How to Prevent Buckling in Flexible PCBs
Prevent Buckling in Flexible PCBs
Flex circuits are often used to replace wire harnesses in a variety of applications where there is a requirement for flexibility in the product or production process. This could be due to space restrictions, weight reduction or to reduce assembly time. However, these flexible connections are not without their risks. If not designed properly they can break or fail prematurely. To prevent this from happening it is necessary to design a PCB that takes into account the bending requirements of the final product.
A flex circuits raw material is a thin polyimide film, which is both rigid and heat resistant. This makes it able to cope with high temperature reflow soldering processes as well as being able to withstand multiple bending cycles. Polyimide is also very tough and can be used in places that are subject to vibration and mechanical shock.
In a multilayer flex circuit board the copper thickness can have an effect on its pliability. Thicker traces are more difficult to bend than thinner traces. In order to increase pliability it is possible to reduce the copper thickness by cross-hatching the ground planes. This reduces the friction between the metal and the flex circuit. It is also important to use teardrop (pad fillet) shaped vias when connecting traces and through holes in a flex circuit board, as these can help to reduce stress concentration points. This can be done by making the via diameter larger than the width of the connecting strand. This helps to spread the strain over a wider area and prevent cracking of the via during bending.
How to Prevent Buckling in Flexible PCBs
Other features that can help to prevent buckling are rounded corners. These can be created by using a CAD tool that allows for the reshaping of a corner and will provide greater resistance to tearing than a sharp 90 degree edge. Also, using hatched polygons rather than solid copper on a flex PCB can increase its tensile strength compared to a smooth square.
Another way to improve flexible pcb reliability is the use of stiffeners. Stiffeners are small bits of rigid materials added to specified areas on a flex PCB to add tensile strength and to avoid the occurrence of buckling. These are especially useful in locations where a high amount of pressure is expected.
Lastly, it is crucial to keep the gap between the flex and rigid sections of a PCB as small as possible. Leaving gaps between the layers can allow a single flex layer to confine other flex layers beyond their natural bending limits, which can lead to fractured circuits. It is recommended that a gap of no more than 1 mil be kept between the flex and rigid portions of a PCB.
Finally, it is important to consider the type of surface finish that will be applied to the flex circuit board. There are many types of finishes that can be chosen from to provide a solderable surface and prevent copper oxidation. It is also recommended that a protective layer be used to protect the copper surface during assembly and storage, which will extend the life of the flex circuit.