> Date:          Mon, 24 Jun 1996 13:55:33 -0500
> From:          [log in to unmask] (ROGER HELD)
> Subject:       FAB: PCB Delamination and Warp
> To:            [log in to unmask]

>      
>      To everyone,
>      
>      We have, in the past, had problems with bare board warp and 
>      delamination in our assembly process (reflow).  We have solved this 
>      problem by instituting a baking process at our vendor or in house 
>      prior to assembly.  My questions are as follows:
>      
1. Is baking a standard PCB fab process (before shipping)?

   Yes and No - If the lamination recipe (asymmetrical) is prepared 
   correctly and cleaning methods are followed you should have no
   problems with warp/twist or delamination.  However, during storage 
   and shipping the boards will absorb moisture.  This is due to 
   laminate and prepreg materials being hydroscopic.
          
   Recommmended baking, say at 230 F for 1 to 2 hours, as shown
   in the HICAM fabrication notes, removes retained moisture
   from the board.  Especially from around the vias and/or
   PTH where moisture can easily work its way through the 
   plated side wall, causing blowholes, which is a unwanted
   defect.   
 
   From an assembly standpoint, large components and tight fitting
   terminal pins on a connector may induce delamination failures.
      
   In your case the pre-bake prior to ship was pushed off to the
   supplier versus in-house.

   Remember also that a preheated board is less likely to fracture
   under stress prior to subsequent heat excursions than unbaked 
   boards.

>      2. If so, what is the standard profile?

see above for baking profile.

      3. If not, why do some vendors' boards need to be baked to prevent     
         warp and delamination while others don't?

   Its sometimes good practice to bake boards prior to shipping and
   bareboard assembly.  Remember the material is hydroscopic, the
   materials characteristics are to absorb moisture.  This absorbtion
   increases as the humidity increases.  

   NOTE: Do not bake boards near the Tg temperature without handling
   procedures in place.  Additional warp and twist are prevalent 
   if not handled properly. 
      
>      4. Does baking cure both delamination and warp or does it just cure    
>         warp?

Baking does not fix and/or cure delamination.  Delamination is a 
Class A defect and is more likely to proprogate further when  
thermal excursions are known to occur.  Again the warp and twist
go back to how the lamination recipe is done.  If you have warped
boards, baking prior to assembly (using weights in the oven helps)
will flatten boards or in spec for SMT placement.  It may or may not
help if the board is two side SMT or multiple passes are required.
Remember also, the additional thermal cycling may cause unwanted 
material fratures that can lessen the boards longevity.

>      
>      5. My understanding is that moisture is getting trapped in the board  
>         during laminate press (or contamination of some sort) which is      
>         leading to the problem during assembly.  Can the moisture           
>         entrapment be avoided?  Should we claim to our vendor that he       
>         better control contamination in the process?

Volatiles from the prior solvents and moisture or resin starvation 
can attribute to delamination.  Its these pockets volatiles that 
attribute to delmaination.

 
>      5. It seems to me that if boards need to be baked before delivering to 
>         a customer that there is some poor process control going on.
Not always.  Read above.


Please call should you need additional information.  Thank you
Roger Dodger.

John Gulley - PE
Compuroute Inc.
214-340-0543 wk
214-847-0343 pgr
>      
>      Someone, please educate me.  Maybe, my perspective is in error.
>      
>      Thanks,
>      
>      Roger Held
>      Hitachi Computer Products (America), Inc.
>      [log in to unmask]
> 
>