design considerations are unique to flexible PCBs
As the name implies, flex PCBs are a combination of rigid and flexible sections. The rigid section is comprised of a layer stack, while the flex section contains the conductive layers. The design process for both sections needs to be considered together, as the conductive layers on a flex PCB must be able to accommodate multiple bendings and temperature variations without breaking.
Rigid and flex PCBs have similar reliability, but the choice of which type to use depends on operating conditions and environmental stresses. The reliability of both types can be improved by designing them with the best possible materials, using good layout practices and ensuring they are correctly manufactured.
The choice of substrate material is key to determining the flexibility of a flexible pcbs. While the most popular option is polyimide, it’s important to select a suitable material for each specific application. A flex circuit can be subject to harsh environmental conditions, including high temperatures and moisture. Knowing these conditions will help in selecting the proper materials and allowing the fabrication shop to produce a robust flex circuit.
What design considerations are unique to flexible PCBs
Conductor layers in a flex circuit are typically copper or an alloy such as aluminum. The leading conductor layer should be flexibility resistant to ensure it can withstand the flexing of the board and its components. The conductive layer is usually coated with an insulation or dielectric. This dielectric layer may be a polyimide, FR4, or epoxy. Polyimide is the most common, as it’s durable, cost-effective, and very heat resistant. It’s also highly tolerant of solder reflow cycles, and will remain stable when subject to temperature fluctuations.
Other insulating and protective layers used in a flex circuit are polyamides, acrylates, and polycarbonate. Depending on the application, these materials can be combined to provide a customized circuit with desired properties. For example, a high-temperature PCB could include a hybrid insulator consisting of a polyimide core with a FR4 or acrylate dielectric.
The conductive layers in a flex PCB are typically made from either electrodeposited (ED) or rolled and annealed (RA) copper foil. RA foil has a column array shape, which results in an even, flat structure and makes it accessible to roughening and etching processing. ED copper has fish scales, which result in an uneven and granular structure. To improve the quality of a flex circuit, it’s important to use RA copper foil when possible.
A flex PCB’s surface finish plays two important roles: protecting the copper from oxidation and providing a solderable surface. The most common finish is an Electroless Nickel Immersion Gold (ENIG), which is applied to about 80% of all boards. It provides a thin, solderable coating on the copper and is a cost-effective choice for most applications.
The thickness of a flex circuit’s copper can significantly impact its flexibility. For this reason, it’s important to keep copper thickness as low as possible. However, this can be a challenge with multilayer flex circuits, as the etch yield for small traces can be lower than on rigid boards. To combat this, a flex circuit manufacturer can use button plating, where copper is deposited only on pads/vias. This can reduce the etch thickness, improve etch yields, and provide controlled impedance.