"Gold isn't a bad surface finish - it just has some disadvantages that
need to be recognized and dealt with in a proactive manner."
Exactly! Somebody finally nailed it. Good job, Dave.
Bev, gold within the solder joint may or may not be a problem. The
concentration is important, but other factors are equally important. It
depends at what temperatures the platelets are formed, what shape they
take, where they are distributed, and the configuration of the solder
joint and the stresses inherent to the joint.
I have numerous failure analysis reports that I have saved over the past
25 years from instances at Honeywell, ADC Telecomm, General Dynamics,
Pemstar (for various customers) and other companies where I have been
involved with fractured solder joints due to gold embrittlement,
primarily from gold on the component leads.
Some factors that affect the amount of embrittlement and/or contribute
to fracture are:
Percentage of gold by volume
Temperature profile during reflow
Thermal cycling after reflow (service loading). This directly affects
the rate of nucleation of the gold within the solder joint. It is a
time-variable phenomena.
Physical dimensions of solder joint (the larger the surface area where
an IMF formation occurs between a gold-plated SMT component lead and a
board pad, the more likely it is to pose a problem, as opposed to a
through-hole connection, for example).
Amount of nickel underneath the gold
Plating parameters
Etc., etc.
More often than not, fractures blamed on gold embrittlement are actually
a combination of several issues, including brittle nickel syndrome,
P-content, amount of nickel oxides, etc.
I have had situations where as little as a 1% content of gold by volume
led to premature fractures, and whenever the gold was removed, either by
changing the part finish or by pre-tinning, the problem disappeared.
Every single time.
Carefully read this:
http://www.binghamton.edu/physics/pub/cottsectc.pdf
I have other evaluations and articles, if anyone is interested.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Thursday, January 17, 2008 7:32 AM
To: [log in to unmask]
Subject: Re: [TN] Solderability of Tin-Gold Intermetallic
Hi Rex - there are a couple of different failure phenomena associated
with the use of gold finishes: 1) gold embrittlement - the formation of
a AuSn4 microstructure phase which induces solder joint degradation by a
brittle fracture in the solder joint bulk or in the Au/Sn phase. This is
a soldering process root cause; 2) black pad - the formation of a
corroded Au/Ni interface region which induces solder joint degradation
by brittle fracture at the nickel interface region. this is a plating
root cause.; 3)solderability failure - the oxidation of the nickel
plating which prohibits the formation of good solder joint wetting at
the nickel interface region. This is a plating root cause or a storage
environment root cause. Gold isn't a bad surface finish - it just has
some disadvantages that need to be recognized and dealt with in a
proactive manner.
Dave
Rex Waygood <[log in to unmask]> Sent by: TechNet
<[log in to unmask]>
01/17/2008 02:41 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to Rex Waygood
<[log in to unmask]>
To
[log in to unmask]
cc
Subject
Re: [TN] Solderability of Tin-Gold Intermetallic
The thread has drifted into brittle fracture due to 'gold
embrittlement'.
We have experienced 'gold embrittlement' at levels (0.25%) which are
well below the threshold said to be 'safe'.
Maybe Murphy did his bit to ensure that the gold didn't distribute
evenly through the joint and therefore the failure initiated at
concentration points. In our situation the important thing was to solve
the problem quickly so we made an assumption it was 'gold embrittlement'
and took to double dipping to get rid of the plated 'excess' gold. The
joints stopped failing after double dipping.
I would now say that we tend to have a belief that gold in solder joints
should be avoided.
Rex Waygood
Technical Manager
PartnerTech Poole Ltd
Benson Road
Poole
Dorset BH17 0RY
United Kingdom
Tel: +44 (0)1202 674333
Fax: +44 (0)1202 678028
DDI: +44 (0)1202 338222
Mob: +44 (0)7887 997403
[log in to unmask]
www.PartnerTech.co.uk
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: 16 January 2008 13:52
To: [log in to unmask]
Subject: [TN] Solderability of Tin-Gold Intermetallic
Technetters,
We all know that the tin-copper intermetallic has terrible
solderability. I also know there are high melting tin-gold solders. My
question is: what is the solderability of surfaces of tin with small
amounts of gold (<4%) in it? As good as 100% tin, better, worse? If
worse, how much so?
Bev
RIM
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