Hi Richard! There  was a considerable discussion several years back on the
"potential" solder joint thermal cycle reliability life improvement by
attempting to create a given solder joint microstructure. However, a number
of industry presentations/papers (by Werner, Klein Wassink, etc) have
demonstrated (based on metallurgical principles) that it is not possible to
"freeze" the solder joint microstructure for a given use environment - the
solder joint microstructure changes even at room temperature. The industry
focus was shifted from solder joint microstructure to solder joint geometry
and wetting attributes. With the implementation of PbFree solder processes
I am guessing the issue will be reexamined.

Dave Hillman
Rockwell Collins
[log in to unmask]




Richard van Beveren <[log in to unmask]>@ipc.org> on 02/21/2002
12:03:47 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to Richard van Beveren <[log in to unmask]>

Sent by:  TechNet <[log in to unmask]>


To:   [log in to unmask]
cc:

Subject:  Re: [TN] Safe processing window


Hi Werner,

Fair comment - I don't claim to be an expert.  Temperature gradient on cool
down is a bit of a mystery to me still - I seem to hear a lot of ideas that
are not really compatible.  It seems logical enough that components or most
dissimilar materials are going to struggle to hold together under a
temperature change exceeding 4 C/sec, but then I've heard discussion that a
fast cool down can produce a better solder joint
metallurgically....should've paid more attention in materials engineering
back at uni or at least gone back to Wassink, shouldn't I?

Anyway, what about wave soldering? How do things hold together here?  On
both entry and exit temperature gradients are clearly extreme..  Should I
be
taking steps to gain control over the cooldown rate as soon as possible
after an assembly leaves the wave and then hold it to something close to 2
/
sec?

Regards,

Richard van Beveren


> Hi Richard,
> In a message dated 02/20/2002 19:00:45, [log in to unmask]
> writes:
> >there are probably advantages in dropping off much faster, particularly
> from
> the reflow peak to well
> >under liquidous (maybe as fast as 4 C/sec?).
>
> No, there is no advantage in dropping off much faster, in fact you risk
> fracturing your just formed SJs due to warping because of the thermal
> shock
> temperature gradients.
>
> Werner Engelmaier
>

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