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
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