What Are Flexible Printed Circuits

The simplest definition for flexible printed circuit: A pattern of conductive traces bonded on a flexible substrate. A better definition would be: The perfect solution to your electronic packaging needs.

Flexible printed circuits are also known as flexible circuits, flex circuits, flexible pcbs and sometimes flexible printed circuit boards. Flex circuits are sometimes regarded as a printed circuit board that can flex. In reality there are significant differences 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 circuit board. See article “five common flexible circuit design errors” by Dave Becker, for more insights.  See this article for  some interesting historical perspectives on flexible printed circuits.

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 bonded to a dielectric layer. Copper is a very common metal, but there are other forms of conductive materials used. Thickness of metal layer can be very thin ( <.0001″) to very thick (> .010″). The dielectric layer is usually polyimide or polyester, but other materials can be used. 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. Please review All Flex Standard material page or its design guide for more information on materials used.

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 impacts the material selections that best meet the functional needs. An experienced All Flex design engineer takes the critical requirements into consideration when designing a circuit to meet your needs.

Basic Types of Flexible Printed Circuits

There is a wide range of circuitry configuration, sizes and functionality, but printed circuits can be classified as one of the following types.

Single Sided Circuit: Consists of a single layer of metal traces on one side of a dielectric layer.
Double sided Circuit: Metal layers are on both sides of a single dielectric layer. Metal layers are often connected by metalized thru- holes.
Multi-layer Circuit: Several copper layers separated and encapsulated by dielectric layers. Metal layers are often connected by metalized thru- holes.
Rigid Flex Circuits: This is a multi-layer circuit where some of the layers are hard board and some are flexible circuitry.

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 packages using hardboard circuits. Assembly and handling costs can be significantly decreased because the entire interconnect system can be built as one integrated part. Add All Flex’s ability for component assembly and testing and the supply chain management becomes greatly simplified.

This tremendous flexibility in design choices leads to electronic packages being smaller, lighter and more functional. See flexible printed circuit benefits.

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 much more common because of they are more robust, cost effective and 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.

Information on circuit imaging

Information on Defining the circuit cutline

Information on Dielectric Coverings for Flexible Circuits

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/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 where the material is dissolved away as part of the process. Standard materials that All Flex uses for surface finishing.

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.

Read this post on Flexible Printed Circuit Boards.