Kathy,

Just a few quick thoughts on the subject of bow and twist before I start
for the day.
Glass cloth is woven just the same as a textile and most of the glass
cloths used in laminate production have more yarns in the warp than the
weft. But it is the warp that usually has the most shrinkage during
board fabrication probaly due to the fact that this is under tension
when the glass cloth is woven.
It is possible to "cross-ply" pre-pregs but it is important to make sure
the build is symmetrical about the centre with respect to pre-pregs,
laminates and copper layers. Perfectly flat boards can be made without
"cross-plying".

If a board comes out of the press flat but shows warp further down the
line then it is recoverable. If it comes out of the press warped then
forget it, you may be able to flatten it for a while but it will retain
its "memory" and revert back.

A few factors which can affect bow & twist (and I bet there will be a
few additions to this list) are:-
From the press
Non symmetrical builds.
Too fast a heat up rate in the press.
Too fast a cool down rate in the press.
Uneven heating across the press. (Heating elements down.)
Uneven cooling across the press. (Cooling channels blocked.)
Uneven heat flow through a stack due to poor heat lagging.
Using pre-pregs with widely different flow characteristics.
Mixing press plates in a stack i.e. steel & aluminium.
In production
Heating above the Tg without proper support (Soldermask cure, or cooling
after solder level.

Wow how do we ever get flat boards?
Hope this helps

OK time for work
Geoff layhe
www.lamar-uk.co.uk

> ----------
> From:         Kathy Palumbo[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Kathy Palumbo
> Sent:         01 December 1999 22:55
> To:   [log in to unmask]
> Subject:      [TN] PCB Warping and Laminate Stack-Up
>
>
>
> Hey techies,
>
> I have three questions for all you PCB technology guru's out there.
>
> 1)  First of all, I was provided (in a PCB technology training
> seminar) the
> following information:
> Because of the way the laminate material is made (it follows the
> textile
> industry equipment), there is always more yarn (glass fibers) in the
> warp
> direction, then in the fill direction.  Because of this you will have
> a more
> stable board in one direction then the other.  When running the
> smaller size
> panels (12" X 36") a lay-up called a "cross-ply layup" of the pre-preg
> material would make the board more stable, and less likely to have
> warping
> issues.
> Can anyone tell me if this is correct or not?
>
> 2) I was also informed of the following information:
>         The warping of the boards will never go away, especially by
> running
> the boards through more thermal cycles.  This will only make it worse.
> The
> Epoxy in the laminate material continues to cross link and with each
> thermal
> cycle the epoxy cross links more and more.  This means everytime you
> run the
> board through an oven you lose some of its ductility.
>         The board is in its relaxed state while in the warped state,
> and
> trying to flatten the board out during a heating process will cause
> even
> more problems, because as soon as the board is heated up again it will
> go
> back to its relaxed (or warped) state again.
>         Usually the warping of the board is caused by the following
> problems:
>         1) Bad design -- uneven stack up of the layers
>         2) The PCB house did not use the same Pre-Preg (stage B)
> laminate
> material as the Cored (stage C) material.
> Can anyone tell me if this is correct or not?
>
> 3)  Can anyone tell me what the other causes for warping of a PCB are?
>
> As always, thanks in advance for your feedback!
>
> Kind Regards,
> Kathy Palumbo
> Manufacturing Engineer
> IPC Certified Instructor
> Viking Components, Inc.
> 30200 Ave. De Las Banderas
> Rancho Santa Margarita, Ca. 92688
> BUS: (949) 643-7255 ext. 352
> Fax:  (949) 459-5360
> CELL:  (949) 422-4145
> E-Mail: [log in to unmask]
> Web: http//www.vikingcomponents.com/
>
> ##############################################################
> TechNet Mail List provided as a free service by IPC using LISTSERV
> 1.8c
> ##############################################################
> To subscribe/unsubscribe, send a message to [log in to unmask] with
> following text in
> the body:
> To subscribe:   SUBSCRIBE TECHNET <your full name>
> To unsubscribe:   SIGNOFF TECHNET
> ##############################################################
> Please visit IPC web site (http://www.ipc.org/html/forum.htm) for
> additional
> information.
> If you need assistance - contact Gayatri Sardeshpande at
> [log in to unmask] or
> 847-509-9700 ext.5365
> ##############################################################
>

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or
847-509-9700 ext.5365
##############################################################