Thoughts on component placement when thermal management is the main concern?

I found that for your setup, leaning heavily on the copper pour for dissipation is prolly your best bet. When I was designing a small power supply for a remote camera rig, I noticed that simply widening the ground plane around the main switching chip did more heavy lifting than any thermal via configuration. It creates a genuien heat spreading effect across the entire board surface, which is exactly what you want when you are trying to keep a profile slim. My approach was to dedicate a large section of the bottom layer to a thick copper pour, connecting it directly to the ground symbol and the power input pads. I used two ounce copper since I was working with an FR4 board, and it made a noticible difference in temperture readings. The copper acts like a radiator right on the board's surface.

I reserve the thermal vias strictly for connecting those massive output traces to the ground plane under the main driver component itself, where the actual switching happens. Trying to rely only on vias when the heat is spreading across the whole component footprint seems insufficient for a BLDC driver. It's about both methods working together, but the copper pour does the heavy lifting for overall cooling.

I disagree with the idea of just going for massive copper pours. When I was integrating a driver for a small vacuum pump, I learned quickly that spreading heat across a broad surface when the surrounding enviornment is tight is often just moving the heat problem to the enclosure itself. My experiance showed that for localized hotspots like the switching chip on a BLDC driver, you need a direct path away from the component, not just a big reservoir on the board. Instead of relying on the ground plane to handle everything, I focused on creating a dense via array underneath the power component. Specifically, using a heavy copper weight like 2oz on the internal layers and punching a grid of vias directly into the internal planes allowed the heat to wick into the core of the board. This drastically lowered the junction temperature compared to just widening the traces.

I found this approach much more reliable when the board was sitting near plastic components, which act as major thermal insulators. For these small power modules, managing the thermal resistance with focused copper connections right at the source is simply a more effective strategy than passive spreading. It keeps the footprint minimal while keeping things cool.

I had a similar problem when designing a small industrial power module, but I wasn't really focused on the trace design initially. The hot spot wasn't where I thought it was. I discovered that increasing the physical thickness of the FR4 substrate made a bigger difference for my thermal issues than fiddling with copper widths. Just moving from the standard 0.5mm board to a slightly heavier 1.0mm version allowed the heat to spread more effectively into the core material itself. It acted like a bulk heat sink just by making the entire layer thicker. You might want to check if your supplier offers thicker base layers for those power zones.

I reckon folks are focusing way too much on the layers and the copper weight, wich is fair, but you gotta consider the actual trace width first. When I was designing a power supply for industrial pumps, I found that narrow power traces were generating more heat than the components themselves. You need to make sure those output paths are beefy enough for the peak current, or you're basicly building a resistive heater. It's funny, the track design often makes or breaks the whole thermal plan, even if the chip is cool. Have you checked the trace width calculation yet?

I think the point about using the copper pour for spreading heat is spot on. For a small profile build like that, you can't rely on surface area on a heatsink unless it sticks out. When I did a similar setup for a power module, the biggest thing I noticed wasn't the copper weight, but how the switching component sat relative to the ground. If the bottom pad of the main chip wasn't making a perfect, flush connection to that massive ground plane, all the heat transfer was basically wasted.

Update: the reply about trace width got me to actually measure my layout instead of guessing. Turns out the trace feeding the driver was skinnier than I assumed, so I widened it and pulled the via array right under the chip pad. Funny thing is the warm spot has now shifted over toward the motor pads, so I'm wondering if I just moved the bottleneck rather than fixing it.