flexible PCB fabrication
The performance of a PCB largely depends on the substrate material. Flexible circuit boards (FPC) use a variety of flexible dielectric film materials including polyester (Mylar), polyimide, and fluoropolymer. These materials must be able to withstand mechanical stresses such as excessive bending, folding, and twisting during fabrication and end-use. They must also be able to conduct electricity without compromising the board’s structural integrity and functionality. Incorporating design criteria for flex circuits that consider these stress points is essential to the overall success of an FPC design.
The etching, copper plating, and coverlay process used to create FPCs is similar to that of rigid PCBs. Firstly, the core material is chemically cleaned and then exposed to photosensitive etch resist. Then the desired mask pattern is applied to expose and develop the resist, before etching it to remove the protective coating and reveal the copper layer underneath. Copper is then deposited and chemically plated to the pads, vias, and lands on the PCB. Typically, a minimum of 1 mil copper plating is used on a flex PCB to add mechanical support to the pad or via, which is important for the reliability of the product.
Another critical factor of a flex circuit is its thermal stability, which must be sufficient to disperse heat generated by the components attached to it. Depending on the application, this may require thermal vias or solid copper planes in the PCB stack-up to improve thermal conduction. In addition, incorporating heat sinks in the assembly helps with thermal management.
What technologies are involved in flexible PCB fabrication?
In order to ensure that a flexible pcb fabrication functions properly, it must be subjected to various tests and inspections. These tests include environmental, functional, and X-ray inspections. During environmental testing, the flex circuit is exposed to different temperature and humidity conditions to ensure that it can withstand the expected environmental conditions. During functional testing, the flex circuit is connected to a test fixture that simulates the intended application and evaluated against the expected specifications. Lastly, X-ray inspection is useful for detecting any hidden flaws in the internal layers of the flex circuit.
A protective layer called a coverlay is applied over the copper traces. The coverlay is typically made of polyimide and serves as an insulating layer while providing mechanical protection. Alternatively, a flexible solder mask can be used to cover the circuitry, depending on the application requirements.
Once the etching, copper plating, and covering processes are complete, the flex circuit is ready to be cut into its final shape. For high-volume production, this is done using a hydraulic punch and die set with reasonably high tooling costs. For prototypes and low-volume runs, a blanking knife is used. A blanking knife is a long razor blade bent into the shape of the flex circuit outline and affixed into a routed slot in a backing board (such as MDF, plywood, or thick plastic like Teflon) to be cut. Finally, the flex circuit is visually inspected to ensure that it meets all of its mechanical and electrical testing specifications before being shipped.
