In PCB design, the topic of "placing vias on solder pads" is always discussed by beginners and experienced engineers alike. Today's question is:

Can vias be placed directly on solder pads? What are the consequences of this design?

Today, we'll explain it clearly with two diagrams and two principles!

01 | Theoretically Possible, but Not Recommended in Practice

Let's look at two basic points:

• First: Theoretically, placing vias on solder pads minimizes lead inductance, which is acceptable.

This seems to be the "optimal connection path" for some high-speed or high-frequency designs.

But the problem lies in the second point—

• Second: From a soldering process perspective, doing so can lead to serious consequences.

Especially if the vias are not properly filled, it can cause solder paste to leak into the holes, resulting in poor solder joints or component lifting.

02 | Common Problems with Vias on Solder Pads: Solder Leakage & Tombstone Effect

• Solder Leakage

Because the vias are not completely sealed, the solder paste flows away through the vias during reflow soldering, resulting in insufficient solder on the pad, ultimately leading to soldering failure or insufficient strength.

• Tombstone Effect

When the two ends of a surface-mount component are heated unevenly, and the solder paste on one side melts first due to leakage or uneven heat distribution, the component will "stand up" due to the unbalanced force.

This is especially common in surface-mount resistors and capacitors and is one of the typical defects in SMT assembly.

03 | Professional Terminology Explained: Lead Inductance and Tombstone Effect

• Lead Inductance

In high-frequency circuits, the wires themselves have inductive reactance, especially the "lead segment" between the via and the solder pad, which is more likely to form parasitic inductance, negatively impacting high-speed signals or power integrity.

Therefore, theoretically, shorter is better.

• Tombstone Effect

Also known as the "Manhattan effect," it is common in the soldering process of surface-mount components. Due to uneven force on both ends, one end of the component "lifts up," resulting in soldering failure.

04 | Recommended Practice: Pull the Via Away from the Solder Pad

Combining theory and practice, we recommend this design:

Pull the via away from the solder pad and connect it with a short trace. Advantages:

• Maintains low lead inductance
• Avoids manufacturing defects
• Improves soldering yield

Design is not just about drawing lines; it's a comprehensive art that considers signals, electrical characteristics, and manufacturing processes.
Don't underestimate the position of a via; it determines whether your design can be successfully assembled and manufactured!