Gold embrittlement is well understood, but not widely understood.
The good news is, the risk of gold embrittlement has been quantified.
It is a function of the solder density and the allowable (maximum) percent
gold. The maximum ratio of gold finish thickness to solder alloy thickness can
be calculated, to help prevent a problem from excessive gold-tin (i.e., AuSn4)
compound. For the equation and its derivation, please refer to a paper of mine,
"The Use of Capillary Action Measurements for Solderability Improvement,"
Soldering and Surface Mount Technology, No. 9, Oct., 1991, Wela Publications,
Ayr, Scotland, Table 5.
It is interesting to note, The Embrittlement culprit, AuSn4 is 29
weight percent gold. So even if one had 100% tin as the solder alloy and 10%
gold were dissolved into the solder, then about a third of the solder joint
would be rocks (i.e., compound) and the remainder would be soft solder. Do not
count on the rocks to bond to the solderable surface. Hence most
recommendations I have seen have been in the 3 to 5 weight % gold range for the
maximum allowable percent.
Gold is dissolved by molten tin based solder, but the gold does not go
into solution (i.e., the result is not a solid solution). For more details on
the mechanism and explanation, please refer to a paper of mine: "Understanding
Gold Embrittlement in Surface-Mount Soldering," ISHM's 1st Joint Technology
Conference, San Diego, March, 1990 (//WWW.ISHM.EE.VT.EDU). Incidentally, ISHM
changed its name to IMAPS, (703) 758-1060.
The above represents my own view, and not necessarily that of my employer.
---------------------------------------------------
Mike Wolverton
Texas Instruments
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------------------
Original text
From: "ddhillma" <[log in to unmask]>, on 3/20/97 5:50 PM:
To: [log in to unmask]
Cc: [log in to unmask]
Hi Tom -
The words gold embrittlement really mean job security for
metallurgists! You would think that as well researched and
investigated the gold embrittlement mechanics of failure has been over
the last 20 years we would stop making the same mistakes. Two rules of
thumb to follow:
a) Is there enough solder volume to allow the gold to uniformly go
into solution?
b) Is the soldering operation of sufficient temperature and time to
allow (a) to occur?
The thickness of the gold finish isn't the only deciding factor of
having a solder joint embrittled by gold/tin intermetallics - the
process is equally important. Also, you can avoid the whole gold
embrittlement issue by keeping the gold below 10 microinches but you
then have to deal with poor solderability on the components if this
thin gold plating is of poor quality (and plating a quality gold
finish below 20 microinches requires good control and knowledge of
one's plating system).
Dave Hillman
Rockwell Collins
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______________________________ Reply Separator
_________________________________
Subject: Gold Embrittlement
Author: [log in to unmask] at ccmgw1
Date: 3/19/97 3:17 PM
To those who have heard enough of this subject I send my deepest apologies,
but to those who still have some energy left to help a double 'E', who has
found himself doing a lot of PCB design lately, I need to try to obtain
some information on Gold Embrittlement.
My question is:
Can SMT components be attached to PCB's, which are fabricated using 50
micro inch 'Hard Cobalt Gold' over 200 micro inch low stress nickle,
without having to worry about Gold Embrittlement, and if so, are there any
special process steps which need to performed or monitored. We are using a
mixed bag of SMT packages with 1mm pitch being the finest.
I have encountered conflicting data on this subject so far.
Thank you
Tom Guerriere
Staff Engineer
Controlotron Corporation
(516) 231-3600 x81
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