Flex Circuits
A flex circuit is a printed circuit board (PCB) that can bend. The basic elements of a flex circuit are a base material, conductive copper layers and a coverlay. The base material is a flexible polymer such as a polyimide. Most flex circuits use standard PI film as the base. This is a thermosetting resin that does not soften when heated, unlike many other adhesives which are also used in PCB construction. Manufacturers also offer upgraded PI films that are more resistant to humidity and tears than standard PI.
Conductive copper layers are etched into the base film to create a conductive flex circuit. A coverlay is then applied to protect the layers from dirt and moisture. A flex circuit can have a single-sided conductive layer, or a double-sided conductive layer. Both versions are a bit less expensive than standard rigid PCBs, however they may not be as durable.
Flex circuits can replace rigid boards in a wide variety of applications, particularly for products that require high-speed connections with multiple connectors. They can be routed in tight confines that would not accommodate the taller and bulkier rigid-board connectors, and they can be integrated with rigid components to eliminate external connectors altogether. The high conductor density of a flex circuit can exceed that of ribbon cables, and they can be designed to form complex shapes in three dimensions with branching conductors to several connectors.
Can Flex Circuits Be Recycled?
The incredibly thin dielectric substrates in a flex circuit allow heat generated by the copper traces to dissipate faster than rigid-board counterparts. This reduces the likelihood of failure due to point-to-point wire failures and increases the reliability of the entire system.
Most circuit board failures occur at points of connection, but flex circuits can be configured to eliminate point-to-point connections completely. This decreases component counts, leads to a smaller package and a significant reduction in overall product weight.
One of the biggest challenges in constructing a flexible circuit is to ensure that the copper will adhere to the dielectric. This challenge is overcome by using a special type of adhesive, typically polyimide based, to create the laminate. Adhesives are also available that can withstand higher temperatures than traditional systems, which improves the performance of the final laminate.
A flex circuit can be constructed with either plated or unplated through holes. Plated through holes are more expensive, but they provide a stronger, more reliable connection between the flex circuit and the components mounted on it.
In order to mount SMT components on a flex circuit, manufacturers must use anchoring stubs and reduced coverlay access openings in order to prevent via breakage. These changes have a negative effect on the density of the pad-to-track arrangement and reduce the number of through hole pads that can be mounted. However, these modifications are necessary to improve the quality of a flex circuit. They also make the process of assembling and soldering a flex circuit much easier. They also help to maintain the integrity of the copper traces and ensure that they will not break or detach under repeated stress.
