As hardware engineers, we often need to solve thermal issues for high-power devices inside tight enclosures. Select a heatsink, choose a fan, prototype, test… If the cooling is insufficient, we have to redo the PCB layout, change solutions, and rerun the whole process. In my experience, many colleagues fall into common traps when evaluating heatsinks – especially ignoring the role of thermal adhesives. Below are three misconceptions I’ve encountered, together with practical advice.
Misconception 1: Only looking at heatsink thermal resistance, ignoring the adhesive layer
We’re used to calculating junction or case temperature using the manufacturer’s thermal resistance Rth:
Tc = Tamb + Q × Rth
Then we pick a heatsink with Rth less than or equal to the calculated value.
But the manufacturer measures Rth under ideal conditions: a standard heat source (e.g., 25.4 mm × 25.4 mm) is pressed directly against the heatsink base – no adhesive in between. In real products, MOSFETs, LEDs, power ICs, etc., must be attached to the heatsink with thermal adhesive – whether it’s thermal grease, a thermal pad, or a thermally conductive structural adhesive.
That adhesive layer adds extra thermal resistance:
R_adhesive = thickness / (thermal conductivity × area)
For example: thickness 0.1 mm, conductivity 1.5 W/m·K, area 100 mm² → R_adhesive ≈ 0.67 K/W. If the device dissipates 10 W, the adhesive alone contributes nearly 7 °C temperature rise. If the layer is too thick, contains voids, or shrinks during cure, the resistance can double or more.
Advice for hardware engineers:
Always include the adhesive thermal resistance in your total thermal budget. When back-calculating the required heatsink Rth from the device’s maximum junction temperature, subtract the adhesive layer resistance first. Prefer adhesives with high thermal conductivity (≥3 W/m·K), low thermal resistance, and a repeatable application process – e.g., thermal gels or phase-change materials. Control the bond line thickness carefully.
Misconception 2: Maximising heatsink surface area while ignoring whether the adhesive can get the heat in
I’ve often heard: “This heatsink has plenty of fins and surface area – it will be fine.” Then test results show the device is still too hot.
The root cause is: No matter how large the heatsink surface area is, if heat cannot efficiently transfer from the device to the heatsink base, all those fins are useless. The thermal adhesive is the “first mile” of the heat path.
Some hardware engineers, to save cost, use a low-conductivity double-sided tape, or apply thermal grease only as a few dots without covering the whole interface. This creates local hot spots, or even air gaps (air’s thermal conductivity is ~0.026 W/m·K – almost an insulator).
Moreover, the rheology of the adhesive affects its ability to fill gaps. High-viscosity adhesives can leave voids on rough surfaces. Adhesives that shrink during cure can warp the heatsink base, lifting it away from the device. I once used a certain structural thermal adhesive; after curing, one corner of the device lifted by 0.05 mm, doubling the measured thermal resistance.
Advice for hardware engineers:
Treat the adhesive as a component during the design phase. Based on the device package, allowable pressure, insulation requirements, etc., choose a thermal pad with suitable conductivity and hardness, or a well‑tuned thermal gel. If possible, use X‑ray or C‑SAM to inspect the adhesive layer for voids and coverage.
Misconception 3: Using the fan’s maximum airflow to estimate heatsink performance, ignoring the “amplifying effect” of the adhesive on airflow demand
Fan manufacturers quote “maximum airflow” at zero back pressure. In reality, the heatsink fins create flow resistance, so the actual airflow is much lower – everyone knows that. But what’s easily overlooked is: a high thermal resistance in the adhesive layer raises the heatsink base temperature, which then demands a higher airflow rate to compensate – often exceeding the original fan’s capability.
Example: device dissipates 20 W, target temperature 80 °C, ambient 40 °C.
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Ideal case (adhesive Rth = 0.2 K/W): allowable heatsink Rth = (80‑40)/20 – 0.2 = 1.8 K/W.
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If poor application increases adhesive Rth to 0.8 K/W: allowable heatsink Rth drops to 1.2 K/W.
To reduce heatsink Rth from 1.8 to 1.2 K/W, you typically need 30‑50% higher air velocity, or denser fins. Denser fins increase pressure drop, reducing actual fan airflow – a vicious cycle. At this point, many hardware engineers suspect the fan is underpowered and upgrade to a larger, noisier, more expensive fan – without realising the root cause is the adhesive layer.
Advice for hardware engineers:
When performing thermal simulation or hand calculation, list the complete heat path: device → adhesive layer → heatsink base → fins → air. If the simulation suggests a very high airflow velocity is needed to meet the target, first check whether the adhesive layer resistance is too large. Sometimes switching to an adhesive with a thermal conductivity just 1‑2 W/m·K higher is far more effective and less expensive than upgrading the fan.
Conclusion
As hardware engineers, we need to understand not only circuits but also mechanics and thermal management. Heatsink selection is never just about datasheet numbers and physical dimensions – the “small material” – thermal adhesive – often determines success or failure of the entire thermal design. Avoid the three misconceptions above, and incorporate the adhesive’s thermal characteristics, process reliability, and compatibility with the heatsink into your design flow. That’s how you get it right the first time – fewer late nights and fewer PCB revisions.
I hope this article helps fellow engineers struggling with thermal issues. Feel free to share your own “adhesive pitfalls” in the comments.
