Via in Pad

      Via in pad is the design practice of placing a via in the copper landing pad of a component. Compared to standard PCB via routing, via in pad allows a design to use smaller component pitch sizes and further reduce the PCBs overall size. With component manufactures pushing smaller parts every year and the demand from consumers for smaller devices, the usage of via in pad practices by hardware engineers have become more commonplace.

      With traditional vias, the signal is routed away from the pad and then to the via. This allows the application of soldermask to prevent solder paste from wicking into the via during the reflow process. In doing so, this prevents a situation which can cause the pad and component connection to fail due to insufficient solder. With via in pad, the drill hole for the via is inside the pad. This means you can not tent the via with soldermask. Leaving the via open to allow the paste to wick into the barrel will cause reliability problems during manufacturing.

      To reduce manufacturing defects and increase yield, it is best to implement via in pad by having your PCB manufacturer cap the via. This is accomplished by first filling the via with a conductive epoxy and then plating over with copper. Most contract PCB assembly manufacturers will not allow via in pad without first filling and then plating over the via.

      When should I use Via in Pad? Trying to route and escape component packages with sub 0.5mm pitch with traditional routing methods will cause Design Rule Check errors due to trace width, annular ring, and drill size limitations. Manufacturing limitations on trace size does not allow for smaller and more compact traces to be made with traditional PCB fabrication methods. For these small pitch components, the only effective way to route them is with capped via in pad as this allows the PCB routing to be as compact as possible.Capped via in pad can simplify routing for complex BGA and LGA packages as well. Instead of the traditional fan out, the signals can go straight into the PCB. Surface routing is minimized which can allow components like bypass capacitors to be placed as close as possible to other components, minimizing parasitic inductance. Also, this enables shorter paths to power and ground planes which will help minimize EMF emissions of high-frequency designs.Vias in thermal pads can also help with heat management. High power surface mount parts typically have a thermal pad that mounts to the PCB. To dissipate the heat more efficiently, it is wise to drop vias through the PCB to the other side of the PCB to increase the copper area for heat release.

      Why not use via in pad for every via then? The advantage gained in reducing component fan out and routing complexity is offset by increased manufacturing complexity and material costs of the PCB fabrication process. If the hardware design can be accomplished with traditional style via routing then it is best to avoid via in pad.