Flexible printed circuits are continuing to grow in popularity and necessity for PCB designers around the world. What is a flexible PCB? In simple terms, it is simply conductive traces that are bonded onto a variety of flexible substrates. Flexible substrates can be made from a polyimide (most common) or even a polyester material. Depending on your application, flex circuit manufacturers can use an adhesive to bond the conductive to the substrate or they can use an adhesive-less bonding process.