Jerry,
we are using OPC coating now. In July, we started printing large
boards with 0.3 mm pitch QFPs. I don't know if we could have printed
this small pitch on HASL boards, though I think the biggest
contribution to our success was a new printer.
The gold question arose on some other boards that are presently OPC
and which have press-in connectors. The connector company recommended
HASL, but the designers chose bare copper. We had some failures where
the press-in pin gouged out part of the through-hole barrel. The bare
boards presently come from Japan, and our customer asked us to buy
them locally. Our design department checked all the options, one of
which was Ni-Au. Probably, in the end, HASL will be chosen since the
finest pitch is O.5mm. But we also wanted to check the possibility of
Ni-Au since it seems to be getting more attention lately.
Thanks again for your input.
Allen Ahlert
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Subject: Re: solubility of gold into solder pot from Ni-Au board
Author: [log in to unmask] (Jerry Cupples) at halsic_ccsmtp
Date: 9/8/95 9:16 AM
Allen, you said:
> Jerry,
> Thank you for your response to my questions about soldering gold
> plated boards. I share your wonder about gold plating when the
> soldering texts say gold is difficult to solder to.
I think it is really a matter of whether the base metal under the gold is
solderable. This is normally going to be nickel, which is more difficult to
activate, and not known for inherently good wettability. The gold is simply
going to dissolve, it will not wet; it is going into solution in the joint
and/or the liquid solder of the machine's pot. The idea whith this coating
is that the gold keeps the nickel from passivating, it's only a sacrificial
protective layer.
> I received a response from J Ahearn at Automata who, I believe,
> fabricates circuit boards, many with gold plate. He said none of their
> customers have experienced solder pot contamination beyond recommended
> limits.
Perhaps true, but as my calculations show, lots of area of a very thin
coating are required to reach even 0.02% in a 200 kg solder pot. You solder
enough boards, you will have contamination and that's a sure bet. Remember
though, that as you solder in volume production, you will "dilute" the
solution with fresh solder bars. So the calculation I showed would really
need to be complicated further by the "drag-out" of solder on boards being
processed, and the addition of new bar solder. This may explain why some
"never" reach the point of having to replace the pot.
> Other responses have been to just monitor the gold level and
> sell the pot for its gold content when the upper levels are reached.
Yes, but I can tell you that the "reclaim" people will say that the level
of gold has to be pretty high to justify recovery of gold, and that level
may be above the point where you would see wetting or graininess problems
in the first pot.
Don't forget my other point, if you are talking about SMT reflow of boards
vs. wave soldering, the gold will NOT be flushed, it stays in the joint,
and the concern could be intermetallics and fatigue life effects in SMT
joints, not solderability issues.
> I'm not sure how quickly our design department will proceed in
> qualifying the gold boards; they are pursuing alternatives as well and
> one of those may be chosen. If we start soldering gold boards, I'll
> try to let you know about our problems and successes.
Good, I'm curious about this. Seems to me that the whole concept of au/ni
protective coating is fairly wierd, but it may be workable. Sometimes those
strange sounding concepts turn out to be poplular in time.
The present problems I have with printing paste on fine pitch QFP patterns
with irregular HASL coating are making me re-think some established ideas.
Have you looked at OSP (such as Enthone Entek) coatings over copper on your
boards?
regards,
Jerry Cupples
Interphase Corporation
Dallas, TX
214-919-9150
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