Totally agree with Peter. Re-HASL board got thicker IMC, it fails early
at end of life with high vib and high TC extreme.
jk
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of
>[log in to unmask]
>Sent: Tuesday, February 18, 2003 5:17 AM
>To: [log in to unmask]
>Subject: Re: [TN]
>
>
>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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