When designing power modules with SMD components, what is the preferred approach for heat dissipation: placing a heatsink on the top (directly on the component) or on the bottom side of the PCB? Given that many SMD devices dissipate heat through the bottom pad, how do you decide the better approach? Also, how much do factors like PCB thickness and copper weight influence this choice?
1 Like
A widely used approach is to combine a heatsink with a thermal gap pad, applied either on the top or bottom side depending on the layout. Top-side cooling becomes tricky when component heights vary across the board, since achieving consistent heatsink contact is difficult. In those cases, bottom-side cooling with thermal vias tends to be more practical, especially for SMD devices that dissipate heat through the bottom pad.
If thermal vias are used, filling them with solder or epoxy improves heat transfer significantly, though this depends on your fab and assembly capabilities.
PCB thickness and copper weight also play a role. Thinner boards with heavier copper (2 oz or more) spread heat laterally more effectively, which can reduce reliance on a heatsink altogether for moderate power levels.
1 Like
It’s also worth considering that internal ground or power planes often act as the primary heat spreaders, especially when thermally connected through bottom pads and vias. This makes the PCB stackup itself a key part of the thermal path, not just a substrate. Top-side heatsinking is also heavily dependent on package construction. Plastic packages typically have high thermal resistance toward the top, meaning most heat is still conducted downward into the board regardless. Unless the package is specifically designed for top-side heat extraction, relying on the PCB planes and bottom-side cooling is generally the more effective approach.
1 Like
One practical way to choose is to compare the device’s junction-to-board and junction-to-case/top thermal resistance values in the datasheet. Those numbers show whether the package is intended to dump heat mainly into the PCB or through the top surface. If the junction-to-board path is much lower, invest in copper area, planes, and thermal vias. If the top path is competitive, a top heatsink can be worthwhile. In short, let the package thermal model drive the cooling strategy rather than using one rule for all parts.
1 Like
Keep the heatsink close to the hot part, use thermal paste for good contact, and make sure air can flow through it easily for better cooling 
2 Likes
Also consider airflow and enclosure layout. A heatsink only works as well as its path to ambient. Sometimes a bottom-side sink tied to the chassis outperforms a top-side sink in stagnant air. Evaluate the full thermal path, not just heatsink location.
1 Like
When power dissipation is significant, prototyping and measuring is often more reliable than theory alone. Real-world variables are hard to fully model upfront, so a quick IR scan on both cooling configurations gives a clearer, faster answer than trying to calculate everything analytically.
1 Like