Ken,

Gold embrittlement is more a concern for fine pitch surface mount joints
where the percentage volume of gold grows as the absolute volume of paste
decreases. That being said, the gold thickness should still be kept low.
The pads which need to be gold will drive the final decision; why are
those pads gold and what does that mean for their necessary gold
thickness? If they are "thick" electrolytic gold, over 30 microinches,
then typically the fabricator would make the board in 2 gold passes: one
pass to put on a light sacrificial thickness of gold; then they would mask
off the whole board except for the pads of interest; then another pass in
gold to plate up those pads thicker - to 30 or 40 or 50 microinches
(whatever is called for). This 2 pass processing adds cost. If the gold
pads can be 25 microinches or less, then they can just gold plate the
whole board in one pass.

For all of this I am assuming that this is electrolytic gold over
electrolytic nickel.

Valerie






Ed Hare <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
10/27/2004 10:27 AM
Please respond to ehare


        To:     [log in to unmask]
        cc:
        Subject:        Re: [TN] Gold Board


Ken,

Yes, there most definitely is a concern.  The wt% gold must be kept below
~
3 wt% or the volume fraction of AuSn4 will be too high in the joint
causing
gold embrittlement.

Ed Hare
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bloomquist, Ken
Sent: Wednesday, October 27, 2004 6:53 AM
To: [log in to unmask]
Subject: [TN] Gold Board


Good morning TechNetters (at least here on the left coast),

We have a small assembly that requires a couple of pads to be gold plated.
We would like to gold plate the entire board but we're concerned about
gold
embrittlement when we solder the components in. The board is completely
through hole and will be both wave soldered and hand soldered.

Do you think we have any concerns? Oh by the way this is a class 3
assembly.

Thanks,

KennyB

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