The shields do conduct the heat faster to the board as you suggest, but
they certainly do not concentrate or increase the heat from my experience.

The simplest suggestion is to thermocouple the components which are in
question using probes installed right into the solderpaste.  It will take
the guesswork out of your problem.


At 12:29 PM 11/11/99 -0500, Michael Forrester wrote:
>We are currently seeing strange component failures in the PLL circuit of
one of
>our assemblies.  The components are as follows:
>
>     1) BB619 Tuning diode SOD-23, defective
>     2) 1500 pF NPO Nickel Barr .080 X .05 Cap, becomes resistive?
>     3) BAV99 Dual Diode SOT-23, PN junction damaged
>
>     All of the parts are surface mount an contained within the walls of a
SMT
>RF shield with just the fence installed during reflow.  The dimensions of the
>fence is 1.75" square at a height of .5".  We use a very similar circuit on
>another assembly that does not have any of these problems nor does it have
a SMT
>RF shield.  I believe the damage to the components is heat related?  One
theory
>I have is during reflow, the shield fence acts as a large heat conductor and
>transfers the heat , through the ground, to the components causing heat
related
>damage?  All but the tuning diode has one pin connected to ground.  I have
never
>heard of a case where something like this has happened.  Could it be heat
>radiated off the four walls of the fence, creating a oven within the area
of the
>walls?  Does anyone have any other ideas or have seen similar problems?  Any
>help would be appreciated!  Thank you.
>
>Best Regards,
>
>Mike Forrester
>LeCroy Corp.
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                                   Phillip Bavaro
                               QUALCO/\/\/\/\ Incorporated
                                  CCA Technical Support
                              Staff, Manufacturing Engineer
                                (858) 658-2542 Voice-mail
                                (858) 658-1584 Fax
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