bake the board flat in an oven.  You must get the board above its glass 
transition temperature.  Bake for a least 3 hrs. and allow a slow cool down 
with the weight still applied.

Some warpadge occurs from the board design, random placement of holes in the 
board, bad distribution of copper layers or component weight.  If this is 
the case a fixture that holds the boards shape may need to be developed for 
wave process.

Other warpedge could be a warning sign that somthing in your process has 
gone wrong.  To high a temperature, bad fixturing during wave etc....

Good luck
 ----------
From: TechNet-request
To: TechNet
Subject: ASSY: Post-wave Warpage
Date: Friday, May 17, 1996 12:28

Can anyone give me some suggestions on returning a board to flat after it 
has
gone over the wave?

I have a new customer who has been using someone else to solder his boards 
and
is experiencing warpage after it has been wave soldered.  The board measures
16
cm x 23.5 cm with two (2) 96-pin connectors (.100" spacing) along the 23.5 
cm
side of the board.  He said the other shop's solder pot is about 480 degrees
and
it is assumed they are running the board with the short side in the grippers
so
as to avoid hand-soldering the 96-pin connectors since they hang off the 
egde
of
the board.

Some additional data:
I.C. orientation: 33 run east-west, 11 run north-south, 1 PLL.  There is one
20-mil pitch SMT QFP device on the board.  The QFP is the only SMT device on
the
board.

Any ideas on how to avoid the warpage in the first place and when you do 
have
it, how to eliminate it?

Thanks to all who may contribute.