safety considerations when using PCB rigid flex
Rigid flex PCBs can provide high-performance interconnects in rugged applications such as foldable wearables, medical devices and aerospace equipment. They can also help reduce package size and weight, increase reliability and simplify assembly. However, it is important to understand the safety considerations when using PCB rigid flex. Poor-quality rigid flex circuit boards can introduce dangerous safety hazards in critical applications and can lead to costly downtime and repair costs. By avoiding these common DFM mistakes when designing rigid flex PCBs, manufacturers and users can enjoy the benefits of a safe and reliable product.
The layer stack is a crucial factor in the mechanical stability and signal integrity of pcb rigid flex. Ensure that your design features an optimal copper distribution and dielectric materials to minimize signal coupling, control impedance, and prevent warpage. Use a minimum of four layers in your build-up, with symmetrical and balanced copper and dielectric distributions across flex sections. Moreover, avoid placing traces directly on top of each other in flex areas. Alternatively, you can use conductive vias in these areas instead. This will reduce the stress on flex copper and increase board strength.
Stacked traces in the flexible and bendable sections of a PCB can cause conductor stress points that can crack or break the copper in these areas, especially when the circuits are subjected to stress in the form of a harsh bending angle. This can also cause thermal issues in the rigid sections of the circuit, which will result in a reduced circuit lifespan. Avoid this by keeping all copper within the neutral axis and following proper bend guidelines.
What are the safety considerations when using PCB rigid flex?
Using heavy copper plating increases the amount of stress that can be put on the copper during a bending process. This can cause the material to fracture, weaken or break under the strain of repeated bending. To reduce this stress, designers should use thinner copper and utilise conductive vias to ensure the signal remains uninterrupted. They should also use curved traces in flex areas to minimise sharp corners.
A flexible PCB requires a large enough bend radius to ensure that the copper doesn’t short out due to the amount of stress applied to it. It is also necessary to keep the distance between sequential bends greater than twice the thickness of the copper being bent.
Another important consideration is the type of prepreg used in the rigid flex circuit’s fabrication process. Standard prepregs can cause air gaps that can create high-resistance shorts and increase the chances of failure in flex areas. To mitigate this issue, it is best to use low-flow prepregs and use adhesiveless cores in the flex section of the circuit board. This will ensure that the stiffeners in the flex section do not come into contact with other layers, and will reduce the risk of failure. A cut back coverlayer or bondply can also significantly decrease the likelihood of via failure by reducing the amount of acrylic resin that is exposed to Z-axis CTE expansion, which can put stress on plated through holes.