Via-In-Pad in Flex Circuit

In flex circuit design, determining how sharply a circuit can be bent without causing damage is paramount. Using IPC 2223 standards and an estimation formula that takes into account the maximum allowable copper elongation and material thicknesses, engineers can determine the minimum bend radius for their flex circuit.

While it may be tempting to use tighter bend radii, there are risks involved. Tighter radii can cause the material to fail under stress. The resulting failure can lead to shorts, cracking, and other defects in the PCB. To mitigate these risks, it is essential to follow the Dos and Don’ts of flex circuit design.

Using the right materials is crucial for a successful flex circuit project. The choice of materials will determine the strength and flexibility of the board, how easily it can be bent, and how much strain is put on the conductors during flexing. While there are many different options, it is generally recommended to use a FR-4 insulator with an ED or RA copper, which provide the highest level of flexibility and ductility. Similarly, the choice of adhesives is important to ensure that the insulator and copper bond well during assembly.

It is also a good idea to use a copper-to-copper ratio of 3:1 or higher, as this will help with the conductors’ impedance. Impedance refers to how much and how quickly electricity travels down the traces, and matching impedance is critical for high-speed signal transmission. Lastly, using a rigid substrate with an FR-4 or FR-5 layer will help the board maintain its shape during bending.

Via-In-Pad in Flex Circuit Design

With hardware becoming increasingly smaller, PCB construction methods must evolve to keep pace. One method that is gaining popularity among engineers is via in pad. While it is not a standard PCB fabrication technique, via in pad allows designers to reduce component fan out and increase density on flex and rigid-flex circuits.

Via in pad is accomplished by first filling the via with a non-conductive epoxy and then capping it before plating it. This process, called “via tenting,” helps eliminate voids and reduces manufacturing defects. Additionally, contract pcb assembly manufacturers can implement this process to reduce the number of holes needed on the component footprint, which increases overall PCB density.

Another aspect of via in pad is ensuring that the pads on the footprint are larger than the corresponding via, to avoid missing components and to prevent solder from flowing into the empty via hole during assembly. This can be done by modifying the component footprint to include an extra covering for the pads, or adding an opening in the mask layer that defines the coverlay area for the pads.

Finally, it is important to understand whether your flex circuit requires panel plating or pad-only plating (button plating). Panel plating uses a thin layer of copper that covers the entire pcb surface. Pad-only plating, on the other hand, only deposits copper on the pads and vias. This can help improve etch yields and pad sizes, as well as control the impedance of the conductors.