Via placement for wearable/IoT flex circuitry is a bit tricky. There are certain rules you must follow. First and foremost, the number of vias has to be limited simply because space is limited. If too many vias are placed, you’re likely blocking critical space for routing traces.
Vias go through a lot of fatigue due to bends and curvature. The more number of layers, the more it becomes difficult for vias to maintain their integrity because they must maintain their adherence with its multiple layers. Let’s take for example a 6-layer flex board. We need to make sure each layer adheres to all others. This means it’s adhering to all 6 layers, internally while the circuit is stationary, as well as when it’s flexing and bending.
It’s important to place the vias appropriately, keeping in mind the flexing movement of the circuitry. Fig. 1 shows via placement so that the vias are kept away from the bend area of a flex circuit.
In terms of spacing, it’s recommended that you keep a 20-mil clearance between the other vias that are being placed on the same board, as well as 20-mils from one of the vias to the edge of the board.
When placing the vias, use certain areas you can define on the PCB layout to assure that these are the areas where the circuit isn’t going to be bent. Or if it’s going to be bent, it has to be very minimal.
A more comprehensive understanding of wearable and IoT flex circuit considerations can be found in our recent article.
In the meantime, here are the tips and hints to follow:
- Clearly define via locations
- Make sure vias are placed there as much as possible
- If you can avoid placing vias that’s best, but usually that’s not possible
- If you can’t avoid placing vias, then minimize the number of vias
- But keep in mind where twist and bend areas are in the flex circuit
- Then place the vias in sections where the flex circuit doesn’t bend much or none at all
If it’s a rigid-flex board, then you can place a minimum number of vias in the flex circuitry and try to keep most or all the vias in the rigid section. Also, when you’re placing the vias you have to make sure. A typical via size that is used for flex circuit is 5-mils but depending on the application and aspect ratio, different via sizes could be used.
The rule of thumb is to keep the vias in the proper place so that they are doing their main function and that is to carry the current. But at the same time, the flex circuitry must have the integrity to keep a connection and be able to withhold the fatigue of bending.