Designing with Ultra-Thin, Flexible Printed Circuit Boards (chapter one )

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Space savings, greater reliability, low mass, and high ductility are some of the advantages offered by flexible PCBs, but designers must also prepare for their complexity.

Flexible printed circuit boards (PCBs) aren’t much different than rigid boards during the design phase, except that the designer must account for the mechanical complexity associated with flex circuits . For instance, a flexible PCB can tear if flexed beyond its capability during installation.

Therefore, it’s very important to create a mechanical model of the PCB and test it for a proper fit, before taking up the electrical design. This would also involve testing the ergonomics of the installation, any misalignments, and servicing. In addition, it necessitates that designers understand the different types of flex circuits available and the way they work.

Types of Flexible PCBs

Depending on the application, several types of flexible PCBs are available, chief among them being the flex, rigid-flex, and the high-density-interconnect (HDI) flex.

Flexible PCBs

These are flexible versions of the commonly available rigid PCBs, with unique capabilities such as flexibility and vibration resistance. The extra features come along with the usual reliability, repeatability, and high density already offered by rigid PCBs. The major advantage over rigid PCBs is flex circuits have the ability to assume three-dimensional configurations. One of the most common applications of flexible PCBs is as a replacement for wire harnesses.

Rigid-Flex PCBs

These PCBs are a blend of the rigid and flex, offering the best of both constructions, while adding some unique capabilities that neither possesses alone. For instance, a typical rigid-flex configuration would be a series of rigid PCBs linked by integrated flex circuits. By integrating rigid areas added to the flexible parts, designers can greatly increase the design capability of their circuits.

While the rigid areas are excellent as hard mounting points for chassis, connectors, and components, the flex areas offer dynamic flexing, vibration-resistance zones, and flex-to-fit. Such blending offers designers multiple options for arriving at creative solutions for the most demanding applications.

HDI Flexible PCBs

High-density-interconnect PCBs are useful when the options offered by typical flexible circuits aren’t adequate. HDI flex circuits offer better design, layout, and construction options by incorporating fine features such as micro-vias. Increased functionality, smaller form factor, and highly dense flex circuitry are among their features.

Although it uses thinner materials, HDI technology provides better reliability, improved electrical performance, and access to advanced IC package use.

Advantages of Flex Circuits

As a replacement for a ribbon cable, or discrete wiring, flex circuits supply customized repeatable routing paths throughout the assembly. Flex circuits can reduce service calls due to their greater dependability.

Flex boards cover their conductors with polyimide, a dielectric layer that protects the circuit better than the simple solder mask. Manufacturers use other base and cover materials to handle a broader range of ambient conditions and harsh environments.

Although flex boards can be very thin, they’re able to weather long duty cycles of flexing. Using suitable design material, they can be made robust enough to withstand millions of flexing cycles, while carrying power and signal without interruptions.

The low mass and high ductility of flex circuits is a huge advantage when facing high acceleration and/or vibration. The stress and impact on itself and solder joints on the flex PCB are far lower than that faced by the solder joints and components on a rigid PCB under the same working conditions.

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