Flex Circuit PCBs
In a flex circuit PCB, copper plays an important role. It is the conductive material that carries electrical signals through the circuits on a flexible base material, such as polyimide. The base material allows the flex circuit pcb to fold and bend while maintaining its structural give.
The thickness of the copper layer plays a big part in the flexibility and conductivity of the circuit. It is also a factor in how much current the flex circuit can carry before overheating or causing other problems. The optimum copper layer thickness is based on the desired conductivity, current-carrying capacity, and thickness tolerances. Using thinner copper can increase the circuit’s flexibility, but it can limit its ability to carry higher currents and may not perform well in high-speed applications.
For this reason, flex circuits require heavy copper plating to enhance its thickness and performance. This process is usually performed using panel plating or pad-only-plating (button plating). Panel plating involves the deposition of copper on all exposed pads/vias and is used for lower-layer traces, power supply connections, and ground planes. Button plating, on the other hand, is a more cost-effective option for higher-layer signal traces. It also improves etch yields in small-etched patterns and helps ensure that conductor traces have consistent copper thickness, width, and spacing.
Copper in Flex Circuit PCBs
To maintain a high quality and reliability, flex circuits must be manufactured with copper that has good adhesion to the PI substrate and coverlay. In order to achieve this, the copper must have a lower z-axis expansion coefficient than the substrate and coverlay. This is especially important for through-holes, as they must withstand stress that could break the via or copper surface. To reduce the risk of broken through-holes, manufacturers should use a high-quality copper with a low-z-axis expansion coefficient, like FR-4 or HASL.
Copper is also prone to fatigue and work hardening during bending movements, which can lead to damage to the circuits. To minimize this issue, designers should stagger the placement of conductive components and conductors across layers. This can help spread the tension bias among different layers to prevent the copper from becoming fatigued or work-hardened during bending. It is also important to avoid laying overlapping traces on adjacent layers.
It is also important to make sure that the drill-to-copper distance on a flex circuit is at least 8 mil. This ensures that the drilled holes are not too close to each other, which can lead to shorting between the conductive pads and the copper surface. Additionally, tin finishing is recommended for the pads and access openings on flex circuits to promote better solder wetting and increased durability. Lastly, it is best to use a turnkey manufacturer that can provide both flex circuit manufacturing and assembly services to eliminate the need for multiple vendors. This can also help prevent miscommunication between the various teams and ensure that all requirements are met for a successful production. This will result in a high-quality, reliable product that is ready to be assembled into an end-product.
