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What are the design constraints of a flex PCB?

design constraints of a flex PCB

A flex PCB can improve design and production efficiencies, increase circuit densities, and eliminate or reduce the need for external connectors. However, a rigid-flex PCB also comes with its own set of unique constraints that must be considered to design a successful board.

Unlike a rigid flex pcb, where copper is embedded within the material and can’t be easily reshaped to accommodate changes in conductor widths, flex circuits are designed with flexible copper foil. To minimize the cost and complexity of flex boards, designers must choose a conductive material with a high coefficient of thermal expansion and a suitable bending radius for their application.

Rigid-flex circuits must have their layer stacks and parts placed carefully to prevent mechanical stresses that could cause them to fail during operation. They can also require additional structural supports such as anchors and teardrop annular rings to help resist shear forces that could separate traces or pads from their plating.

What are the design constraints of a flex PCB?

To maximize the strength of a flex circuit, it is recommended to stagger traces rather than stacking them one on top of another. Stacking traces like this creates an i-beam effect that makes the circuit weaker, and it can also restrict its flexibility.

The underlying polyimide substrate and the coverlay of a flex circuit are not as strong as FR4 or other commercial pcb materials, so it’s important to keep in mind that the flex PCB can be damaged by excessive force or overheating during assembly. In these cases, a flex PCB may need to be strengthened with adhesives or other stiffening agents. This involves dispensing and curing liquids that can add time and cost to the fabrication process. If your design calls for a stiffener you should clearly document the location of this area on your fabrication drawing and collaborate with the fabrication and assembly engineers to make sure they have enough space to work.

In addition to adhesion issues, flex PCBs can be susceptible to environmental damage caused by moisture and chemicals. This can cause corrosion and shorting between layers, and it’s essential to design a robust and reliable circuit to ensure long life.

Copper is the standard material used for the conductive traces in Flex PCBs. The copper layer can be applied to the substrate through various methods, including electrodeposition or lamination. The thickness of the copper layer can vary depending on the application, but it is typically thinner than the copper used in rigid PCBs to maintain flexibility. After the copper layer is applied, it is patterned using photolithography, a process that defines the circuit pathways by exposing the copper to light through a mask.

A flex PCB can be made to withstand harsh environments with proper preparation and design. To protect the underlying copper foil from environmental damage, you should use a higher-grade rolled-annealed (RA) copper. This type of copper is more resilient to work hardening and fatigue, which makes it a better choice for a flex circuit that will be bending or rolling all the time. To mitigate electrical problems, you should also consider using hatched grounds for maximum reliability and impedance control. Alternatively, you can also route return traces directly under the traces of concern to avoid exposing them to excessive stress.

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