Flexible Circuit Boards: A Primer

To be able to understand flexible circuits, one must be familiar with metal foils, plastic films, and adhesives and must understand the interactions of a variety of combinations. Each type of material has a different operating temperature base on the adhesive system.

Below is a list of basic types of flex materials:

Single sided: Having conductors on one side of the base material, coverlay on the same side.

Rigid-Flex: Several layers of a variety of combinations of either single or double sided materials bonded together in a stack configuration, with areas being rigid, and other areas being flexible.

Double-sided: By definition double-sided boards have circuity on both sides of the boards. The have plated through holes to interconnect the two sides. These are the workhorses of consumer electronics, automotive electronics, and the RF/Microwave industry. They are the lowest cost choice of consumer products. Laminate materials range from resin-impregnated paper to blend of low cost Teflon for RF applications. The most common material used is FR-4 because of its proven reliability and its low cost. Even with technology advances in material applications double sides are here too stay.

Electrical interconnections for products are too frequently an afterthought and chosen when many opportunities for true cost reduction have already been lost. The application and use of flex circuits should be evaluated in the beginning. Products made with flexible printed circuits tend to be more reliable because of fewer interconnections and the flexibility of the circuits.

All About Multilayer Boards

By definition multilayer boards have three or more layers. Multilayer boards are the mainstream of electronic devices, including consumer products such as portable cameras and cellular phones. As personal computers become more powerful, mainframe computers are being replaced in many applications by smaller machines. As a result, the use of highly sophisticated multilayer boards which have a larger layer count and have incorporated surface mount technology to enhance these capabilities. As the pwb have had to address the issues of higher speed, higher density, and the rise of surface mount components that use both sides of the board, the need to communicate between layers has increased. At the same time the space available for vias has decreased, causing a continuing movement towards smaller holes. With more holes on the board comes the decline of the use of holes that penetrate the entire board, which use space on all layers. As a result, the use of buried and blind vias has started to become a standard part of the multilayer board technology.

The manufacturing cost of these sequential technologies is not necessarily directly cheaper than conventional multilayer technology, However an example of eight layer conventional structure can be reduced to four layer structure, reducing the cost for packaging density.

Modern electronic packaging has become very complex. The choice of which packaging technology to use is governed by many factors. The main factors are cost, electrical requirements, thermal requirements and density. Material selection is the final consideration. Multilayer printed circuit boards play an important role in electronic packaging.

Rigid-Flex Circuit Boards: A Primer

Rigid-flex circuits have been used for the last 30 years primarily in the military and aerospace markets. Rigid-flex circuits have a distinctive advantage over conventional rigid circuits. They provide a flexible medium that delivers signals and power to areas of a system that may be around a corner or located in a tight space. Engineers and designers have the freedom to position components and other devices so that the system builder does not have to change the package configuration (a costly task).

Due to the size and packaging constraints of today's products, engineers are being forced to seek alternatives in trying to cram high tech function into less space (remember ENIAC)? Eniac was replaced by a handheld calculator. In most rigid-flex boards the circuit consists of multiple flex layers bonded together. The flexible substrate layers are usually a polyimide film with a cast adhesive, using either a modified acrylic or epoxy adhesive. The rigid portion of the board is typically bonded using an epoxy no-flow prepreg. The flexible portions consist of a base material in varying dielectric thickness .001 through .005 thousand's of an inch. Adhesive and rolled annealed copper foils are laminated onto the polyimide base on one or both sides to form the first surface that can be processed. The copper side of the base material is then coated with a photoimagible material.

The coating allows for an image to be developed and then exposed to etching process. This process is the same for rigid boards except that flex requires a certain amount of special handling. Etched layers are then laminated with a protective cover layer of adhesive backed Kapton base material that offers insulation, dielectric and oxidation protection.

Rigid core standard material is then bonded to the outer surfaces of the flex to form a sandwich that adds stability and strength to the final assembly of the complex board. The package is now ready for further processing and is treated like many standard boards. Drill, copper plating, etching. Etc... Increased real-estate on PC boards demands total utilization of the packaging surface area.

Rigid-flex gives you the technology to capture and maximize cross-coupling technologies of packaging and processing of special materials. The advantage to this is a solution of flexibility, reduction of package size, cost-effectiveness, and reliability for technology customers.