Different from functional pads to establish electrical connections, non-functional pads are not connected to a trace on the layer they are on. They are typically used on inner layers of PCBs and are not necessary for electrical functionality.
Figure 1: Functional pad vs Non-functional pad
Why keep non-functional pads?
(Preventing over-expansion) When a PCB is heated, the Coefficient of Thermal Expansion stresses can cause the board to warp or delaminate. Non-functional pads can help to prevent this by providing a physical barrier that prevents the board from expanding too much.
(Strengthening copper adhesion) In addition to preventing warping, non-functional pads can also help to strengthen the copper adhesion on hole walls. When copper is plated through a hole in a PCB, the plating can be weak at the hole walls. NFPs can help to strengthen the plating by providing additional metal attachment points. This can help to prevent the plating from cracking or delaminating.
(Balance copper weight) Finally, non-functional pads can also be used to balance the copper weight in a PCB. PCBs often have copper planes or traces on multiple layers to carry high currents or provide specific signal paths. However, the copper weight on different layers of a PCB can vary. NFPs can be strategically placed around these copper planes to balance the copper weight distribution, preventing the PCB from warping or twisting during manufacturing.
Why remove non-functional pads?
(Routing density) Non-functional pads can pose challenges when designing boards with a high level of internal routing. Their presence reduces the available space for trace routing, making it more difficult to find suitable paths for the traces. This is especially true when vias are spaced very close together. In these cases, it may be necessary to open up a channel between the vias to accommodate a thin trace. However, it is important to note that while eliminating non-functional pads is an effective way to reclaim valuable real estate on PCBs, this practice should be approached with caution when dealing with flex and rigid-flex printed circuit boards. Copper plating does not bond strongly to flexible materials as well as it does to rigid materials. If all non-functional pads are removed causing the gap between functional pads becomes too great, the plating may start to separate from the hole wall.
(Signal integrity) Another reason for removing non-functional pads is that they can cause telegraphing. Telegraphing is a phenomenon where the copper stack-up of a via is visible on the surface of the PCB. This can happen when there are nonfunctional pads on the via, as the copper can spread out and create a raised area on the surface of the PCB. Telegraphing can cause problems with signal integrity, as it can create parasitic capacitance and inductance. For example, if a signal is traveling through a trace that is next to a nonfunctional pad with telegraphing, the telegraphed signal can couple with the signal in the trace and cause signal reflections. This can lead to errors in the signal, which can affect the performance of the circuit.
