It sounds like your wave soldering process is not set up correctly.  The 
boards should be evenly heated through before wave soldering, then cooled 
uniformly.

What you describe happens when the board is heated mostly on the solder 
wave side, causing softening of the resin and differential expansion top to 
bottom.  As soon as the board leaves the soldering station, it is probably 
cooled rapidly by ambient exposure or forced air cooling.  This is a recipe 
for guaranteed warping.  Your multilayer boards are more tolerant of the 
abuse because they have more thermal mass ( they can't heat or cool as 
rapidly throughout the bulk material ).  The inner layers are essentially 
holding things together for you; however, I'll bet the four layer boards 
are also warping - just not as noticeably.  If you want to see the physics 
in action for yourself, try taking a scrap board and heating just the top 
side with a heat gun - you'll make a 'Frisbee' of it fairly quickly.

Material, layup, copper volume, and differential temperature are the 
ingredients you need to mix carefully to avoid warp, bow, and twist.

Regards - Harry

At 11:52 AM 7/20/01 -0400, you wrote:
>On normal 2 layer PCBs, I've seen my boards stay flat & fine through the 
>SMT mounting process, but after the wave pass, they would have an arch in 
>them.  When I changes the design to a 4 layer board, it no longer 
>happened.  I think the material on the 4 layer was of better refinement 
>than the 2 layer, even though both boards came from the same manufacture.
>_____________
>Brian Guralnick
>
>----- Original Message -----
>From: "Dennis Saputelli" <[EMAIL PROTECTED]>
>To: "Protel EDA Forum" <[EMAIL PROTECTED]>
>Sent: Thursday, July 19, 2001 6:49 PM
>Subject: Re: [PEDA] Bow & Twist
>
>| I've occasionally had boards received nice and flat from board fab, and
>| then have substantial bow and twist after assembly
snip

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