Number of Layers in a Flex Circuit
A flex circuit is a multi-layered printed circuit board (PCB) that combines rigid and flexible sections. Its flexibility is accomplished by using a combination of materials to support bending in various directions while maintaining good conductor and dielectric integrity. The most common material used in flex PCBs is polyimide. It is tough, can withstand many solder reflow cycles, and is thermally stable. It also has good electrical properties and is a good choice for high-frequency applications like RF and EMI.
Flex PCBs are available in single layer, two-layer, four-layer, and six-layer configurations, with the latter a common choice for most designs. It is possible to build more layers, but each additional layer adds more cost and complexity to the manufacturing process. In addition, the more layers there are in a flex circuit, the more likely it is that stresses will occur in the copper layers. This can result in work-hardening and fatigue fractures, which can be costly to repair.
The layer structure of a flex circuit is defined by the number of signal layers and ground planes. These layers are typically separated by insulators, such as PI or PET. In some cases, the insulators are made of soft gold, silver, or a combination of both. The insulators are needed to prevent the signal layers from touching each other, and to protect them from damage during assembly, handling, and soldering.
Limitations on the Number of Layers in a Flex Circuit
In order to minimize the insulator thickness and increase flex PCB bendability, the copper traces in flex circuits should be as thin as possible. This can be achieved by reducing the signal layer thickness and cross-hatching the ground planes on either side of the signal layers. This technique can also help to improve the flex PCB’s electromagnetic interference (EMI) performance.
Adding stiffeners to the flex section of a PCB can help to make it more resilient to mechanical bending. The stiffeners can be added in a variety of ways, such as by dispensing epoxy, acrylic, or PSA (Pressure Sensitive Adhesives) onto the flex area. However, adding these stiffeners can also limit the flex PCB’s ability to be folded or rolled and increases the overall cost of production.
Another way to reduce the flex PCB’s thickness is by using an epoxy-based prepreg. This material is more cost effective than the more expensive network epoxy resins that are typically used in rigid PCBs. However, the prepreg is not as dimensionally stable as a typical rigid PCB material, and can shrink and move with changes in temperature. This can lead to gaps in the flex area of the circuit, and can cause traces to short out.
To address this issue, the flex PCB manufacturer can use plated-through hole anchoring stubs and reduced coverlay access openings. This allows the flex pads to have smaller footprints, and is more compatible with SMT component footprints. This approach can lower the density of the flex PCB’s component mounting, but will provide sufficient strength and reliability for most flex circuit applications.