Bob and fellow Technetters,

The situation you are describing usually refers to cross- graining 
the core material (usually short) vs the prepreg (usually long). 
You are only going to be compensating the core material (C-stage)  
Thus you will be maintaining the most stable configuration for the
core in its' longest direction.   At one time due to glass widths and
laminator sizes there was an economic advantage to buying material
this way for 18 X 24 panels, i.e. you got to use the edges of the 
glass for prepreg but not for core material because the laminators 
have to trim off the edges after lamination just like we do.  I 
mention this only because it could have been the cross-plying got 
started.  One of those good accidents.  The last time I did an 
informal census of what people were running it went about 1/3 
cross-plying, 1/3 straight-plying and about 1/3 who didn't know or 
didn't care. 

MY OPINION  (did'nt used to be that of my employer but is now)

At one time we were forced to run straight-ply on 20 x 24 panels 
due to availability.   ln certain cases we would encounter, that most
dreaded of process problems saddle warp, due to unbalanced
board designs (usually odd shaped internal ground areas).  After
the laminators increased their press sizes we were able to convert
everything to cross-ply and the problems with the larger panels went
away.  I also feel like the overall dimensional stability of the 
panels, as encountered  when trying to register soldermask, improved.
It also seems plausible that drill accuracy  may  improve slightly  
on smaller holes. 

 Why, because most glass styles are not even, more threads/inch
in one direction than the other.  Given a balanced construction, 
cross-plying gives the entire package  the same amount of 
glass going one way as the other.  Of course there are a lot of other
warpage issues (both process and material related)   And no doubt 
other solutions.   I can only say that there are certain part types 
we will not even consider running without cross-plying. 

Hope this makes sense and helps:


Robert E. Welch
Process Specialist
Email: [log in to unmask]
Modem: 804-239-9120
Fax:   804-237-3048

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