Via-in-Pad: The Space Saver That Quietly Drains Your Solder Joints

Vias placed directly on SMD pads are a design choice that can lead to weaker solder joints. The issue lies in the behavior of the hole after plating; during the reflow process, solder flows into the via and runs down its barrel, resulting in a joint that has less solder than originally intended. On larger pads, this creates a weaker connection, while on smaller components or fine-pitch leads, the joint may be entirely absent, leading to intermittent connections that are difficult to detect.

This phenomenon can cause dog-bone fan-out structures, via-in-pad configurations, and QFN thermal vias to become problematic. In some cases, it can also lead to a specific failure known as tombstoning, where small parts stand upright on pads containing a single via, while those without a via lie flat. The decision to use via-in-pad depends on several factors, including the availability of space for a via alongside the pad on a short trace, the presence of thermal pads under QFN or power packages, and the need for fine-pitch BGAs.

However, small passive components or ordinary fine-pitch leads do not require an open via. The impact of via-in-pad extends to manufacturing processes, as filled and capped vias, often referred to as VIPPO (Via-in-Pad Plated Over), must be specified to the fab to ensure proper joint formation. While via-in-pad isn't a layout trick, it is a manufacturing decision that requires careful consideration and communication with the fabrication partner.