Bow and Twist of Printed Circuit Boards
Bow and twist of printed circuit boards routinely rank among the highest levels of falsely identified non-conformance because it is perhaps the least understood. Envisioning a perfectly flat rigid circuit board as the standard is a fallacy believed by many incoming inspectors. Understanding the reasons and causes for PCB bow and twist can help resolve the issue at the board design stage.
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.
Factors affect levels of bow and twist in circuit board manufacturing, including: individual part number characteristics, higher circuit board layer counts, mixing of materials, and mixing of copper weights.
Mixing of copper weights has a negative effect on bow and twist, as copper has a high coefficient of thermal expansion. Higher density copper will expand toward the area of least resistance at greater force than a low density copper area.
A balanced stack-up allows opposing thermal expansion values to work against each other to help maintain an even bow and twist force. By unbalancing the stack-up, the side with the greater thermal expansion value will influence the complete board. Solid layers of copper plane will expand differently than signal layers, specifically if the signal density is light. Having all signals on like layers on one side of the board, with all planes on the opposite side, is a recipe for disaster.
PCB fabricators are also responsible for doing their part to ensure proper storage of raw laminates to keep materials flat, as well as the handling of work in progress to prevent production panels from being awkwardly stacked.