Flexible Circuits Accommodate
A flexible circuit, also known as a flex PCB, consists of a metal layer (the traces) bonded to a dielectric, which can be either polyimide or polyethylene. The copper traces can be as thin as.0001″ or as thick as.0010″ and are typically applied to the surface of the dielectric using an adhesive, although some manufacturers are now utilizing “adhesiveless” laminates that deliver improved performance.
Despite their diminutive size, flexible circuits are capable of carrying significant amounts of data and power. As such, they are often used as an alternative to wire harnesses in applications where weight and space savings are critical considerations. In addition, flex PCBs can be integrated into a rigid board to eliminate the need for bulky ribbon cables or solder connections.
The industry trend toward shrinking semiconductor device packages and increasing I/O capability places considerable demands on the printed circuit boards that they connect to. In order to keep up, board designers are seeking a solution that can accommodate high-density interconnects without increasing cost and layer count. Multilayer flex circuits are the ideal solution to address these requirements.
Can Flexible Circuits Accommodate High-Density Interconnects?
However, designing a multilayer flex circuit is not as simple as simply stacking multiple double-sided flex circuits together. There are a number of additional design issues that must be addressed in order to achieve the desired performance and manufacturability objectives.
One of the most important considerations in the design of a multilayer flex circuit is establishing the appropriate via size and spacing for the application. Smaller vias require careful design, as there is a risk that they may be subject to stress cracking during the flexing process. This is mitigated by filling or capping the vias, which helps to distribute the stress over a larger area and thus reduce the probability of failure.
Another issue is the selection of a proper copper material. Static flex applications that will only experience tight bending during installation typically use electrodeposited (ED) copper, which is less expensive than Rolled Annealed copper (RA). Conversely, dynamic flex applications, such as flip-style cell phones and laptop arms, require RA copper for better flexibility.
Lastly, the design of a multilayer flex must consider the dimensional requirements of the connectors that it will interface with. Both single- and double-layer flex circuits can be used with zero insertion force (ZIF) connectors, but stiffeners created from pieces of FR-4 fiberglass and pressure-sensitive adhesive are necessary to stabilize the ends of the flex circuit in order to maintain the needed flexing during assembly.
Ultimately, the key to a successful multilayer flex circuit is to understand the limitations of the design and then utilize the right tools to overcome these challenges. The combination of a high-performance laminate, a proven design tool, and an experienced supplier can provide the right solutions for your unique product requirements.
