Paul
Unfortunately, I don't have the published literature handy, but it dissolves
fast! It will dissolve to saturation. And if it is not fully dissolved,
you will have remaining gold on the surface, which is now contains a low Sn
amount. I think it is the brittle, 90 weight% Sn "phase" of Au-Sn compounds
(again, I can't remember the phase diagram correctly, but I think it is
AuSn5 or something like that...I used to live with that diagram on my
wall...) that is the killer to bond strength. Any amount of gold that leads
to a couple of percent (I've been corrected to say 4%, but that is still too
much because that is a theoretical situation based on equilibrium
thermodynamics- in other words, aging for a million years).
That said, I have used 500A Au-on-Ni (my substrate/board) and interfaced it
with 5000A Sn-on-Cu (my contacting TAB leads). This amount of gold is
dissolved within milliseconds (I was doing some weird thing) of soldering
time. It withstood aging tests with no degradation to pull strength (the
leads would break before the joint failed). I've also used 25um Au pads on
my board and soldered those same leads on with similar success. In the
latter case, there is concern of reliability unless the Sn is completely
consumed in the Au. For using low Sn, this was close to the issue, but I
saw publications that made me stay away from that (besides it's not
cost-effective).
I agree with Inge, not to use more than a fraction of a micron if you have
to. If the Au is plated, its nonuniformity is suspect.
Carey
-----Original Message-----
From: Paul Klasek <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Wednesday, October 13, 1999 12:40 AM
Subject: Re: [TN] Soldering gold terminations
>Werner would you have any statistics of how much Au goes onto the base
metal
>?
>We did experience LED's with over 1um Au .
>True , they solder without problem ; but they did not stay on for long in
>cycling .
>Same with cer cap's .
>I think Gaby referred not as much as to solderability as reliability ;
>did you experience any caps falling off prematurely due Au content ?
>
>Paul Klasek
>ResMed
>
>-----Original Message-----
>From: Werner Engelmaier [mailto:[log in to unmask]]
>Sent: Wednesday, 13 October 1999 13:58
>To: [log in to unmask]
>Subject: Re: [TN] Soldering gold terminations
>
>
>Hi Gaby,
>There is no such thing as a "gold termination"; you have some metallization
>that is covered with gold to protect it, most likely, from oxidation. Gold
>goes into solution in liquid tin too fast for this. So the real question is
>how to best solder to the metal comprising the layer underneath the gold.
My
>advice would be to try your standard soldering procedures; they most likely
>will work.
>
>Werner Engelmaier
>Engelmaier Associates, L.C.
>Electronic Packaging, Interconnection and Reliability Consulting
>7 Jasmine Run
>Ormond Beach, FL 32174 USA
>Phone: 904-437-8747, Fax: 904-437-8737
>E-mail: [log in to unmask], Website: www.engelmaier.com
>
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