Coordinating QFN Thermal Vias with Solder-Paste Design

The exposed pad beneath a QFN is a shared design problem for thermal performance, electrical connection, PCB fabrication, and soldering. Adding more vias without reviewing the paste pattern can create a different assembly result from the one assumed in the thermal model. Treat the land, holes, surface treatment, and stencil as one coordinated detail.

Start from the exact package

TI’s array-QFN guidance explains that thermal vias connect the top pad to other copper layers and discusses solder loss into those vias. It also describes filling, plugging, and tenting as process choices with tradeoffs. The dimensions and recommendations in that guide apply to its illustrated designs; they are not universal values for every QFN. TI: array-QFN design guidance.

Collect the component’s recommended land pattern, exposed-pad electrical function, power dissipation assumptions, and assembly notes. Check whether the thermal calculation assumes a particular copper area or via arrangement. If the pad is also an electrical connection, verify that thermal-relief settings or plane clearances have not changed the intended connection.

Make the via treatment unambiguous

Describe whether holes are open, tented, plugged, or filled and capped, using terminology agreed with the fabricator. State which side is treated and whether a planar solderable surface is required. These treatments are not equivalent. A drawing that says only “via-in-pad” leaves the assembler to guess what surface will arrive.

Then overlay the proposed paste apertures on the finished pad geometry. Look for apertures placed directly over open holes and for large uninterrupted paste regions. Ask the assembler to review deposit volume and gas escape for the selected paste and stencil process. Avoid applying a fixed percentage reduction from an unrelated package as if it were an acceptance standard.

Run a focused first-build review

1. Confirm the fabricated via treatment before printing paste.
2. Inspect representative paste deposits for the exposed-pad pattern.
3. Record the assembly setup and reflow profile used for the trial.
4. Evaluate solder-joint evidence with an inspection method suited to the hidden pad.
5. Compare thermal and electrical performance under representative operating conditions.

For example, a board may show acceptable perimeter joints while the hidden pad has inconsistent coverage. Increasing the total paste volume is only one possible change and may introduce other problems. First compare the actual hole treatment, stencil pattern, and process conditions with the approved design.

Preserve what was qualified

Keep the accepted pad detail, via process, stencil revision, and inspection criteria together. If the assembly supplier changes paste or the fabricator changes the via process, determine whether the first-build evidence still applies. A thermal simulation and an X-ray image answer different questions; neither alone proves the complete assembly meets its electrical, thermal, and reliability objectives.