Jason,

We have a military program with rather large assemblies where the PWB's are at 
least .093" thick impedance control boards (14-16 layers, lots of plane layers) 
and external aluminum heat sinks bonded to the PWB.  When rework is required, we 
first bake the board, and then work with it hot on the plates.  This greatly 
improves the ability to solder & desolder.  If we don't there has always been 
problems where damage to the board or component would occur.

Cindy Kemp
Lockheed Martin
Orlando, FL
(407) 306-6873
email:  [log in to unmask]
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Subject: Preheating of Ceramic Chip Capacitors
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Dear Technet,

We are a military assembly house that employs preheating 
plates for ceramic chip rework to reduce the risk of 
internal chip delamination from thermal shock. 
This has been done per IPC recommendations.   However, this step in the 
rework process is time consuming and cumbersome.  We have done 
minor and informal tests to determine the real value of this 
practice and found that it really is not needed if the operators take 
care in their soldering.  If the tip size and heating time is 
controlled, there doesn't seem to be an immediate effect or a 
temperature cycling effect on the chips.

I am interested to see how widespread the use of preheating
plates actually is.  I would like to hear from anyone having
any experience indicating that the process is actually necessary.
(or not necessary)

Thank you in advance for any input.

Jason Spera
Manufacturing Engineering
General Atronics Corporation

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