My 2 cents on this topic.  I spent several years suppliying laminate to 
the printed circuit industry.  The overwhelming majority do not cross 
grain.  The main reason they do not is it may induce warp.  However, 
some major pwb suppliers have cross plied all or part of their designs 
with no warpage problems.  I agree that when faced with an unbalanced 
construction trying anything to keep the boards flat is worth a try.
As for uneven thread count this is a good point.  For many years the 
most popular glass styles (7628, 1080, 2116) were victims of this 
design.  In recent years glass suppliers have worked in conjunction with 
laminators to produce square weave glass styles that improve dimensional 
stability.  However a word of caution.  This may not have a direct 
impact on warp because when you cross ply you are rotating the axis of 
the machine direction of the glass cloth.  This refers to the direction 
that the cloth was woven on the loom.
My advice is to contact your laminate supplier before making wholesale 
changes.  There are many excellent tech service engineers who are very 
well versed on this topic.
On Jan 21,  6:11pm, Robert Welch wrote:
> Subject: Re: FAB:cross grain construction
> 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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> 
>-- End of excerpt from Robert Welch



-- 
Thomas Ingham
Eastern Regional Sales Mgr.
Details Inc.
714-630-4077
[log in to unmask]

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