Flex Circuits
The various elements of flex circuit construction must reliably work in concert to meet the demands placed upon a flex circuit for the lifetime of its application. The design process begins by determining the specific requirements of a particular product, then creating a flex circuit that can consistently deliver those functions.
Flex circuits are used in a variety of applications to connect electrical components. They can be used as point-to-point wire replacements, particularly in dynamic applications such as folding cell phones and automotive airbags, or they can be used to connect sub-assemblies, replacing traditional bulkier wire harnesses. The flexibility of a flex circuit increases its reliability, as it allows the assembly to withstand vibrations and movements that would otherwise disrupt the connections of traditional rigid-circuit boards or hand wiring. In addition, the lower thickness of a flex circuit allows it to dissipate heat more effectively, as well as increasing the surface-to-volume ratio of the copper conductors.
A flex circuit is typically constructed from several different layers of materials. The base layer is typically a glass-epoxy FR-4 or polyimide. The copper is then either pre-laminated onto the base layer by the material vendor (a more common approach) or chemically plated on to the seed layer using sputtering. Once the copper has been deposited, a photosensitive etch resist is coated on the surface of the foil and developed, then the copper is etched off using one of the many available methods to create the desired conductor patterns.
How Are Flex Circuits Made?
Once the conductive patterns are complete, the substrate is laminated to an adhesive-based polyimide coverlay that has been pre-punched to expose the copper pads. A double-sided flex circuit has copper conductors printed on both sides of the polyimide, which are connected by through holes that have been plated in a similar manner to the through-holes on a rigid PCB.
The next step is the creation of the insulators that separate the conductors from each other, and in some cases act as support. The most commonly used insulator is polyester, but other materials such as polyimide and polyethylene naphthalene are also commonly used. A sealant or solder mask is then applied over the top of the insulated substrate, which protects it from contamination during the manufacturing process.
Lastly, the plated through-holes are filled with copper, which is often called button plating. This helps to create a strong connection between the bare copper and the surface mount component, as well as adding mechanical strength to the pad or land termination point.
After the insulator is applied, the final stage of fabrication involves cutting the flex circuit into its appropriate sizes and shapes, which is done using a tool known as a blanking knife. The high-volume cost effective method is to use a hydraulic punch and die set which requires substantial tooling costs, however for prototypes or low-volume runs, a manually operated mechanical blanking tool is utilized. The etched, plated, and coverlayed flex board is then laminated to the glass-epoxy rigid panel along with any other flexible sections of the PCB using additional adhesive, heat, and pressure.
