Printed Circuit Board vs Flex Circuits

It is common for many designers and engineers to think of a flexible circuit as a printed circuit board (PCB) that can bend.  While it is true that a flex circuit and PCB perform the same electrical function, the structural and mechanical differences are significant, and not understanding them can result in poor yields and field failures.    It is important to note the different terminologies that are used in the electronics industry.  Flexible circuits have been called flex, flex circuits, flexible pcbs and flexible printed circuit boards.  Printed circuit boards can also be called PCB’s or simply hardboards.

Let’s start by stating the obvious difference:  a flexible circuit can bend, fold and twist, a printed circuit board cannot.   This key difference requires a flex to be manufactured with a different set of materials, design rules and different manufacturing processes.

Perhaps the biggest difference is the materials.   Both printed circuit boards and flexible circuits have similar construction:   In the case of a single sided circuit, the construction is dielectric base layer, adhesive layer; copper or conductive material layer and perhaps a protective overlay material.

Base Construction
basiccktconstruction

Base Layer:  For a printed circuit board the base layer tends to be rigid and usually contains glass reinforcement, FR4 is probably the most common material.  This gives the material excellent dimensional stability, thermal resistance and mechanical strength, but very little ability to bend.   Flexible circuit base materials are most commonly made of polyimide.  The material has excellent flexibility properties, but it does not have the same mechanical support for components and is less dimensionally stable vs. the glass epoxy FR4.

Adhesive:  Since printed circuit boards don’t flex, the requirements on the adhesives are limited to chemical and thermal properties.   On the other hand, a flexible circuit has an additional requirement to allow bending.    An adhesive that works well for a hard board, would crack or fracture in a flex application.   An adhesive for a flexible circuit needs to “stretch” a bit.   The result is that an adhesive for a flex will have different thermal, chemical and mechanical properties from adhesives used in PCBs.

Copper:  There are 2 basic types of copper film, Electro Deposited (ED) and Rolled Annealed (RA).   The key difference is that RA copper is much more flexible and is needed in dynamic flex applications.   ED copper does have some flexibility and may be used in some circumstances, and is almost exclusively used to produce rigid pcb’s.   There is also a HDED (high ductility) copper that is electro-deposited with a treatment that makes it more flexible, but not quite as good as RA.   Considerations for copper are seldom an issue for a rigid printed circuit board since it doesn’t require flexing.   Copper choice in a flexible circuit can be critical to performance, both in dynamic flexing and flex to install applications.

The before mentioned material differences create a whole set of added complexities in design and fabrication of a flexible circuit.  Registration tolerances, chemical and thermal properties and a host of other factors mean design specifications for a flex will be different vs. a rigid printed circuit board.

The key differences discussed in this blog pertain to common constructions in flexible and hard board circuits.   Both are available in alternate materials for special applications.  Most notably, flexible circuits are available in adhesiveless constructions.