Concur, I think also that you non materials people using technical terms in
a conversational way and getting them jumbled, losing their specific
technical meaning. For example think of wave soldering literature and the
lists of impurities and their effects, people tend to carry those terms and
meanings into reflow soldering with some confusion.
Regards
Mike Fenner
Applications Engineer, European Operations
Indium Corporation
T: + 44 1908 580 400
M: + 44 7810 526 317
F: + 44 1908 580 411
E: [log in to unmask]
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Leadfree: www.Pb-Free.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Dave Hillman
Sent: Tuesday, February 18, 2003 2:01 PM
To: [log in to unmask]
Subject: Re: [TN]
Hi Peter! I can take a stab at the thick IMC Old Wive's tale origination! I
think we can assign the metallurgical engineers of the world some of the
blame for the promotion of the tale and the fact that under very extreme
conditions sometimes the IMC does fail (but as Werner said, it is very
unusual). IMC's have been studied for their basic properties for numerous
application uses and their overall mechanical properties are described as
brittle in nature. When a solder joint cracks its a pretty short path to be
looking for something in the solder joint microstructure which would crack
and the IMC get "accused" immediately. However, when you look at a failed
solder joint you find that the crack has progressed through the solder
microstructure and not at the IMC/solder interface. Some times
metallurgists compare IMCs to thick Chrome plating in terms of brittleness
- and many folks have experienced the brittle cracking issues with thick
chrome plating so an inadvertent association gets formed.
Dave
[log in to unmask]
OM.SG To: [log in to unmask]
Sent by: TechNet cc:
<[log in to unmask]> Subject: Re: [TN]
02/18/2003 04:17
AM
Please respond to
"TechNet E-Mail
Forum."; Please
respond to
peter.duncan
Cheers, Werner! I guess that cleared the air a bit, but I wonder from where
the Old Wive's Tale originated if it isn't true(?) I have read quite a few
articles that refer to the brittle nature of intermetallic compound layers.
In products where the operating environment is fairly benign, the IMC layer
can probably be as thick as the entire solder joint without failing. Solder
joints on Class 3 boards, however, operating in environments where
temperature cycling and vibration are normal, do, in my experience, suffer
higher fracture rates in older age, often either through the IMC layer or
on its border. For that reason, I argue against applying more heat to a
board than is totally necessary.
Peter
[log in to unmask] 18/02/2003 02:27 PM
To: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group@ST Domain,
[log in to unmask]
cc:
Subject: Re: [TN]
Hi Peter,
You repeat what I regard as an unsubstantiated old-wives-tale: "Time and
temperature increase the thickness of the brittle intermetallic layer that
is
the solder joint, rendering the solder joints increasingly susceptible to
fracture."
In all my years in this business, I have never seen a SJ failure caused by
'too thick' an IMC layer. Of course, IMC layer thickness will increase with
time and temperature, but (1) IMCs--with the exceptions of AuSn and AgSn
IMCs--are much stronger than solder even though they are brittle; I have
NEVER seen a fracture in the IMC. (2) It takes long exposures of about 150
hours at 155C--I have seen the results from up to 1,000 hours at 155C
[TUMunich & TI]--to get to thicknesses I would get concerned about, and not
so muchabout the IMC layer but the structure of what is left of the SJ.
I certainly would agree that eposure to elevated T's of PCAs should be as
imited as possible, but nnot because of concerns for the SJs.
Werner Engelmaier
[This e-mail is confidential and may also be privileged. If you are not the
intended recipient, please delete it and notify us immediately; you should
not copy or use it for any purpose, nor disclose its contents to any other
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