Flexible Printed Circuits
There are many different ways to design a flexible printed circuit. There are also a number of fabrication and assembly options available that can give the end-user flexibility in how the circuit is assembled. With these unique properties a flex circuit allows electronic designers to create interconnect systems for a huge variety of applications that are otherwise not possible with rigid PCBs.
The minimum feature size achievable in a flexible printed circuit is determined by the material used and how the board is constructed. There are two basic materials for a flex circuit: FR-4 and polyimide. Polyimide is more expensive than FR-4 but it offers much more flexibility. For a flex circuit to perform properly it must be designed with the correct bend radius in mind. This is a critical factor that must be considered early in the design process and is defined by IPC-2223.
Single sided flex circuits are one of the most common types of flexible circuit boards. They consist of a substrate layer, a conductive metal layer and a protective solder mask layer. The conductive metal layer is generally bonded to the substrate layer but can be bonded between dielectric layers. This is called a laminated structure.
Minimum Feature Size Achievable in Flexible Printed Circuits
These single sided flex circuits can be either dynamic or stable flex boards. The difference between the two is that dynamic flex circuits are designed to be bent and twisted regularly during use while stable flex circuits can withstand static bending for long periods of time. Stable flex circuits are often used in printers and other devices that must be able to bend and twist without damage.
When designing a flexible circuit it is important to consider the number of layers in the board as well as the thickness and density. Typically, these circuits are built to be as thin as possible in order to handle dynamic flexing and still provide the desired performance. Typical flex circuits are made up of three or more copper layers but can have as few as two. The thickness of the circuit is usually limited by the availability of the raw copper. Thicker circuits are available but these are more costly and have a lower electrical conductivity.
The number of layers in a flex circuit can also affect the minimum feature size that can be achieved. The more layers in a flex circuit the more space is required for the conductors. This is why flex circuits cannot be as dense as their rigid counterparts.
When the number of layers is high it is necessary to make sure that vias are placed in areas where flexing will not occur. This can be done by creating “rooms” in the layout and allowing the placement of vias only in those areas. Another method is to utilize anchoring stubs and reduced coverlay openings to support the pads of SMT components in a flex circuit that will be subject to dynamic flexing.
Flex circuits are an ideal choice for replacing wire harnesses, connecting moving or rotating parts and providing connectivity in a variety of other applications. With the proper construction they can withstand dynamic flexing and allow designers to create a variety of electronics packages that are smaller, lighter and more functional than traditional rigid circuits.
