The question concerns baking of MLB's

Many old timers remember the advice "When in doubt, bake it out". 
Years ago the MLB process contained many bakes.  The purpose of these bakes 
was to remove moisture and/or complete the epoxy cure.  More recently 
materials and processes have improved and many of these bakes have been 
omitted.  Let me comment on several bakes.

Pre-lamination  - Many fabricators bake innerlayers prior to lamination. The 
theory is that dry layers have more consistent lamination shrinkage and the 
bake will remove moisture responsible for blisters.  This may be true, if 
innerlayers are allowed to sit for extended periods in an uncontrolled 
environment.  However our experience is that this bake is unnecessary.  
Indeed, we did experiments in which we purposely added moisture to innerlayers
prior to lamination.  This experiment found no difference in either shrinkage 
or blisters.  We currently do no baking prior to lamination.
MANY customers still require bake prior to lam, compaq et al
Post-lamination - Many fabricators bake MLB's after lamination.  The purpose 
of this bake is to complete the epoxy cure and eliminate warpage. Our 
experience is that this bake is unnecessary. A properly run lamination cycle, 
with controlled cooling, will produce warp-free panels.  The post-lamination 
bake may actually cover-up intrinsic warp that will return in a later thermal 
cycle.  Similarly a good lamination cycle will achieve full epoxy cure, so 
additional baking only serves to thermaly degrade the board. We have 
eliminated this bake. 

Post-drill bake -  A bake after drill has been shown to eliminate pink ring.  
Our work shows this is a dangerous fix.  Pink ring is caused by a chemical 
attack on innerlayer surfaces that are exposed by cracks introduced at drill. 
The bake relaxes the epoxy enough to close the crack, but it cannot reseal the
crack.  Thus it is at best a cosmetic fix.  However in addition, the bake 
hardens epoxy smear and makes it difficult to remove.  A much better solution 
to pink ring is to use a good innerlayer surface treatment that prevents 
cracking at drill.

Post-solder mask and/or post-nomenclature bake -  Most solder masks require a 
bake to complete the soldermask cure.  This bake is in the 300 to 350 F range 
for one hour.  This bake has the added benefit that it will complete the cure 
if the lamination cycle was inadequate. We use this bake.

Pre-HASL bake - Some fabricators bake prior to hot air level.  The purpose of 
this bake is to remove moisture that might cause blisters. We have found that 
the bake at soldermask effectively dries the boards and for standard process 
delays, an additional bake is unnecessary.

Pre-ship bake  - Some fabricators bake prior to ship to minimize moisture 
problems at assembly.  We find that with modern materials, the rate of 
moisture uptake is so slow that this bake is also unnecessary. 

In summary, the only process bakes that I recommend are the ones required to 
cure soldermask and/or nomenclature.  In my opinion additional bakes may 
actually do more harm than good.  The secret is a fast manufacturing process, 
that minimizes the time that etched boards are exposed to the environment; 
coupled with modern materials, that have low moisture absorption.

R. R. Holmes Phd
Viasystems  (Formerly CircoCraft Technologies, formely Lucent; formerly AT&T)
804-226-5114
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------------- Begin Original Message -------------
Date: Fri Jan 24 15:03:18 PST 97
Subject: Post Baking Process

     Hi,
     I have questions about "Post Baking Process".
     
     Q1. Is post baking a standard practice in lamination dept. of PCB
         fabrication industries ?
     
     Q2. How does it helps to improve quality of PCB ?
     
     Q3. What are the common equipment used for post baking ?
     
     Q4. Is there any MIL SPEC for curing of PCBs ?
     
     Thanks,
     
     Riken Suthar
     

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