Flexible PCB’s give you the freedom to wrap your Printed Circuit Board around corners, create flat board to board connections and manufacture custom membrane keypads. In this article, I shall give some background to the materials used in flex pcb's and suggest an introduce approach to low cost prototypeing.
Typically there are two types of material used for flexible PCB manufacture. The traditional and more widely used material is Polyimide or PI which is usually referred to as Kapton or PyraLux (trade names of Dupont). The most common alternative is Polyester film, or Mylar.
The following table indicates some of the important properties of the materials commonly used in Flexible circuits. Tg=Transition temperature.
| Substrate (Selection)
| Tg [°C]
| Moisture Absorption
| Polyimide (PI, Kapton)
|| 1...3 %
|| Standard resin for flexible printed circuits. Good resistance against strong acids, organics, but poor against strong bases.
| Polyester (PET, Mylar)
|| < 0.4 %
|| PET is a low cost, optically clear substrate that is suitable for many applications. Good resistance against strong acids, organics, but poor against strong bases.
| Polyethylene Naphthalate (PEN)
|| approx. 120
|| < 0.5 %
|| PEN is a low cost substrate designed to offer properties ranging between polyester and polyimide. PEN also has excellent resistance to solvents and chemicals.
| Composite (Epoxy/Glass etc.)
|| Good resistant against strong acids, organics, but fair against strong bases. Lower flexibility than other substrates.
The Flexible circuit is constructed of layers of material bonded together. The inner core is the substrate, the outer layers are copper and a protective film.
An alternative construction uses an adhesive layer to bond the copper tot he Substrate.
An adhesiveless laminate has the copper directly attached to the polyimide core . Eliminating the adhesive bond layers allows for thinner constructions and more flexible design with vastly improved reliability. In addition adhesiveless copper clad laminates have higher operating temperature ratings, higher copper peel strengths, and the desired reduced Z-Axis thermal expansion stress on vias.