In the world of electronics, necessity is the mother of all inventions, holds best applicable to the invention, evolution and development of flexible circuits in all types of electrical and electronics gadgets. The flexible circuits have just recently come of age as an interconnection technology, although it was originally developed around two decades ago.
In short, a flexible circuit is “a patterned arrangement of printed wiring utilizing flexible base material with or without flexible cover layers.”
Let us first understand the necessity of such circuits and the constraints posed by the earlier technology of printed circuit boards, which led to its invention.
Increased application of electronic devices such as car stereos, heart pacemakers, disk drives, digital cameras etc., requires greater flexibility of circuit designing and installation, to maximize space constraints. Since size of all gadgets is reducing exponentially, the need arose for designing circuits, which are functionally more capable and compact in size. Therefore came up flexible circuits to replace bulky wire attachments.
Miniaturization is the buzzword in the world of circuit designing. Great emphasis is there in reducing the size of the circuits without compromising on the performance. This required the engineer to improve the functionality and reliability of the circuits along with its flexibility. Flexible circuits are thus the solution to several spatial and orientation constraints earlier faced by fixed printed circuit boards.
How flexible circuits score over the rest?
1. A flexible circuit is made up of a flexible polymer film, which is laminated onto a thin sheet of copper that is engraved to produce a circuit pattern. The advantage of the polymer film is that the circuits can be designed and etched on both sides of the film. Another polymer overcoat is added to insulate the circuit and provide environmental protection.
2. The polymer film used for designing flexible circuits is KAPTON, which has several favourable characteristics and make it the best bet. These include heat resistance, dimensional stability, flexural capability. KAPTON’s excellent thermal stability provides a better base for surface mounting than hard boards.
3. The flat nature of such circuits offer considerable weight and space savings as compared to traditional wire harnesses. Thickness as low as 0.10mm and weight reductions of over 75% can be achieved.
1. Flexible circuits can be used in designing several single or double-sided circuits with complex interconnections, shielding, and surface mounted devices in a multi-layer design. These multi-layer designs can also be combined with rigid circuit boards to create a rigid/flex circuit capable of supporting devices as needed.
2. Flexible circuits also give designers a third dimension to work with. As the name itself indicates, these circuits provide flexibility with which one can bend and shape around circuits along two or more planes during installation. This property can be used in complex and tight-fitting assemblies where it would be impossible to accommodate several rigid boards and harnesses.
3. Flexible circuits provide excellent means of reducing assembly time of a product due its properties of flawless integrate form and flexibility, thereby reducing number of assemble operations and testing time.
Thus, now the time for extinction of the printed circuit boards is fast approaching whereas development of flexible circuits is about to reach its pinnacle.
TYPES OF FLEXIBLE CIRCUITS
Flexible circuit is a pattern of conductors created on a bendable film, often polymers, which acts as an insulating base material. The top is coated with an insulating cover layer. A flexible circuit is in fact a counterpart to a rigid printed circuit board.
There are four basic types of flexible circuits, varying in degrees of complexity which can be used in different combinations to solve almost every interconnection design problem.
1. Single-sided flexible circuits –
This is the simplest of all types and provides maximum flexibility for dynamic applications. These simple circuits are also the most easily adaptable to SMT (surface mount technology), TAB (Tape Automated Bonding), and other developments in circuit technology. It is called single sided because the circuit allows access from one side only. Its applications are in the field of optical pick-up for computers, camera, camcorder, VCD players, vibration motors for mobile phones etc.
They are the most least expensive, and produced in the greatest volume. Single-sided circuits find maximum application in appliances where dynamism is required like in case of printers and disk drive heads.
Single layer circuit is created using a technique known as back baring or double access. This technique allows access to the metal conductors from both sides of the circuit. This method eliminates requirement of plate-through holes as required in multilayer circuits.
2. Double-sided flexible circuits –
As the name suggests, these circuits have two conductive layers that are usually accessible from both sides. Their main features include component assembly on both sides. Although, their capacity to flex reduces because of their thickness and complexity in design but their ability to interconnect between sides using plate-through hole helps to implement complex designs without compromising on flexibility.
These types of circuits are used when circuit density and layout cannot be routed on a single layer. Also used in shielding applications and dense surface mount assembly.
3. Multi-layer flexible circuits –
These circuits are ideal for complex, highly dense design requirements. Large numbers of conductors can be interwoven into a small package. Flexibility may be reduced somewhat, depending on the number of layers in the design. Multilayer circuits are the ideal technology to overcome design challenges like unavoidable crossovers, specific resistance requirements, elimination of crosstalk in sensitive circuits, additional shielding of ground planes, and high component density.
4. Rigid-flex circuits –
These are combination of rigid circuits and flex circuits. So it has the advantages of both. Rigid-flex circuits are a hybrid construction, consisting of rigid and flexible substrates laminated together into a single package and electrically interconnected by means of plate-through holes.
Rigid-flex boards are normally multilayer designs, but double-sided (two-metal layer) constructions are possible as well, and, in fact, have been selected for certain microelectronic chip-packaging applications, such as in the construction of hearing aids.
Such circuit types have been encouraged by military product designers, but, in recent years, this type of construction has gained popularity in the commercial world, as well.
All the above-discussed types of flexible circuit technology offer numerous viable solutions for those challenged with packaging electronic products. The technology has come of age and become a strong contender in the arena of electronic packaging technologies.
Only sky is the limit in the advancement of electronics packaging with flexible circuits.