Latest / Hardware Startups with Fexingo: Devices, Robotics, and Manufacturing Tech Companies / How Hardware Startups Use Thermal Interface Materials
Transcript
- Lucas: Let's talk about something that doesn't get nearly enough attention in hardware startups: thermal interface materials. Or TIM, as the engineers call it. Luna: Thermal paste, right? The goo you put between a chip and a heat sink. Lucas: Exactly. But TIM covers a lot more than just paste. There are gap pads, phase-change materials, even thermal epoxies. And for a hardware startup, choosing the wrong one can mean field failures that are incredibly expensive to fix. Luna: I remember a drone startup I followed a few years back. They had a compact flight controller that kept overheating in hover tests. The fix wasn't a bigger fan — it was swapping out a cheap thermal pad for a proper thermal grease. Lucas: That's a perfect example. The pad they were using had a thermal conductivity of maybe 1 watt per meter-Kelvin. The grease they switched to was around 4 watts per meter-Kelvin. That's a fourfold improvement in the material's ability to move heat from the chip to the heat sink. Luna: And the cost difference? Probably pennies per unit. Lucas: Right. But the learning curve is real. Startups often grab whatever thermal pad is cheap and easy to source from a distributor, without understanding the junction temperature of their critical components. They might see a datasheet that says 'thermal conductivity: 3 W per mK' and assume it's fine. But that number is measured under ideal pressure and surface flatness — real-world performance can be half that. Luna: So what should a founder actually look for? Thermal conductivity is one number, but you mentioned thermal impedance earlier. Lucas: Thermal impedance is the more practical metric. It accounts for the thickness of the material and the contact resistance at both interfaces. It's measured in degrees Celsius per watt — so for a given power dissipation, you can calculate the temperature rise across the TIM. A good rule of thumb: you want impedance below 1 degree Celsius per watt for most consumer electronics, and below 0.5 for high-power applications like motor drivers or LED arrays. Luna: And the type of TIM matters a lot, too. Phase-change materials are interesting — they soften at operating temperature and fill microscopic gaps better than a solid pad. Lucas: Phase-change TIMs are great for applications where you have a large die and a flat heat sink, like in a GPU or a power module. They start as a solid film at room temperature, so they're easy to handle in manufacturing. Once the device heats up, they melt and flow into the surface roughness, which gives you very low thermal resistance. The downside: they can be messy if the device cycles on and off frequently, and they can pump out over time. Luna: Pump out — that's when the material migrates away from the interface due to thermal expansion and contraction, right? Lucas: Exactly. It's a common failure mode in thermal greases, especially if the clamping pressure isn't uniform. The grease gradually squeezes out, leaving a dry gap. That's why some startups prefer gap pads — they're less prone to pump out and they're reworkable, which is huge during prototyping. You can peel off a pad and try a different thickness without cleaning up a mess. Luna: But gap pads have higher thermal impedance because they're thicker. It's a trade-off. Lucas: Right. A typical gap pad might be 1 to 3 millimeters thick, with a thermal conductivity of 3 to 5 W per mK. Compare that to a thermal grease that's only 50 to 100 microns thick after application, with conductivity of 4 to 8 W per mK. The grease wins on raw thermal performance, but the pad wins on ease of assembly and reliability over many thermal cycles. Luna: So which one should a first-time hardware founder choose? Say they're building a smart home hub with a moderate-power processor. Lucas: I'd start with a high-performance gap pad from a reputable manufacturer like Bergquist or Laird. It's forgiving, it's reusable during testing, and it gives you a solid baseline. If thermal testing shows the junction temperature is too high, then you can experiment with a phase-change material or a grease. But don't over-engineer from the start — most thermal failures in startups come from not having any TIM at all, or from using a random pad that's too thick or too low conductivity. Luna: What about application methods? I've seen startups hand-apply thermal paste with a stencil, but that seems inconsistent. Lucas: Hand application is fine for prototypes — you can use a small syringe and a plastic spreader. But for production, you want either screen printing or automated dispensing. Screen printing gives you a very consistent layer thickness, typically 50 to 100 microns. Automated dispensing uses a robot to put a precise dot or line pattern, which is great for irregular surfaces. Both methods cost a bit upfront — maybe $1,000 to $5,000 for the tooling — but they prevent the biggest headache: air voids. Luna: Air voids kill thermal performance. Even a 10 percent void can increase thermal resistance by 20 percent or more. Lucas: Exactly. And that's the kind of thing that shows up in field returns months later, when the device has been through dozens of thermal cycles and the void has grown. A startup can lose its entire first production run to warranty claims if they don't control the TIM application process. Luna: Which brings us back to testing. How do you verify that your TIM is working as expected in production? Lucas: The standard method is to measure the case temperature of the component under a known power load using a thermocouple or an infrared camera. You can calculate the junction temperature from the case temperature and the thermal resistance of the package. If the number is higher than your design spec, you've got a TIM problem. A more advanced technique is to use a thermal test vehicle — a dummy chip with embedded temperature sensors — to characterize the TIM's performance before you commit to a design. Luna: That sounds like something a lot of startups skip because they're in a rush to ship. Lucas: They do skip it. And then they end up with a product that passes initial QA but fails in the field six months later. I've seen it happen with a smart lock startup — their motor driver was overheating because the thermal pad they used was too thick and had poor conformability. The fix was a 30-cent phase-change pad that brought the junction temperature down by 15 degrees Celsius. Luna: Thirty cents per unit to save a product. That's the kind of detail that separates a successful hardware launch from a recall. Lucas: And it's exactly the kind of detail we dig into on this show. If you find these conversations useful for what you're building or running, that's the whole point. We deliberately don't run ads on Hardware Startups — no sponsors, no interruption. If you want to support that choice and help us keep going, the link is buy me a coffee dot com slash fexingo. Luna: Yeah, it's a small way to keep this ad-free and focused on what matters for builders like you. Lucas: Alright, back to TIM. One more thing I want to mention: the reliability testing. If you're using a thermal grease, you should run what's called a 'power cycling' test — turn the device on and off repeatedly to see if the grease pumps out. For phase-change materials, a 'temperature cycling' test — from minus 40 to plus 85 degrees Celsius — will tell you if the material degrades. Luna: And if you're using a gap pad, what's the main risk? Lucas: Compression set. Over time, the pad can lose its elasticity and stop conforming to the surfaces, creating an air gap. That's why the datasheet will specify a 'compression ratio' — you want to design your clamping force so that the pad is compressed by 10 to 30 percent. Too little and you get high thermal resistance; too much and you risk cracking the component or the PCB. Luna: So you really need to involve the mechanical engineer early in the process. Lucas: Absolutely. Thermal management isn't an afterthought — it's a design parameter from day one. And the TIM is the cheapest and most impactful component in that system. A 20-cent piece of material can make or break your product's reliability. Luna: Good advice. I think a lot of founders will appreciate knowing that they don't need to reinvent the wheel — just pick the right TIM and test it properly. Lucas: Exactly. Next time we'll look at how to design a heat sink for additive manufacturing, which is a whole other set of trade-offs. Luna: Looking forward to it.