I have experienced this problem in the past and have found the problem
to be very tenacious. This applied to products built to Bellcore
standard. There have been two cases of solder on gold that were
noticed: solder on gold after reflow where it was assured there was
none before SMT and solder on gold upon receipt of the boards from the
fab board supplier. It sounds like you have the former problem.
The problem was not controlled until we paid extra attension to make
sure the gold finger do not get anywhere near solder paste. Solder
paste particles get everywhere, are particularly difficult to see, and
remove from work areas and work surfaces. To help confirm this
hypothesis, solder powder was obtained from our paste supplier,
individual solder particles placed on gold fingers, reflowed, and the
spot size measured. The dot diameters were compared to that found on
production boards and found to represent the same population. Taping
(Kapton) the fingers within the surface mount work area was tried, but
did not eliminate the solder on gold problem. It was assumed that
this was due to solder particles contaminating the work area. They
were then Kapton taped in an area removed from the SMT area (in the
stock/kitting area) and the solder spots were significantly reduced.
Dave Shaddock
GE Corp R&D
> ----------
> From: Annie Laberge[SMTP:[log in to unmask]]
> Sent: Tuesday, February 03, 1998 5:47 AM
> To: [log in to unmask]
>
> I am not shure I can help you but at least I can share with you what
> we are
> experiencing over here: The same thing as you are.We have solder
> spot on
> gold fingers (We assemble PCBs).
>
> The board is a double sided 0.062" SMOBC board with goldfingers( 30
> microinches of gold over 100 microinches of Nickel) The assembly
> process
> consists of screen printing of solder paste, pick and place SMT
> machines ,
> Reflow, manual insertion of through hole
> components and finally wave solder. The problem manifests itself
> after IR
> reflow. There are tiny silverish/grey spots on the goldfingers, on
> solder
> side and componant side.
>
> What is strange is that we don't see any tendancy in terms of
> Manufacturers
> or part numbers. At least 6 manufacturers are involved with this
> issues,
> and almost all part numbers has this issue on and off.
>
> We also put Kapton tape BEFORE assembling to see if the problem
> comes from
> the board or the process. And after SMT assembly, we remove the
> tape and
> find solder spot on gold fingers. So it definitively comes from the
> PCB.
> At incoming inspection, we don't see those solder spots (the
> inspection
> may not be as rigorous as on the assembly line). What we think it
> could be
> is tinny solder flakes on the boards which with manipulation fall on
> gold
> fingers during assembly (before reflow oven).
> We will investigate futher for this point.
>
> But if anybody can give us more input, it will be greatly
> appreciated.
>
> ALaberge
> Quality Eng.
> Matrox Electronic systems
>
> ____________________________________________________________________
> _________
>
> At 05:04 PM 1/29/98 EST, [log in to unmask] wrote:
> >We are PCB manufacturers and seem to be having a enigmatic problem
> on one of
> >our boards we manufacture for one of our customers.
> >
> >The board is a double sided 0.062" SMOBC board with flash gold
> goldfingers(
> >1-3 microinches). The assembly process consists of screen printing
> of solder
> >paste, pick and place SMT machines , Reflow, manual insertion of
> through hole
> >components and finally wave solder. The problem manifests itself
> after IR
> >reflow. There are tiny silverish/grey spots on the goldfingers.
> Suspecting
> >that the spots were caused by solder paste particles which were
> either
> >deposited because of contaminated equipment or degassing from the
> PTH causing
> >splattering of solder paste we ran the next batch of boards using
> Kapton tape
> >on the gold fingers. Unfortunately we saw the same spots again. The
> straw
> >which broke the camels back was when a recent datecode boards fresh
> out of
> the
> >cartons were run without Kapton tape and exhibited the same
> symptoms.
> >
> >Our dilemma:
> >
> >We manufacture 30 P/Ns for this customer. The problem is only on
> this P/N.
> The
> >same lines are used to run all the boards.
> >
> >We have manufactured thousands of these boards without any problem
> and now
> >suddenly we have this issue.
> >
> >The same assembly line used to assemble this board is used to run
> other
> boards
> >we manufacture and no other P/N exhibits this problem.
> >
> >Hope somebody has seen such a manifestation and can advise.
> >
> >Thank you.
> >
> >Richi
> >
>
> >
>
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