Once a board warps, there is very little that can be done to remove the warp. The board has memory of stresses that were incurred during multilayer lamination. These stresses will be relieved anytime the board experiences a thermal cycle after lamination. Don't point the finger at the fabricator of the board just yet. I have rarely seen a board warp that was designed with a symmetrical stackup. First, look at the cross-section stackup of the board and look for the following: 1. The dielectric thicknesses should be symmetrical about the center of the stack. 2. The copper weights should be symmetrical about the center of the stack. 3. Signal and Ground/Power layers should be a mirror reflection about the center of the stack. 4. If multiple dielectric materials are used in the board, the dielectric materials should be centered about the center of the stack. 5. The circuitry should be the same density as much as possible in the x-y plane on each layer to minimize twist. The further away from symmetry the board is, the more it WILL warp and twist. If the design is unbalanced, there is very little that the fabricator or assembler can do to flatten it out for any extended period of time. Baking the board with weights on it will attempt to flatten the board out, but once it is heated back up again in IR reflow, wave solder, etc., the warp is back. If the board was designed with symmetry, then the fabricator has some evaluation to do. Is there any missing prepreg, wrong copper weight, layers in the wrong order, fast cool down in the press, etc.???? Darren Hitchcock [log in to unmask] [log in to unmask] (Jerry Schwartz) Wrote: | | | > | >Greetings, | > | >Does anyone have a good procedure for removing a | warp/twist from a pcb? I | >am dealing with a .093 thick assembly that is 7 | inches by 20 inches. The | >board currenly has smt components placed and | soldered (topside only) but, | >the through hole components have not yet been | mounted/soldered. I have | >tried heating the board and letting it cool while | applying weight, that did | >not do the job. Any suggestions ? please respond. | > | >Thanks, | > | >[log in to unmask] | > | > | > | If it came to you like that from your supplier | SEND IT BACK! | Also a pre bake of the bare board to reduce | moisture should help | warp later in the process. | | Regards, | Jerry Schwartz | [log in to unmask] | "May The Schwartz Be with You" |