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.

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 PCB’s 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 that the All Flex team has termed “flex-izing” and have worked hard to perfect over the last 25+ 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 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

Single sided circuits consist 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 through- holes.

Multi-Layer Circuit

Several copper layers separated and encapsulated by dielectric layers. Metal layers are connected by metalized through-holes.

Rigid Flex Circuit

This is a multi-layer circuit where some of the layers are hard board and some are flexible circuitry. These are used when components are mounted on both sides of the rigid section.

Maxi Flex™

All Flex has trade marked our special flex circuit prototype offering that allows an application to be produced at larger than normal sizes. 

Fine-Line Maxi Flex™

Fine-Line Maxi Flex™ technology allows designers to increase the density of the FPC, allowing for an increase in the electrode count from 8 per unit to 12, 15, and even 18.

Sculptured Flex Circuits

Sculptured flexible circuits have circuit traces that vary in thickness across the circuitry pattern. By having different thicknesses a circuit can have thicker copper.

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 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.

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.

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.

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