Reliability in PCB Flex Circuits

PCB flex circuits have an incredible combination of tensile strength, physical flexibility, and bendability that makes them ideal for many applications. For example, they can be used to replace traditional wiring harnesses in automobiles and medical devices. Removing these hand-built harnesses helps lower assembly costs and improves quality, since they can be prone to failure from poor workmanship.

These specialized circuits use an incredibly thin substrate made of polyester or polyimide material—films that can be only 12-120 microns thick. Conductive material traces are etched on these layers, and a coverlay is applied to protect them from moisture, dirt, and damage. Coverlay can be PI, PET, or glass fiber, and it can be either flexible or rigid.

The thickness of the substrate and the materials used for a pcb flex are important, as they dictate the minimum allowable bending radius that can be achieved. It is also important to ensure the copper plating has a low tolerance for stress cracks and fatigue, which is why PCBWay recommends using higher-grade, rolled annealed (RA) copper foils in flex circuits. This process elongates the copper grain structure, which increases its flexibility and resistance to stress cracks.

How to Ensure Reliability in PCB Flex Circuits

As the designer, you can help ensure your flex circuits meet these criteria by designing them to have as few layers as possible near the flexing zones. This will reduce the amount of strain placed on them, preventing the layers from delaminating. Additionally, the conductors on different layers should not be stacked directly on each other, since this can cause them to slip and lose contact with each other during flexing.

Another way to ensure your flex circuits can withstand repeated bending is to use stiffeners. These are areas of the board that have rigid dielectric material added to them. This can be a glue or adhesive, and it is best to use these in the flex areas of the design. This will increase the manufacturing cost a little, but it can be worth it if you need the circuit to stand up to mechanical stress and flexing over time.

To prevent traces from touching each other as they bend, you can use a controlled impedance design that shields both sides of the trace. This will keep the impedance constant as the flex circuit is bent, which will eliminate any stray capacitance that can lead to noise or other errors.

Mechanical testing is an important step in ensuring that your flex circuits can withstand the stress of everyday use. It’s recommended that you use a three-point bend test, which will evaluate the ability of the circuit to resist deformation and damage from various angles. You can also perform torsional testing to see how the circuits respond to twisting forces.

A final step you can take to help your flex circuits last is to limit the number of soldering contacts. This can be done by reducing the number of through-hole pads on your flex circuit and by designing them to be less dense than you would for a rigid board. In addition, you can use anchoring stubs and reduced coverlay access openings to minimize the number of places where the pads could be soldered onto.