (25 - 50 microinches), that way less gold is there to dissolve into the molten solder.

Paul Stransky
- Putnam, Connecticut


(1996)

Saleem, if it is not a manufacturability problem, you may consider pre-plating the solderable leads with Sn-Pb instead of gold. Also realize that if the gold layer you are attempting to solder to is plated simultaneously with any gold die-attach/wire-bond surfaces in an overall fashion, reducing the gold thickness to promote solderability on the leads may(will) kill any die-attach or wirebonding that is expecting 100 microinches of Au to be there.
Good Luck,

Dave Kinghorn
Dave Kinghorn
Chemical Engineer
SUNNYvale, California



(1996)

Dear Saleem, You do not mention if this is soft or hard solder but I am assuming it is soft by the 63/37 ratio of Sn-Pb. You do not mention any use of flux which is essential for proper flow, and I just happen to have a formulation for a flux which leaves the flowed solder nice, shiny and smooth. Being in the jewelry industry I see it done more often than I would like to.
Sincerely,

David Guleserian
- Rhode Island



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