In message  <[log in to unmask]> [log in to unmask] writes:
> Thanks, Glenn ... But I'm having a difficult time visualizing what you are
> doing ... the solder joint is made, then the epoxy is applied then the part
> is injection molded?  My situation is a resistor with the leads soldered in
> PTH on an FR4 double sided PWB.  The bottom half of the resistor, including
> the leads, is then potted or bonded to the board with an epoxy adhesive.
> The epoxy was probably 3/8" inch thick.  The entire assembly is then potted
> with a thermally conductive silicone.  During temp cycling, the solder
> joints are failing.  The reported failure (although I haven't seen it to
> verify) is that the solder joint is failing and the lead is pulling out of
> the board.  I believe the leads have a stress relief loop in them, but I'm
> not sure it does any good when the whole thing is encased in epoxy.  Anyhow,
> the electrical guy thinks that if we change to an epoxy with a glass
> transition that is above the upper limit of the temp cycle, the CTE won't be
> as bad and the parts will survive.  I'm not sure I buy that cause I don't
> think we've identified the root cause of the solder joint failure properly.
> I was trying to see what experiences other have "endured" in this area.
>
> Sheila Smith
>
>
> At 09:00 AM 9/23/97 PDT, you wrote:
> >Hi Sheila,
> >
> >        Hope this helps.  We have an SMT process where some connectors are
> >given a bead of epoxy along the solder joint to give it additional mechanical
> >strength.  The part then goes through an injection molding process.  Tests
> >have shown the epoxy adds considerable strength to the joint.
> >
> >        Let me know if you need more info, and I'll supply what I can.
> >
> >Glenn Pelkey
> >Quality/Reliability Engineer
> >Maxtek Components Corp.
> >
> >Sheila Smith <[log in to unmask]> Wrote:
> >|
> >| Hi!  Does anyone have any experience with using epoxy to
> >| encapsulate
> >| components, including the leads, soldered in PTHs?  The
> >| entire assembly is
> >| subsequently potted with a thermally conductive silicone.
> >| The goal is to
> >| keep the solder joint from moving during temperature
> >| cycling.  Any
> >| suggestions would be appreciated!  Thanks, Sheila Smith @
> >| Tracor AES

Sheila,

Could it be that the epoxy is _causing_ your problem, not solving it?

A low stress situation may occur when the CTE of the epoxy, the leads, the
component body and the board substrate are all equal, or nearly. This is
virtually impossible. Another way is to use a material which has a low modulus,
when the CTE of the material and the glass transition temperature become much
less relevant (trivial?) to the stress equation.

Why use the epoxy at all? What happens when you just pot with the silicone? The
inherent stress relief loop of the component lead may then still function
somewhat. If you need more structural strength than just the silicone (may have
no real adhesion), then I agree with Susan Mansilla - use a less rigid, more
resilient material. We have good success with one of our customers who uses a
urethane to pot the leads/underfill the typically problematic TSOP component,
which is subject to joint damage from CTE mismatch.

Peter
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Peter Swanson                                              Oxfordshire, England
INTERTRONICS
[log in to unmask]                      http://www.cygnetuk.demon.co.uk

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