Hi Jan -  Gold embrittlement was "discovered" and investigated in the
1960's - we just seem to be really slow in implementing preventative
measures to stop "rediscovering" the same phenomena! Think of avoiding gold
embrittlement as having to satisfy these two criteria:

1) Is there enough solder volume present  for the amount of gold that is
present to allow the gold to completely diffuse into the solder joint?

2) Is the soldering process being used hot enough (i.e. greater than 183C),
long enough to allow the diffusion process to occur?

In  most cases there is sufficient solder to allow the gold to diffuse into
the solder joint thus avoiding the formation of the brittle (crack causing)
Au4Sn phase. The problem is criteria #2 - some soldering processes are too
fast and create zones/areas of gold embrittlement. Take a look at the
following references - these are very informative.

"Manufacturing Concerns When Soldering With Gold Plated Component Leads or
Circuit Board Pads, Mark Ferguson, IBM, IPC-TP-1103"

"Effects of gold on the Reliability of Fine Pitch Surface Mount Solder
Joints, Circuit World , Vol.18, 1992, J. Glazer"

Good Luck
Dave Hillman
Rockwell Collins
[log in to unmask]





Jan Satterfield <[log in to unmask]> on 09/08/98 12:03:32 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
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Subject:  [TN] Soldering to Gold Plated Pads




I have just spent the last 2 hours going through the TechNet archives
reading correspondance regarding to soldering to gold plated pads.  I am
now more confused then when I began.  We have just finished reflowing
and wave soldering our first gold plated pwb (mixed technology).  It
looks terrible.  Talk about dull and grainy solder connections!  The
wave soldered connections look worse than the reflowed connections.

The suggestions I've read in the archives include:
Altering reflow profile (longer/hotter).
SEM Analysis and microsection.  (we are only building  5 CCA's at this
time).
"Quit looking at the solder joint appearance".
A remedy of better preheat, slower conveyor speed and higher wave solder
temperatures.
Use 2% silver in your paste.
Use more solder paste.

Are these dull connections acceptable?  What constitutes true gold
embrittlement?  Is gold embrittlement a legitimate defect?  How are
other companies dealing with gold plated pads.  Do you need to change
your process?

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