Why Does IPC Allow Bow Or Twist?
A circuit board with a bow issue will lift off the surface plate despite all four corners of the board making contact with the surface plate. Twist occurs when three of the PCB corners are in contact with the surface plate while the fourth corner is elevated. Both conditions have requirements, per IPC, to determine if the degree of bow or twist is conforming or nonconforming. Have you ever thought why does IPC allow for any bow or twist?
The answer is due to the materials and processes involved in printed circuit board manufacturing. PCB laminates are made with layers of fiberglass cloth and epoxy, each containing unique thermal expansion properties. Add a layer of copper to one or both sides of the PCB laminate and you must account for additional thermal expansion properties.
When the circuit board fabricator exposes the materials to various etching and thermal processes, there is no guarantee that the laminates will express uniform reaction across all samples.
Since the reaction differs from sample to sample, despite PCB laminate manufacturers following a strict fabrication process for all laminates, IPC has set an allowed tolerance. To account for normal variance, the finished product is expected to fall within defined parameters, or tolerances, rather than an exact number. These preset tolerances allow for a small degree of bow and/or twist that won't affect performance of the circuit board.