Moisture Affect the Performance of Flex Circuits
The performance of flex circuits is driven by the integrity of all the materials and components that make up the flexible printed circuit board (FPB). As such, it’s critical that each material and its associated processes work reliably in concert. The primary elements in a flex circuit construction are the base material, copper foil and coverlay/bond plies. The base material provides the primary physical and electrical properties that define the characteristics of the flex circuit.
The base material is a thin flexible polymer film which is typically copper clad polyimide. The copper foil is either a rolled or electrodeposited material. Rolls are a traditional choice for flex PCBs, but thinner films which are electrodeposited and etched to create conductive material traces are becoming increasingly popular. The etching process itself is similar to the traditional PCB manufacturing processes, with panels being drilled, plated, photo-imaged and developed before they are etched and laminated into the final flex circuit board design. Once the etching and plating is completed, a layer of the bond/coverlay material is applied to protect the copper and conductive traces from moisture.
Once the bond/coverlay and etching process is complete, the flex PCBs can be fabricated into one of several variations. Single-sided flex circuits have two conductor layers while double-sided flex circuits can be fabricated with or without plated through holes, depending on the design requirements of the application.
How Does Moisture Affect the Performance of Flex Circuits?
When a flex circuit is exposed to mechanical stress, it can be degraded by excessive bending or twisting that results in cracks in the conductive traces or insulation. Exposure to corrosive chemicals and gases can also cause damage to the flexible circuit material.
The ductility of a flex circuit enables it to absorb vibration and shock, which can improve the reliability of an electronic product in harsh environments. It also reduces the weight of an assembly which can have a positive impact on a product’s end-use application.
Another key factor in the performance of a flex circuit is its thermal stability, which helps it operate at high temperatures without damaging its materials or affecting its electrical properties. The flex circuit’s exceptional thermal stability allows it to be used in high-reliability military and medical applications.
Lastly, a flex circuit’s smooth surface finish can improve the aesthetics of an assembled product. This is a crucial element when delivering products to medical professionals who may use the product in clinical settings, such as hand-held cauterizing tools. The sleek flex circuits of these devices are not only pleasing to the eye, but they have been shown to increase patient confidence and satisfaction in the procedure.
Baking both flexible and rigid-flex circuit boards before assembly is an essential step for achieving the highest quality and durability standards required in demanding electronic applications. Pre-assembly baking eliminates defects that can be caused by trapped moisture and enables these circuit boards to withstand rigorous environmental conditions while maintaining consistent performance standards. Learn more about the importance of this pre-assembly baking process by downloading our free whitepaper.