What Is PCB Bow and Twist?

Ideally, your PCB should be perfectly flat, however, this is not always possible. IPC has defined the acceptable limits a PCB is permitted to bow or twist, 

this is defined in IPC-A-600 under section 2.11. Below is an excerpt from IPC-A-600 section 2.11.

(From IPC-A-600 – 2.11 FLATNESS)

Flatness of printed circuit boards is determined by two characteristics of the product; these are known as bow and twist. The bow condition is characterized by a roughly cylindrical or spherical curvature of the board while its four corners are in the same plane.

Twist is the board deformation parallel to the diagonal of the board such that one corner is not in the same plane to the other three. Circular or elliptical boards must be evaluated at the highest point of vertical displacement. Bow and twist may be influenced by the board design as different circuit configurations or layer construction of multilayer printed boards can result in different stress or stress relief conditions. 

Board thickness and material properties are other factors that influence the resulting board flatness.

Bow, twist, or any combination thereof, shall be determined by physical measurement and percentage calculation in accordance with 
IPC-TM-650 – 2.4.22 Bow and Twist.Panels containing multiple boards that are assembled in panel form and later separated shall be assessed in panel form.

Acceptable – Class 1, 2, 3