The key word here is "IF" (the operators..etc.).  A rework or a repair
should be designed, like any other process, to accomplish the intended
function without degrading the design reliability of the item. 
- You need to look first at the maximum rate of temperature change which
chip components can tolerate without damage (most component manufacturers
provide that information).  Capacitors are especially sensitive to high
rates of change and should be preheated prior to installation, rework or
repair.  
- It seems that most rework/repair actions are conducted in something less
than a "planned" manner and frequently operators are encouraged to "get
these 20 assemblies finished before lunch".  In those conditions the "IF"
seems to be put on the shelf until the rush is over.  You can make a strong
argument that the rework and repair operations need to be a more closely
controlled process than the initial manufacturing process because of the
higher number of variables involved.  Preheating both the assembly and the
component are considered essential process steps here at the EMPF, and are
reflected as such in the repair classes we teach.  Failure to preheat
engenders the risk of two distinct liabilities: 1) damage to (either or
both) the component and printed wiring board during the procedure and, 2)
latent damage (micro fracture) of the component (not detectable during
post-rework/repair visual inspection) which will negatively impact reliability.
- You need to be guided by your customer's quality needs.  The Class of
equipment you build (IPC ANSI/J-STD-001 definition) is a pretty good
indicator of how you should design the rework/repair processes.
- No, as far as I know we do not have a paper or study documenting the need
for preheating.  Regards, Jim Moffitt/EMPF

At 09:48 AM 10/10/96, you wrote:
>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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