thickness of a typical flexible PCB board
The overall thickness of a flex or rigid-flex printed circuit board (PCB) can vary from 12 mm for single layer constructions to 180 mm for multilayer boards. Thinner flex PCBs are more cost-effective and lighter in weight than rigid PCBs. However, there are design and manufacturing challenges that must be considered when designing a thin flex or rigid-flex PCB.
Generally speaking, thinner flexible pcb board are more vulnerable to mechanical damage than thicker ones. Flex PCB thicknesses of 50 mm (2 mil) and less can be reliably produced, but this is dependent on the design and fabrication capabilities used.
Ultra-thin flex PCBs require more precision in manufacturing to ensure that the drill holes are aligned properly with the copper traces. The extra care in drilling and alignment ensures that the circuit boards meet quality standards. In addition, flex materials are more prone to contraction and movement during manufacture than rigid PCBs, so the drill-to-copper distance should be at least 8 mil. The use of panel plating instead of pad-only-plating (button plating) is also crucial for ultra-thin flex PCBs. With button plating, copper is deposited only on pads and vias, rather than the entire etched pattern, which improves etch yields in tight traces and increases control of impedance at high speeds.
What is the thickness of a typical flexible PCB board?
For ultra-thin flex PCBs, the thickness of the base substrate is also important to consider. The most popular flex PCB base material is polyimide, which has excellent durability and thermal properties that allow for extremely thin flex boards. The base substrate is paired with an ultra-thin rolled annealed or electrolytic copper foil for conductors. A flexible dielectric coverlay is laminated over the conductors for insulation and protection.
The conductive pattern is then patterned using a subtractive process to create the desired connections (known as traces). A solder mask coats the surface of the traces for insulation and anti-oxidation. Finally, a hot air solder leveling (HASL) coating is applied to the exposed areas of the traces to prepare them for assembly.
With proper design, a robust flex or rigid-flex circuit can withstand millions of bending cycles over the product lifetime. However, critical design considerations are needed for aspects such as trace spacing and bend radius, adhesive bond strengths, stress propagation, tear resistance and structural backing support.
The final inspection of a flex or rigid-flex circuit is critical for ensuring that it meets design and production requirements. This includes a comprehensive electrical test that examines the circuit’s performance for short circuits and internal faults. The test results are compared to the target specification for the flex or rigid-flex circuit to verify that it is ready for the assembly line. It is also a good time to make sure that all the components and wires are attached and in place. This will ensure that the finished flex or rigid-flex circuit is free of flaws and is ready for shipment. The flex or rigid-flex circuit should also be inspected for signs of heat damage, which can occur due to excessive temperatures during the reflow soldering process.
