Compensating for lack of via-in-pad for buck converter thermal design

Question

I am designing a buck converter, based on a TI chip. Both the datasheet and an excellent document "Five steps to a great PCB layout for a step-down converter" show vias in the exposed pad, under the IC.

The application note states:

Starting with ground, vias are best placed directly under the IC so that the exposed thermal pad conducts its heat down into the PCB layers. This is required to achieve the IC’s best thermal performance.

And the example layout looks like:

Via-in-pad is an expensive service from most PCB manufacturers, especially at low volume. The design I hope to achieve will occasionally (but not continuously) run at the top of the converters capability at 5V / 8A (40W).

How important are these vias for thermal performance, and are there any ways to compensate for the lack of via-in-pad?

Answer 1

I designed this board with four vias, which was the closest I could get within JLCPCB's design rules. This is a four layer board with an inner ground layer.

The important things I learned were:

1. It's possible to have vias in pads - as per comments from earl and Matt.
2. Poured planes help to radiate heat.

The final design looked like:

I could not directly compare with / without vias due to cost, but I did some thermal testing to check maximum temperature rise at ~55% load. I tested with a 50W 2R2 resistor as a dummy load, drawing approx. 2.2A / 11W at 5V. Every 15 minutes I checked temperatures using a thermal camera.

The top copper zone for power output is clearly visible in the thermal photo, with the cut out underneath the inductor. The hotspot on the right is the controller chip (only 3mm x 3mm) and the hotspot on the left is the output connector.

I also logged temperatures for an hour. The controller chip peaked at 37°C (about 18°C above ambient). This is comfortably within the maximum ratings of the parts involved, and shows that closely following the recommended design is a sensible way forward.