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Mechanical Stress on Flexible PCBs

PCBs have long been at the heart of consumer electronics, power tools and medical devices. These products have gotten increasingly complex and compact over time, which has been possible thanks to innovative technologies like flexible PCBs. These specialized boards allow manufacturers to pack more features into small packages that are lightweight, durable and easier to assemble. However, the design process for these types of boards can be tricky due to their unique properties.

One major factor is mechanical stress, which can damage the copper traces and result in poor conductivity or even failure. To avoid this, there are a few design tips to keep in mind when designing a flexible pcb board.

The first is the importance of a good drill-to-copper distance. Flex materials are more prone to movement and contraction during manufacturing, so the drill-to-copper distance should be a minimum of 8 mils to prevent errors. Another crucial tip is to stagger traces on the front and back of the circuit to avoid I-beaming. Stacked traces can cause stress concentration which can lead to copper thinning and breakage at the bend radius. Keeping traces perpendicular to the overall bend radius will also minimize this stress.

Calculating Mechanical Stress on Flexible PCBs

Stiffening a flex circuit is another way to reduce the amount of mechanical stress. This can be accomplished by adding localized rigid material (such as FR-4 or thicker polyimide with adhesive) to a flex section of the board. This will increase thickness, strength and rigidity, which can help reduce mechanical stress on the flex circuit.

When designing a flexible PCB, it is important to understand how many times and how much the circuit will be flexed. If the circuit is going to be static and will only be bent once, then the design can be more lenient and you can choose to use thinner layers of copper, higher etch densities, etc. However, if the PCB will be dynamic and subject to regular bending then you will need to take more precautions to ensure the integrity of the flex section.

For example, if the flex circuit will be bent over and over again, it is important to have a high-quality stiffener in place to avoid stress on the copper and other components. Also, the use of anchoring stubs for SMT component pads will decrease the number of coverlay access openings and reduce the risk of stress from unsupported solder pads.

In addition, it is important to minimize the number of vias on a flex circuit and to use tin or soft gold plating to protect them during the assembly process. This will protect the vias from abrasion and corrosion and ensure that the solder joints are strong. It is also important to ensure that there are no sharp corners on a flex circuit and to have tear-relief holes, tangent curves or slots in place instead of slits or straight lines. This will ensure that the circuit can be pulled out of a connector without the glue leaking.

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