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Subject:
From:
Guy Ramsey <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Guy Ramsey <[log in to unmask]>
Date:
Thu, 6 Sep 2018 10:15:58 -0400
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Not linked to a root cause, but observed as unacceptable,  too thin and
inconsistent.
soak time (150 to 200) = 150 sec
reflow time (above 217) = 75 sec
peak temp = 250
Seven thermal couples placed at various places on and in the board all
within 3 degrees delta.  The board is heavy, Megtron 6 with 34 copper
layers. Most of the thermal load is the board. There would be very little
difference from pad to pad on any given component.  Solder mask defined
pads conform to MFG recommendations.  We learned that the DDR packages are
several years old. Maybe the solder spheres are hosed?

On Thu, Sep 6, 2018 at 9:38 AM Wayne Showers <[log in to unmask]>
wrote:

> IMC has been the bane of my existence more than once.  So here is my take
> from experience backed up with some lab evidence.
> I have had failures attributable to insufficient IMC.  Almost all of these
> were corrected by either ramp-to-peak or increased time above liquidus /
> intimate contact time / temperature.
> For SAC, the intimate contact temperature I have been able to dial in on
> is time above 208C after liquidus is achieved.
> For SnPb, the intimate contact temperature I have been able to dial in on
> is time above 177C after liquidus is achieved.
> I have found that if I can back end a little more time above these
> temperatures on the profile, I have more consistent joints.  I have not
> devoted a whole lot of 'science' on these numbers just 20+ years of trial
> and error coupled with white papers, IPC BOK, and other research.
>
> I have yet to have a problem or a return tied to excessive IMC.  I am sure
> that at some point excessive IMC may cause or have caused a failure mode, I
> just have not seen it.  The few times where it may have been a contributing
> cause, I also cooked the parts making cross-sectioning of the IMC moot.
>

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