Printed circuit board (PCB) warpage and distortion are usually among the highest levels of misidentified failures because it may be the least understood. Imagining a completely flat rigid circuit board as a standard is a fallacy that many newcomers believe. Understanding the causes and causes of PCB bending and bending can help solve problems in the circuit board design stage.

The difference between bow and twist
First, let's discuss the difference between bow and twist terms. Although all four corners of the circuit board are in contact with the platform (imagine the shape of a bow weapon), the circuit board with the bow problem will be lifted off the platform.

Distortion occurs when three PCB corners are in contact with the platform and the fourth corner is raised. According to IPC, there are requirements for both conditions to determine whether the degree of bowing or twisting meets or does not meet. Prevent bow and twist on the PCB circuit board

Why does IPC allow any bows or twists?

Because of the materials and processes involved in the manufacture of printed circuit boards. The PCB laminate is made of multiple layers of fiberglass cloth and epoxy resin, and each layer has unique thermal expansion properties. A layer of copper is added to one or both sides of the PCB laminate, and additional thermal expansion characteristics must be considered.

When the circuit board manufacturer exposes the material to various etching and heat treatment processes, there is no guarantee that the laminate will show a uniform response on all samples.

Due to the different reactions between different samples, although PCB laminate manufacturers follow strict manufacturing processes for all laminates, IPC has set allowable tolerances. In order to resolve the normal variance, it is expected that the finished product will fall within a specified parameter or tolerance range, rather than an exact quantity. These preset tolerances allow for small bends and/or twists, which will not affect the performance of the circuit board.

Additional effects of bow and twist

Other factors affect the bow and twist levels in circuit board manufacturing, including:

Add individual part number features

Higher circuit board layers (other materials = additional heat treatment)

Material mixing (that is, use standard FR4 high-frequency PTFE laminate to control the impedance value, resulting in unbalanced laminate)

Mixing of copper weights Because copper has a high coefficient of thermal expansion, the mixing of copper weights has a negative effect on bow and twist. The higher density copper will expand to the least resistance area under the action of greater force than the lower density copper area.

Balanced stacking allows opposite thermal expansion values ​​to interact to help maintain uniform bow and twisting forces. With unbalanced stacking, the side with the larger thermal expansion value affects the entire board. The solid layer of the copper plane will expand to be different from the signal layer, especially if the signal density is low. Putting all the signals on the same layer on one side of the circuit board and all the aircraft on the other side is a disaster.

The printed circuit board manufacturer is also responsible for ensuring the correct storage of the raw material laminate to maintain the flatness of the material, as well as ongoing work processing to prevent the inconvenience of stacking production panels. The manufacturer may also suggest stealing copper when the design is easy to bend and bend.

In conclusion

Understanding PCB bending and twisting provides us with acceptable standard parameters and key prevention methods. Understanding the main causes of printed circuit board bending and bending is important for manufacturers, but they do not rule out the possible root causes of circuit board manufacturers. With a better understanding, a better and more efficient circuit board design and manufacturing process.