Bow & twist measuring help

[petehouwen]

I am trying to measure a board for bow and twist, and I have a question about an interptretation of IPC-650.

I understand the bow measurement. What I'm trying to interpret is what constitutes twist.

When I measure the bow, all four corners touch the surface, and the center of the board is raised. The bow in each direction is nearly equal. This means that when the board is flipped over, all four corners are off the surface. Is this twist, or is it just bow, upside down? If I consider this twist, and measure according to IPC-650, the board fails, even though it passes for bow.

Am I correct to assume that this board fails due to my interpretation of twist being out of spec, or is it just upside down bow?

[randychase]

Simply.. BOW is bending in one plane. If you laid the pcb on a flat surface, one end or the middle is not level with the rest.

TWIST goes beyond BOW in that the pcb is bent in more than one plane. A simple twist would be that one corner is not on the same plane, or two corners.

[petehouwen]

So I'm assuming that a bow in both directions, though equal, would be twist?