Flexible Printed Circuits

What are Flexible Printed Circuits?

Flexible printed circuits, also known as flex circuits, are sometimes regarded as a printed circuit board (PCB) that can bend, when in reality there are significant differences between PCBs and flex circuits when it comes to design, fabrication and functionality. One common mistake that designers make is to design a flexible circuit using the same rules as a PCB. Flex circuits require a unique set-up and have their own set of design rules. The All Flex team has termed this process “flex-izing,” and we have worked hard to perfect it over the last 30+ years.

The word “printed” is somewhat of a misnomer as many of the manufacturing processes today use photo imaging or laser imaging as the pattern definition method rather than printing.

A flexible printed circuit consists of a metallic layer of traces, usually copper, bonded to a dielectric layer, usually polyimide. Thickness of the metal layer can be very thin (<.0001″) to very thick (> .010″) and the dielectric thickness can vary from .0005″ to .010″. Often an adhesive is used to bond the metal to the substrate, but other types of bonding such as vapor deposition can be used to attach the metal.

Because copper tends to readily oxidize, the exposed surfaces are often covered with a protective layer; gold or solder are the two most common materials because of their conductivity and environmental durability. For non-contact areas a dielectric material is used to protect the circuitry from oxidation or electrical shorting.

The number of material combinations that could go into a flexible printed circuit are nearly endless; current, capacitance, chemical and mechanical resistance, temperature extremes and type of flexing are just some of the criteria that impact the material selections to best meet functional needs. An experienced All Flex design engineer takes the critical requirements into consideration when designing a circuit to meet your needs.

Flexibile Circuit
Flexible Printed Circuit Design Advantages

The fact that a flex can be bent, folded, and configured in just about any shape or thickness imaginable gives the designer tremendous options when creating an electronics package. Size and space limitations are far less of an issue than traditional design using hardboard circuits. Assembly and handling costs can be significantly decreased because the entire interconnect system can be built as one integrated part. Add in All Flex’s ability for component assembly and testing, and supply chain management becomes greatly simplified.

This tremendous flexibility in design choices leads to electronic packages being smaller, lighter, and more functional.

Fabrication

There are two basic categories of processes for manufacturing a flexible printed circuit: subtractive and additive.

In a subtractive process, one starts with a solid area of metal, and the unwanted areas of metal are removed to form the traces. Screen printing and photo imaging are the two most common processes used for defining the circuitry pattern.

In an additive process, one starts with a bare dielectric layer and the metallic traces are added only where needed to form the circuit. The conductive layer can be printed, plated, or deposited in a variety of manners.

The subtractive processes are more common because they are more robust and cost effective, and they allow greater choices in final product configuration. The circuits created by the additive process have less current-carrying capability and environmental resistance than circuits created by the subtractive processes.

Finishing and Assembly

Surface finishing is usually required to assure the printed circuit surface is ready for subsequent bonding such as SMT assembly, wire bonding, or pressure connector insertion. Nickel or gold, tin, silver, and solder are excellent metals for this purpose. Organic coatings can also be used to protect the copper until the bonding process, which dissolves the material away.

There are countless assembly options for a flexible printed circuit. In addition to electronic components and connectors, a variety of electrical or mechanical devices can be attached to a flexible circuit. The circuit can also be easily bonded to a curved surface or formed to any 3 dimensional shape. With proper construction a flex circuit can handle dynamic flexing, making it the ideal interconnect solution for electronic packages that connect moving or rotating parts.

The true potential of a flexible printed circuit may only be limited by the imagination of the designer! Contact an All Flex design engineer today to learn more about the amazing possibilities.