Flexible circuits boards

01 October 2010

Flexible circuits can have a significant effect on product cost, but their use can provide cost savings when consideration is given to the total cost of the equipment
Flexible circuits can have a significant effect on product cost, but their use can provide cost savings when consideration is given to the total cost of the equipment

Flexible circuits can have a significant effect on product cost, but their use can provide cost savings when consideration is given to the total cost of the equipment.

If considered in isolation, they are more expensive than rigid circuit boards. However, when the designer considers the first steps in his new product he must consider the basic materials that will be used, including conductors, dialectics and adhesives.

Single side flexible
This is the simplest of constructions and incorporates conductive tracking on one side of the dielectric/substrate. It is generally covered with a separate layer of dielectric to protect the conductors during the assembly and soldering operations, and photo resist is more commonly used for cost reasons. This type of circuit is the simplest to manufacture from the conductor point of view used in consumer and professional consumer products. The single sided circuit is often used to directly replace hard wiring in equipment.

Double sided flexible
Double sided circuits will have conductor patters running on both sides of the flexible, with interconnection formed with the use of plated through holes. The manufacturing stages are very similar with the creation of conventional through plated boards achieved with extra laminating and bonding stages. Due to high moisture absorption characteristics, the material does go through additional baking stages to avoid the possibility of delamination during assembly.

Multilayer flexible circuits
The multilayer features a number of layers of circuit tracking similar to the conventional board. The flexible is made up of layers of conductors laminated with dielectric, and these may be flexible or rigid. The layers may include polyimide as flexible supports and epoxy as the rigid layers. A modern circuit includes traditional through plated holes, and in some cases, the use of buried and semi-buried via holes.

Careful consideration needs to be taken at the design stage as the use of these materials require different tolerance considerations, as some of the materials used are not stable during the manufacture stages and are prone to movement. The difference in expansion of the materials also needs to be considered in the Z-axis due to the possible impact on through plating reliability. Insulating areas around all tracking and through hole interconnection should be as wide as possible. The insulating gaps in ground or voltage planes should be as wide as possible to aid manufacture tolerance, and via hole lands should also be large to aid registration for drilling.

Flexirigid circuits
The flexible circuit can be single, double or multilayer, and can be manufactured as a part rigid substrate which can benefit the assembly operation for component mounting. The rigidising is achieved by bonding an extra layer of cover coat material to the circuit or through the use of a piece of rigid laminate such as FR4. In this case, the component mounting may be conducted quite simply on the fixed area; simplifying the assembly and soldering operations.

A further method of rigidising the circuit is through the use of a copper earth plane, which may or may not be part of the circuit. The extra copper thickness provides further support without a further bonding operation. Alternatively, an assembly pallet is used for flexibles during printing, placement and reflow soldering.

Each of the materials used in the manufacture of flexibles must be considered for their electrical, environmental and application requirements. The flexible material may have weight saving benefits in comparison to hard-wired interconnections. This in turn leads to benefits of repeatable connection, eliminating wiring errors.

In many cases, the use of flexible circuits makes curtain products feasible, but in the case of photographic equipment or hand held portable goods, they would not be possible. In the case of controlled impedance circuits, both technologies have supported the products’ feasibility. In the case of the conductors, the selection is limited to two types and the actual thickness of the copper used. Their functional application and whether the flexible is to operate in a dynamic where it is to be continually flexed will affect the selection.

Copper is the most common material used for conductor tracking, and low temperature polymers in the electronics industry has given a new dimension to circuit manufacturing.

The use of polymer connections is generally only used as jumper tracks and to produce conductive touch pads. For switch operations, the polymer should not generally be used in a flexible mode. The copper may be either electro-deposited or rolled annealed.

Electro-deposited copper is the most cost effective and may be used for most applications; especially in cases where extreme flexibility is required. The difference in the manufacturing techniques causes a change in the grain structure, which in turn affects the long term flexibility.

The copper weight is normally selected for its flexibility, but the electrical requirements are another consideration. The dielectric is the material that forms the base substrate for the circuit as an insulating surface for the conductive circuitry.

The dielectric types used for flexible circuits are polyimide, polyester and PTFE (Poly Tetra Fluoro Ethylene). When considering all the uses of flexible circuits, Polyester is probably the most widely used, but not for SMT due to the higher soldering temperatures.

Polyimide is the most widely used for the production of these circuits because it has very good mechanical, electrical and chemical properties. The material can withstand high soldering and operational temperatures, but it is difficult to assemble automatically due to the flexible nature of the circuit. In this case, circuits are often rigidised for production.

Polyimide, or Kapton as it is known, is generally available in thicknesses of 25 microns (0.001in) to 125 microns (0.005in) with 50 microns being the most cost-effective selection for most applications. Kapton is the most expensive of the dielectric materials, being about five times the price of polyester.

However, improvements in material technology are happening all the time and suppliers do have materials that will compete alongside Kapton for modern applications. The adhesive systems available for flexible manufacture are limited. Polyester is used for polyester laminates in conductors and dielectrics.

In the case of polyimide, both epoxy and acrylic based adhesives are available. The acrylic is generally used due to its superior flexibility and the adhesion qualities during extended soldering operations.

Adhesive systems need to be considered in detail with regard to the product specification. In many cases adhesive selection needs to be made in the light of the flame-retardant requirements.

Of course, the selection depends upon the final product application because it is very easy to design the ‘unbuildible board’ with the many combinations of conductor, dielectric, and adhesive systems available.

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