Printed Circuit Board Warping

What is it?

PCB warping is the bending and sagging of circuit board substrates caused by expose to temperatures higher than their glass transition temperature (Tg).

Why does it happen?

The polymeric materials used in most circuit board laminates are typically based on epoxide resins which are reinforced with woven glass fibres. At room temperature these materials are quite rigid but at higher temperatures they pass through a transition known as the glass transition temperature (Tg), where they change from being rigid to more rubber-like materials. For many common PCB laminates the Tg is usually in the region of 135 to 150°C. The higher the temperature the board is exposed to above the Tg, the more rubber-like it becomes and problems such as warping and sagging become more apparent.

What problems can it cause?

Circuit board warping can lead to interconnect failures on the affected circuit boards and even a small amount of warping can make the placement of components difficult. It may also cause problems with components already mounted on the board. For example, the cracking of multi-layer ceramic capacitors.

How can it be prevented?

The choice of circuit board laminate is important and laminates with higher Tgs will be less prone warping and sagging as well as exhibiting improved thermal stabilities. Another key factor is the board thickness, with thin boards likely to perform worse than thicker boards.

Component loading and the position of components should also be considered. Large, heavy components placed in the centre of the board will encourage sagging during transport through the soldering ovens. One way to circumvent this type of problem during reflow is to use a centre board support conveyor system. This can mean that the circuit boards may need to be redesigned in order to be able to accommodate the support.