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Bow and twist are terms used to describe the amount of deformation or warpage in a printed circuit board (PCB) that can occur during the manufacturing process.

Bow refers to the deformation of a board along its horizontal plane, meaning it is curved along either its x or y axis, resembling the shape of an archer's bow.

Twist refers to the deformation of a board along its vertical plane, meaning the board has a twist from one end to the other, similar to the way you'd twist a wet cloth to wring out water.

The bow and twist in a PCB can be caused by several factors, including:

Uneven distribution of copper across the layers of the PCB

Uneven distribution of heat during the manufacturing process

Differences in the coefficients of thermal expansion of the materials used in the PCB

Physical stress during the manufacturing process, such as during depaneling

Excessive bow or twist can cause issues with the assembly and operation of the PCB. For instance, it can lead to problems with component placement and soldering, particularly for surface mount devices. In severe cases, it can even lead to mechanical or electrical failures.

To minimize these issues, industry standards such as the IPC-600 (Acceptability of Printed Boards) and IPC-6010 series (Performance and Quality Specifications for Printed Boards) define acceptable limits for bow and twist.

The IPC standard generally allows a maximum of 0.75% bow and twist for rigid boards, but the value can vary depending on the specific application and class of the PCB. For example, boards used in high precision or high reliability applications may have stricter requirements.

If you're seeing issues with bow and twist in your PCBs, it might be worth discussing with your manufacturer. They might be able to adjust their processes or materials to help minimize these issues.