Flex circuits general Guide

Definition of flex circuits

The Institute of Printed Circuits (IPC) defines flex circuits as "A patterned arrangement of printed wiring utilizing a flexible base material with or without flexible coverlayers." 

Flex has different connotations according to how many times a circuit is required to flex during its lifetime. Some flex circuits are only flexed a few times during assembly (referred to as “bend and stay”), while others are required to flex a few hundred times in their life (flexible circuit), and still others are flexed thousands of times during their life (Dynamic flex). Each of these has design implications. 

The common flexible base materials encountered are polyimide (PI), polyester (PET), polyethylene napthalate (PEN), and thin bendable semi rigid material (Bendflex). 

The printed wiring is etched in high elongation copper (ED), roll annealed copper (RA), or printed in silver conductive ink. Constantan and many other metal foils can and have been used to create flex circuits including, stainless steel, beryllium-copper and gold, to name just a few. 

Coverlayer films bonded onto the flex circuits with adhesive are normally are polyimide (PI), polyester (PET), polyethylene napthalate (PEN). There are also photo-imageable dry film solder masks available and these are applied and processed similar to dry film resists. Covercoats are available for silk screening over the flex circuits using screen stencils, and photo-imageable covercoats can be applied through screening, curtain coating, air spraying and electrostatic spray. The flexibility of the dry film and photo imageable covercoats vary a great deal and dynamic flexing applications still normally use coverlay films. 

Here is a more accurate definition “A bespoke component designed for use as a flexible interconnect, or as an electronic assembly with integral flexible interconnects and capable of being modelled in 3-D space” 

A colourful description rather than a definition came from the chief mechanical engineer of one company who described flexible circuits as “The spaceman’s friend - the answer to small enclosures”

Types of flex circuits

Single Sided flex circuits
This is a single layer flexible base material with printed or etched wiring on one side. It may have a coverlayer or covercoat on top of the printed wiring. The coverlayer or covercoat normally has apertures in it which dictates the component placement. It may have stiffeners and/or rigidisers applied to facilitate component support

Dual Access flex circuits
This is a single layer flexible base material which is pre-routed or punched. A layer of metal foil, usually copper is then laminated onto the base material using pre-routed adhesive sheet. The printed wiring is then etched from the metal foil. A pre-routed or punched coverlayer is then laminated onto the parts. The apertures in the base and the coverlayer allow access from the top or bottom sides for soldering etc. Again stiffeners and/or rigidisers can be applied to facilitate component support.  

Double Sided flex circuits
This construction consists of a flexible base material pre-laminated on both sides with a metal foil. The printed wiring is created by etching off the foil on both sides. It may have a coverlayer or covercoat on top of the printed wiring. The coverlayer or covercoat normally has apertures in it which dictates the component placement. It may have stiffeners and/or rigidisers applied to facilitate component support.
It is common to make via connections from one side of the printed wiring to the other, using drilled holes which are then metallically coated and plated through. The term PTH is commonly used for the latter type of board. 

Multi-Layer flex circuits
This construction combines multiple single sided and /or double sided flexibles laminated together using sheet adhesives. Layer counts of up to 30 layers have been achieved. Drilling and plating operations are used to give the necessary interlayer connections and it is common for the outside layers to be used as screens for Electrical purposes.
Where there are high layer counts involved it is usual to vary the length of the layers to facilitate bending.

▲

Rigid flex Multilayer circuits
These are rigid multilayers internally interconnected with flex circuits where required. The flexible interconnects form an integral part of the construction along with the rigid layers and the drilling and plating is done as a composite. The elimination of connectors gives repeatability and reliability gains to the equipment designer.

The advantages of flex circuits

Space.
Flex is able to occupy three dimensions. It can be bent around corners and over itself in order to fit into a much smaller device enclosure. This contrasts with the clutter and bulkiness often seen in rigid interconnects.

Weight.
Because flex is significantly thinner and lighter than standard rigid boards, products incorporating it will naturally be lighter and thus more portable. High track densities are achievable and this has led to weight reductions of over 75%
 

Enhanced Reliability.
Rigid-flex multilayer applications can remove the need for cabling and connectors. Removal of these interconnection points gives a much greater reliability. Because the track routing is determined by artworks, wiring errors are completely eliminated. Electrical characteristics such as capacitance, impedance, and cross-talk are consistent from batch to batch.

Better for Surface Mount Technology (SMT)
Thermo-mechanical failures at the solder joint interface due to thermal expansion is a chronic problem in bonding SMT devices to rigid PC boards. Flex circuitsoffer a strain-free bonding site for the SMT component. The flexible base materials absorb the thermal stresses and thus decrease solder joint failure. In tests consisting of 1,000 thermal cycles from -55 to 125°C, flex circuits showed no failures until 2,500 cycles while rigid boards failed after as little as 10 cycles.

Interconnection between moving parts
The most reliable way to interconnect moving parts is to use a dynamic flex circuit. Flat ribbon cable can only be used  where a limited number of flexures are required. Properly designed flex circuits with correctly selected high elongation copper or rolled annealed copper conductors designed to be in the neutral plane have proven to be reliable where millions of flexures are required.

Legal Notice

This information is supplied in good faith by Flexible Technology Limited. We hope you find the above information helpful. It is by no means a complete guide to flex circuits and is only offered as an aid for flex circuit users. The use of any information contained in this site at your own risk.

For information on where to purchase flex circuits visit flexible circuits manufacturer Flexible Technology Limited 

Copyright FTL Webmaster © JAN 2006