What you didn't mention is the relative rate this phenomenon occurs.  Having
it occur on the same board type built by two different assemblers could just
be a coincident.  If it is, then I have seen this and it is an elusive
problem that can be very difficult to eliminate.  There are many sources of
gold finger contamination, but I only have a short list.
1.  traces of solder paste on the board support tooling in the screen
printer.  (Check there first!)
2.  accumulated solder paste in the stencil wash redepositing on boards
washed there.
3.  not washing misprinted boards in an ultrasonic stencil wash, thus
allowing solder powder to surface from every crevice on the board.
4.  traces of solder paste on the underside of the stencil allowing solder
balls that find their way onto the edge fingers.
5.  aggressive reflow ramp rates that cause solder balls which get onto the
fingers.
6.  other unidentified causes.

If it is not a coincident that both vendors have this problem, I'm at a loss
for explanations.

Kind Regards

Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]


> -----Original Message-----
> From: Bev Christian [SMTP:[log in to unmask]]
> Sent: Friday, September 14, 2001 8:34 AM
> To:   [log in to unmask]
> Subject:      [TN] Technical question - immersion gold
>
> Here's the situation:
>
> We have immersion gold over nickel boards made by two different board
> shops, populated by different companies and in both cases we have spots of
> reflowed solder on the gold contact pads after reflow.
>
> The board shops use no tin/lead in their gold/nickel processes.  The
> boards did not come in with this problem.  These pads were not screen
> printed with paste.  Both ovens use edge fingers.  Both ovens are
> convection.  Some boards were actually run through the oven up-side down
> and the problem was still observed.  The problem is only seen on the one
> board type, although we have others that are immersion gold over nickel.
>
> Has anyone every seen anything like this?  What do you think caused it?  I
> have my suspicions, but I will keep them to myself and see what you have
> to say first.
>
> regards,
> Bev Christian
> Research in Motion

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