What is PCB Bow and Twist?

PCB Bow and Twist are parameters that determine alignment errors in a PCB board based on its flatness. The PCB Bow tests the spherical or cylindrical curvature of the board, when its four corners lie in the same plane. The PCB twist tests if any one corner of the PCB is different from the others four in terms of flatness. A board with a bow issue will lift off the surface despite all corners of the board making contact with the plane. The twist occurs when three of the PCB corners are in contact with the surface while the fourth corner is elevated.

According to the IPC-A-600 standard, boards with less than 1.5% of bow and twist are acceptable. In the case of the boards with SMD components, this percentage needs to be less than 0.75%. 

PCB Bow and Twist can be used by a number of factors that include high temperature exposure, heavy thermal shocks during soldering etc. The board design, thickness, layers and materials also play a role in the board showing Bows and Twists over various fabrication, assembly or operating conditions.