Flexible PCBs are a tremendous innovation in the world of printed circuit boards. Rather than having to adhere to a rigid structure, these printed circuit boards can be bent, shaped and twisted as need be to fit your application. Some modern applications would be impossible without them. However, as you may have discovered if you work with flex PCBs, some flexible circuit board issues can arise.
Why Do Flex Printed Circuit Board Issues Happen?
In most cases, flexible circuit board issues arise because the designers are used to designing rigid circuit boards, so they design flex circuit boards according to the same principles. Of course, flex circuit boards are not rigid circuit boards, and they require their own flex-friendly design.
What Are Some Common Types of Flex Printed Circuit Board Issues That Occur?
Some of the most common flexible circuit board issues are:
- Solder Mask/Coverlay Openings Not Wide Enough: Some change to the shape of flexible circuitry during fabrication is to be expected. Processes like copper plating, etching and pumice scrubbing are bound to have some effect. However, those used to designing rigid circuit boards may not allow for larger tolerances for coverlay, die cutting, stiffeners or the adhesive squeeze resulting from the coverlay dielectric lamination. Because there are so many complex processes to consider when designing a custom flexible circuit, it’s important to make sure the openings in the coverlay are wide enough.
- Solder Joints Too Close to Bend Point: The point where the solder is bound to the copper trace is the solder joint, and they are much more rigid than the rest of the copper trace due to the addition of the solder. In a rigid circuit board, this is usually not an issue. In a flex board, if the solder joint is too close to where you will bend the substrate, you are going to have a cracked solder pad or lose the lamination, either of which will probably cause the board to fail.
- Spacing Between Solder Pads: Since you need to make the solder mask/coverlay openings larger in a flexible circuit board, you risk exposing the edges of adjacent conductor traces when you route them too close to the solder pad. The result can be shortouts of solder bridges between connector pads or pins. It’s important to make sure you design your flexible circuit board with greater space between each solder pad and its adjacent conductive trace to account for the larger coverlay openings.
- Stress Points in Conductors: A common problem that designers of rigid circuit boards often find with flexible circuit boards is in creating stress points. Trace arrangements that are perfectly suitable for rigid circuit boards, such as ones with acute junctures at the base of solder pads or sharp corners, will not work with folding, flexing flex boards once the user applies that board to their application. Whenever the user flexes that area where the stress point is, fracture or delamination is likely to occur.
- Stacked Traces: It’s important to make sure the copper stays in the neutral axis of a bend. When traces on either side of the dielectric stack right on top of each other, traces on the outside of the bend radius will often crack when one bends the circuit if the traces on opposite sides are perfectly lined up. The farther the tension forces these traces away from the neutral axis of the folded area, the more likely there is to be a fracture. By designing the region as one conductive layer, you can sidestep this problem.
When it comes to avoiding flexible circuit board issues, it starts with design. You want your flex printed circuit boards designed and manufactured by flex PCB experts, who have designed these types of boards many times before and know what the correct protocols are for this particular board.