We assemble some pretty large boards that have 20 mil pitch parts as far as
28 inches apart on the diagonal.  On a number of occasions we've had to
order 'shrunk' stencils to match the boards.  I've yet to run across a
board that was 'stretched'. 

Your stencil vendor's photoplotter accuracy becomes rather critical for
very large stencils.  We measure the actual artwork on the boards and give 
the vendor a scale factor for each axis.  Typically the scale factor will
be slightly different in each axis.  

Cheers,

Doug Ondricek
Convex Computer Corporation


 
> From: Kelly Kovalovsky, PCB Quality Engineering  
> ~     EMail:[log in to unmask]            
> Subject: PCB vs. Paste Stencil Comp            
> We have a fairly large circuit board that we are assembling. The card
> has fine pitch SMT at extreme ends. We have noticed a mismatch between
> the solder paste stencil and the printed circuit board. The circuit  
> board features are actually closer together than the stencil.       
>                                                                    
> My question to any card assembly site is whether it is common practice
> to compensate a solder paste stencil for shrinkage of a PCB?         
>                                                                     
> IBM Microelectronics Division                                      
> 6800 IBM Drive MG12/251                                           
> Charlotte, NC  28262-8563                                        
> 
>