Hi Guy - this is one of my soapbox topics as the "thickness" of an IMC is
not an issue for standard soldering processes regardless of the alloy type.
In 30+ years of chasing solder problems and working soldering processes, I
have never seen an IMC issue unless the soldering process parameters were
abused (excessive rework or excessive solder temperatures/times). As others
have detailed, the creation of an IMC is the result of a number of factors
including soldering time, soldering temperature, surface finishes, solder
volume and solder alloy. The majority of the time I see the mention of an
IMC issue, it is from someone trying to promote a marketing angle or
someone who "read about an incident" but does not have all the process
facts. It is true that IMCs are brittle in nature and if they would become
thick, could be a source of solder joint degradation. However, the
formation of an IMC is required to form a proper metallurgical solder
connection.  Dr. Paul Vianco does a outstanding job of discussion IMC
formation in the AWS Soldering Handbook, ISBN 0-87171-618-6 and I highly
recommend that reference as reading material on the topic. The IMC
thickness we achieve in a solder joint is because we used a
validated/characterized soldering process - trying to create a defined IMC
thickness is just process backwards. Ok, sorry for the rant, I'll go get
another Diet Coke.

Dave Hillman
Rockwell Collins
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On Wed, Sep 5, 2018 at 11:58 AM, Guy Ramsey <[log in to unmask]> wrote:

> Recently, I was reviewing a lab report. It concluded that the manufacturer
> should increase the IMC thickness as a part of process changes . . .
> It stated that, while there are no industry specifications for IMC
> thickness it s generally accepted that for Pb-free assemblies the IMC
> thickness should be in the 20 to 120 uin range. It seems to be critical of
> a process that produces IMC between 10 and 70 uin on pads across a single
> device.
> Does anybody have reference papers or texts that would support this target
> and process critique?
>