limitations to the design of flex pcb design
The design of a flexible circuit board can be tricky, but not impossible. It can be created in the same way as a standard PCB, and most of the same IPC rules apply. The main limitations are the materials used and the maximum bend radius that is acceptable. Also, the number of layers can limit the flexing of the circuit.
The minimum bending radius must be determined for each flex section of the flex pcb design, based on the thickness and material properties of the copper and substrate. The manufacturer’s data sheets and guidelines should be consulted for this information. Conductors should not be stacked on top of each other in a flex area, as this can create stress points in the copper that could eventually crack the circuit. Also, sharp angles in a flex area cause strain, which can damage the circuit board and shorten its lifespan. Therefore, large curved angles are preferred.
It is important to understand the difference between plated vias and blind vias in a flex circuit board. Plated vias are metalized through holes that connect the conductive layers of a circuit board, while blind vias only extend through the surface of the copper and do not connect the internal layers. The use of blind vias can help prevent copper cracking, which is common in high-repetition flex circuit designs.
Are there any limitations to the design of flex pcb design?
Another issue with flex circuits is the tendency of the adhesive to squeeze out around pads after lamination. This can occur if the pad land and access opening are not sized correctly to allow for adequate solder wetting. Using the correct pad size and a larger access hole will minimize this problem.
Routing traces near the flex areas of a circuit is also challenging. This is because the flex material can move and compress more during manufacturing, so it is important to leave enough space between the drill-to-copper distance and the end of a trace. Leaving an 8-mil margin will ensure that the flex material is not damaged.
In addition, slits and cutouts in the flex area can increase stress and lead to failure in some applications. To reduce this, it is best to add tabs and anchors to these slits and cutouts to assist in stopping peeling. It is also good practice to stagger traces in a multi-layered flex circuit to avoid stress points and ensure that they are running perpendicular to the neutral bend axis. This will also decrease the chances of l-beaming, which can harm the copper and cause stress fractures. Also, reducing the width of the traces to taper down from thick to thin can decrease stress and reduce the possibility of damage.
