Mike,

Have you tried running this very same board without the fence on some
quantity of boards to see if the problem still occurs.  This can help
isolate the causes and you should be able to hand solder the fence in
afterwards so you can still use those boards if no failures occur.

Good luck.

-Steve


Steven K. Lustig
Process Engineer
EMS Technologies, Inc.
Norcross, GA
(770) 263-9200 x4714
[log in to unmask]


-----Original Message-----
From: Michael Forrester [mailto:[log in to unmask]]
Sent: Thursday, November 11, 1999 12:29 PM
To: [log in to unmask]
Subject: [TN] SMD shield damaging parts thru Reflow?


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.
[log in to unmask]

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