Hi Ben - the major difference between the Au/Sn intermetallic (IMC), the
Sn/Cu IMC and the Sn/Ni IMC is the structure of the IMC itself. All IMCs
are brittle materials in comparison to solder joints. Both the Sn/Cu and
Sn/Ni IMCs form as coherent structured layers at the interface between the
pwb plated surface and the solder joint. The Au/Sn IMC is a needle-like or
block-like structure that is located in the solder joint matrix or forms
at the pwb plated surface/solder joint interface then intrudes into the
solder joint matrix. That explanation is very very simplified (and yes,
there are some other forms of the Sn/Cu and Sn/Ni I am leaving out) but it
gives you a relative idea on how the IMC structures differ. These IMC
structure differences result in very different cracking modes. Also, you
must form either the Sn/Cu or Sn/Ni IMC to have a proper solder joint. The
Au/Sn IMC forms only when you get approximately 3-4 wt% Au in the solder
joint matrix and is not necessary for good solder joint formation. Hope
this helps.
Dave
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Re: [TN] Au embrittlement on Copper vs Nickel
Dave,
I'll see if I can track down a copy of the AWS Soldering Handbook, but
the IPC-STD-001 HDBK doesn't go into the detail that I'm looking for.
Are you differentiating between the brittleness caused by AuSn4 in the
bulk solder and the brittle intermetallic layer that tends to form with
aging of the joint? I can understand the bulk solder brittleness being
the same, but are you saying that the effect of Au on the strength of
the intermetallic layer is not significantly different?
Ben
________________________________
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Friday, October 24, 2008 11:29 AM
To: TechNet E-Mail Forum; Gumpert, Ben
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Subject: Re: [TN] Au embrittlement on Copper vs Nickel
Hi Ben - gold embrittlement is a function of the amount of gold in the
solder joint and not dependent on the pwb pad being copper or nickel.
The IPC-STD-001 Handbook has a very good and short tutorial on gold
embrittlement in section 5.4.1.1 on page 63/64 which would be useful.
Also, there is good coverage of the topic in the AWS Soldering Handbook,
section 2.2 Contamination, pages 182-183. FYI - the AWS Soldering
Handbook, ISBN 0-87171-618-6, edited by Dr. Paul Vianco, Sandia National
Labs, is one of those references that everyone must have on their desks
(IMHO). It covers a huge range of solder and soldering process topics in
a well written format.
Dave Hillman
Rockwell Collins
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10/24/2008 09:58 AM
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[TN] Au embrittlement on Copper vs Nickel
Technetters,
Anyone care to share some comments on Au embrittlement?
Specifically, what is the relative difference between joints on copper
vs nickel? (For a SnPb joint)
Does Cu inhibit AuSn4 from forming in the bulk solder? Does this have
any effect on Au in the intermetallic layer?
Do the intermetallic compounds containing Au of each joint result in a
similar brittleness?
Do the joints have the same propensity for growth of intermetallic
compounds containing Au?
Any thoughts are welcome.
Ben
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