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Bow And Twist Issue on PCBs


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What are Bow and Twist Issues on PCBs?

Bow and twist issues on Printed Circuit Boards (PCBs) can cause registration errors in components and parts during the PCB assembly process. When dealing with Surface Mount Technology (SMT) or Surface Mount Devices (SMD), these issues can introduce significant challenges to assembly and installation.

IPC Standards and Industry Requirements

The IPC-6012 standard defines the maximum allowable bow and twist for circuit boards as 0.75%, with other PCBs generally not exceeding 1.5%. Electronic assembly plants typically allow for bow and twist between 0.70% and 0.75% for double-sided or multilayer PCBs. In practice, many rigid PCBs (with a thickness of 1.6mm) require bow and twist to be less than 0.5%, with some factories even requiring less than 0.3%.

Preventing Bow and Twist on PCBs

1. Design Phase:

Arrange prepreg layers correspondingly within the PCB layers.

Use laminate and prepreg products from the same vendor.

Utilize independent grids for large areas without copper.

2. Baking Before Lamination:

Baking the laminate before cutting; the drill bit can exert pressure on the board, potentially causing bow.

3. Pre-pressing Multilayer PCBs:

Pay attention to the longitudinal and latitudinal directions of the prepreg.

4. Baking Before Drilling:

Typically bake at 150 degrees Celsius for 4 hours.

5. Measuring Rigid PCB Bending:

Heat at 150 degrees Celsius for 3-6 hours using a smooth steel plate with heavy pressing, baking 2-3 times.

6. Post-HASL Cooling:

Place the FR4 PCB on a smooth marble or steel plate to cool naturally to room temperature.

7. Baking Before Vacuum Packaging.

Techniques for Handling Warped PCBs

Heat at 150 degrees Celsius or use heat pressing for 3-6 hours, employing a smooth pressure plate and baking 2-3 times.

Additional Recommendations

SysPCB suggests that bow and twist issues can arise not only in the PCB manufacturing process but also due to mistakes in circuit layout. For example, uneven copper thickness across layers in multilayer PCBs can cause these issues. PCB design engineers should also pay attention to stack-up design; an asymmetrical stack-up can lead to serious bow and twist issues. Therefore, we always recommend using as symmetric a stack-up as possible for multilayer PCBs.

By following these measures, you can effectively prevent and address bow and twist issues in PCBs, ensuring optimal performance during assembly and use.

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