What is a Via-in-Pad?
As electronic products become increasingly miniaturized, multi-functional, and highly integrated, PCBs need to accommodate higher routing densities. To meet this demand, via-in-pad design is commonly applied. Via-in-pad design refers to placing a via directly under a surface-mount component pad, instead of routing the trace around the pad. The via connects the component pad on the top layer to the internal layers of the PCB.
Figure 1: Via-in-Pad
Via-in-Pad Filling
In the via-in-pad process, the vias are typically filled with a non-conductive epoxy, capped, and then plated over, to avoid short circuits or empty soldering caused by tin leakage. This method ensures the vias are completely filled, thereby reducing the risk of voids and improving plating consistency. Note that this process is different from "via tenting," which uses the solder mask to cover the via and prevent solder from flowing into it during assembly.
Figure 2: Via-in-Pad Filling
Designing for Via-in-Pad
To design a successful via-in-pad on a PCB, it’s important to consider four key factors. These include choosing the right via size to prevent excessive resistance or unnecessary capacitance, determining the proper via placement to prevent signal noise and reflection, considering the appropriate size of the annular ring to maintain the via's electrical and thermal properties, and designing the solder mask to prevent solder from wicking into the via, which can cause issues like solder voids or solder balls.
When to Use Via-in-Pad in Design?
Is via in-pad design suitable for every PCB layout? When using a via-in-pad for your design, it's important to consider whether the following conditions are required for your specific design.
Increased Density
When you need to fit a large number of components and traces in a small area, via-in-pad design can help you save PCB space by reducing the number of vias on the surface of the board.
Improved Thermal Performance
Via-in-pad design can help transfer heat away from heat-generating components, such as power devices, by providing a direct thermal path to a copper plane.
Improved Electrical Performance
If you need to improve the electrical performance of your circuit, via-in-pad can provide a low-impedance connection between the component and the inner layers of the PCB.
High-frequency Applications
For high-frequency applications, via-in-pad can be useful for controlling the impedance of the connection and improving signal integrity.
Via-in-pad can also have some drawbacks, such as increased manufacturing complexity, higher cost, and potential thermal stress on components. Overall, the suitability of via-in-pad design depends on the specific requirements and constraints of your design.
