Via in Pad Work
A via in pad is a way to increase component density on a PCB by placing the pads on the surface of the board on which the component sits and adding a via directly underneath the component pad. It works by creating a new path for signal to go to the power or ground planes of the board instead of being routed around the components using dog bone fan outs and shorting paths. This helps minimize parasitic inductance which is a problem for high frequency nets.
Via in pads can also help with heat dissipation as they allow for the via to be plated to copper on both sides making it electrically and thermally conductive. This can be especially important in high power applications or when it is important that the PCB can withstand temperature stresses from the component.
The process used to make a via in pad is the same as for traditional vias except that when the drill hole for the via in the pad is made it must be capped and filled with epoxy. This prevents solder paste from wicking into the via barrel and creating a poor connection between the pad and the component. The epoxy used to fill the via is usually non-conductive but conductive epoxies are available for applications that need better heat dissipation.
How Does a Via in Pad Work?
When a via in pad is used on a PCB the pad on which the via is located will be larger to accommodate the extra metal. This makes it difficult to tent the via with soldermask but it is possible if a smaller pad is used on the component. In most cases a smaller pad is preferred because it will not impose as much stress on the component connections.
There are a number of ways to implement via in pads and the choice depends on the specific requirements of a particular PCB design. The primary benefit is that the via can be placed directly on the component pad which opens up a lot of routing possibilities not possible with dog bone fan outs or other methods. This can simplify complex BGA layouts and in many cases reduce the amount of surface routing needed for signals to get from the chip to the power or ground planes.
The main disadvantage is that the additional steps required to make a via in pad can add to the manufacturing cost of the PCB. This is because an extra manufacturing file is required that calls out which vias need to be capped and filled. Having two separate files can avoid confusion during the fab process and may save on the cost of a second drilling operation if a mistake is made. A good PCB design software should have a feature to create this extra manufacturing file. If you are considering via in pads for a project please consult with your PCB designer to find out how the technology can be implemented and what additional costs might be involved.
