Hi Daan! The solderability of a board can be influenced by prebaking if the
surface finish system (base metal + barrier finish + final finish, in your
case copper + nickel + immersion gold) has problems. A couple of areas to
investigate: (1) was the immersion gold deposited in such a manner that it
is relatively porosity free? A porous immersion gold finish will allow the
nickel to diffuse to the immersion gold surface during prebaking and make
soldering very un-fun. (2) was the nickel barrier finish deposited in a
solderable manner? If the immersion gold is fine but the surface of the
nickel was tarnished you will have soldering problems. (3) Is the copper
diffusing to the board surface? If the nickel and the gold are messed up
then the copper might be getting to the surface of the immersion gold.
Investigate your board finish system to make sure that it is not the source
of your solderability problems before chasing down any possible processing
source. I use the same solder paste for HASL and immersion gold finishes
without problems. Good Luck.

Dave Hillman
Rockwell Collins
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"d. terstegge" <[log in to unmask]> on 09/10/99 07:17:48 AM

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Subject:  [TN] soldering to immersion gold boards




Hi Technet,

I'm having some problems lately with soldering of pcb's with an electroless
nickel/ immersion gold conductor finishing, which brings me (after some
discussions with our board supplier) to the following questions :
Is solderability influenced by pre-baking the boards ? Does the first
soldering step have an effect on solderability for the second soldering
step (double sided reflow) ? How about the time between all these thermal
excursions ?
Is it possible that a solder paste that performs excellent with HASL-boards
(and normally also with immersion gold boards) is a bad choice for
soldering to immersion gold ?

Greetings,

Daan Terstegge
Unclassified mail

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