Thermal interface materials enabling new applications
Stephen LawProduction / Materials Contract Manufacturing thermal
While there are obviously similarities, different applications have different key requirements of thermal interface materials (TIM), according to a market report from IDTechEx. In all cases, the TIM needs to minimize stress, prevent electro-migration of its fillers and maximize adhesion. IDTechEx’ report explores in detail the relationship between the form factors and characteristics of different commercial product offerings and their applications, focused on new opportunities and gaps in the market.
With multi-app usage, consumer devices can get 50% hotter than can reliably be cooled. Low power cannot support the desired usage. With small form factors, there is less natural convection. For light devices, lower density materials are desired but tend to have lower conductivity. In thin devices, heat from the CPU can distort the screen, so heat spreaders must be used. Metal heat spreaders fatigue under bending, but graphite heat spreaders have been used up to 2000 cycles.
These devices are including more and more functionalities. Wireless charging is very inefficient and produces a lot of heat. 3D cameras need a fixed distance between the IR projector and sensor, so thermal expansion must be very small. 4K will only bring high data rates, and cannot be throttled without a very noticeable reduction in quality.
For two-in-one devices (laptop-tablet hybrids) the thermal management can be changed depending on the mode, and retractable heat spreaders can be used.
In the consumer markets, the design cycle is short, product life is short, products are high volume and low cost. These markets will not spend money on TIM research, but use materials developed in other sectors.
Highly conductive materials are typically not flexible, and desired shapes of wearables are complex.
Thinner form factors also mean that skin temperature is a key concern. First degree burns occur at 43°C, but the user is uncomfortable far below that, as low as 38°C when the device is touching their face.
Surface area to dissipate heat is also very small. In a watch or bracelet, the stack sequence can be used to make sure the heat is dissipated on the world-facing side, not the hand-facing side. Components are spread out on single-sided boards. Movement does give an additional 20-30% of forced convection.
Many forms of renewable energy have time-dependent thermal loads. Solar and wind power both have 24-hour temperature cycles, while electric vehicles are heated over a matter of minutes. All the packaging materials must be able to handle cycling changes in ambient temperature.
Electrification of vehicles
IGBT are used in high speed trains and electric vehicles. They have a junction temperature of 175°C, which will rise to 200°C for the next generation, so are liquid cooled. The cost of thermal management in vehicles is too high to be sustainable and it is slowing the rate of electrification. In avionics, parts have a 20 year lifetime with 2000-3000 service hours. Deutsche Barn are driving IGBT development, demanding 30 year lifetimes (13 years is currently the norm). In geothermal exploration, these electronics must run at ambient temperatures up to 300°C and in highly corrosive environments.
In a white LED, only 50% of the input electrical power is emitted a light. 90% of the heat emitted is conducted away through the solid. LEDs are more sensitive to temperature than standard solid-state electronic components so more attention must be paid to the thermal architecture.
Often, if a material with a lower thermal conductivity is used due to cost or manufacturing reasons, the design will demand either shortening the length of the thermal path or increasing the heat transfer area.
Despite the lighting industry’s ability to provide more lumens per watt than ever before, an LED’s reliability and performance depend greatly on how it is packaged and mounted. The main drivers of technology development in this area are directly related to increasing lumens per watt while decreasing overall costs.
Performance is the number one requirement for thermal interface materials in data centres.
Facebook decommissions every server after three years. They run them fast and hot to save cooling power while maximizing computing power, but then must change them often. This also means they can take advantage of new microprocessors.
The voltages at which GaN can be operated is limited because the heat that can be dissipated from them is capped at 1kW/m2. For high performance embedded computing, the chips are standard, but the packaging is the limiting factor and differentiates. Packaging should be lightweight, reliable, CTE matched, hermetic, stable against corrosion, cheap, and route the power and coolant.
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