John, I would be concerned if I were you (customer viewpoint). We would not accept boards in that kind of shape regardless of how they have flattened out under weight. The degree of twist indicates that additional stresses have been placed on the material which could affect fit after assembly. When placed under additional stress on the reflow/wave solder line the board could twist up even more (or it might flatten out). I would be scared to try and place any high density QFP's on that board (my QA and MFG departments would kill me if I tried to send them something like that). We have had circumstances where even if a board is heavy enough to flatten itself (or is screwed down by ~20 screws per side to a stiffener) then the board still won't fit with other boards in the assembly unless it was flat to begin with. Currently, with some of our suppliers, we are having them bake our boards after final test to ensure there will be no warp when received (based upon past problems). I don't really care what the accepted (or industry standard) specs might be, if there is an obvious problem, I will reject it. Regards, Roger Held Manager of Electrical Component Engineering Hitachi Computer Products (America), Inc. [log in to unmask] ______________________________ Reply Separator _________________________________ Subject: Bow and Twist Author: [log in to unmask] at Internet-HICAM-OK Date: 5/8/96 6:17 PM A question on interpretation of the IPC method IPC-TM-650, 2.4.22. I have a batch of boards 400mm square, which are quite clearly twisted when held free in my hands. When I lay a board horizontal, it straightens under its own weight. All corners touch the plane and I am left with a small residual bow of 1 to 2mm, say 0.25 to 0.5%. My supplier says it is in specification, but my customer (who cannot carry out the prescribed test on the populated board) comments on the degree of twist. Questions: 1) Is this the correct interpretation of the test and specification? 2) Should I be concerned? Regards John Loveluck Tadpole Technology Cambridge United Kingdom [log in to unmask]