Manufacturing Rigid-Flex Circuits
Rigid-flex PCBs are increasingly popular in applications like wearables and foldable devices where mechanical reliability is important. As these types of PCBs become more common, however, designers need to understand the specific challenges that they face when designing and building rigid-flex circuits. These issues can range from simple aesthetic defects to catastrophic delamination. Whether they’re caused by design errors or manufacturing mistakes, these quality issues can impact the function of rigid-flex PCBs and lead to expensive repair costs.
rigid flex circuits assembly requires unique considerations that differ from the standard conveyor belt-style processes used for rigid PCBs. Rather than the standardized dimensions of rigid boards, which can be assembled with high speed and accuracy using conveyor-belt-style manufacturing, rigid-flex circuits must be assembled by hand with care to ensure that they’re built correctly and will function as intended.
This means that designers must be aware of the challenges involved in creating these types of circuits, and work closely with their fabricators to avoid costly mistakes during production. To help, Sierra circuits has put together this blog highlighting the most common quality issues seen in rigid-flex circuits.
Challenges in Manufacturing Rigid-Flex Circuits
Insufficient hole-to-flex spacing can cause several problems with rigid-flex PCBs, including mechanical stress concentration, copper trace thinning in the bend radius, signal integrity concerns, solder joint reliability issues, and electrical shorts. The best way to address this issue is by ensuring that the vias on the rigid-to-flex transition area are spaced at least 50 mil from each other and are not aligned with the underlying layer. This will allow for the insertion of stiffeners and other structural components to provide extra support for these regions during flexing.
Stacked traces in the flex or flexible section of a rigid-flex PCB can increase stress on these parts when they are bent, potentially leading to thinning of the copper trace at the bend radius. To prevent this, designers should use a staggered routing pattern and avoid overlapping traces in the flex and/or bendable sections of the board.
Maintaining robust connectivity between rigid and flex sections of a rigid-flex board is vital for ensuring that signals are transmitted properly in both areas. This can be a challenge when working with these types of circuits due to the varying CTE expansion rates of rigid and flex materials. By ensuring that the rigid-flex sections are fabricated from materials with similar CTE expansion rates, this can be mitigated.
In addition, designers should make sure that the transition between the rigid and flex areas is made from materials with good thermal properties. This can be achieved by incorporating a thermal pad between the rigid and flex sections of the circuit or by using heat-sinks. Lastly, it’s crucial to consider how the rigid-flex circuit will be handled in the real world during operation and assembly. This will require careful planning of signal routing and grounding techniques to avoid crosstalk or electromagnetic interference.
