Feature: Thermal protection
with high thermal conductivity for a ceramic, is oſten used as a filler material. Termal conductivity is the heat flow through a material. Termal resistivity, a related property, is the heat flow across an interface. Bond thickness determines which property dominates the heat flow. If the bond line thickness is greater than 100 microns, thermal conductivity dominates. If the bond line thickness is less than 100 microns, thermal resistivity dominates. Termally-conductive silicones can
retain their flexibility even with high levels of filler content. Modulus, a measure of how easily a material can be stretched, is generally lower in silicones than in most other thermal management materials. A low modulus provides stress relief and protection against shock and vibration. Among their advantages, silicones
have low surface energy for good wetting properties. Tey also make reliable contact with uneven surfaces and have broad chemical resistance. Importantly, silicones provide strong hydrolytic stability, for resistance to moisture degradation. In addition, they are available in formulations without the toxicities of competitive non-silicone products, to help support sustainability initiatives and environmental health and safety.
Silicones vs other thermal management materials In terms of device reliability, silicones tend to outperform other thermal management materials as reliability requirements become more demanding. If reliability testing is performed too late in the device commercialisation process, problems with TIMs may require urgent replacement and lead to lost time that could have been prevented. If release testing is shortened, it may not reveal problems with TIMs that could severely affect performance later in a device’s life. In turn, this can affect warranty liability. Te benefits of thermally-conductive
silicones are evident in comparison to other thermal management materials. Specifically, silicones outperform in terms of hydrolytic stability, modulus and stress relief, bond line thickness and cure exotherm. Advanced silicone technologies
Silicones deliver reliable protection of sensitive components and systems
also address several challenges specific to thermal greases. Hydrolytic stability is critical for
today’s electronics because 5G will require more towers, base stations and transmitters that are exposed to outdoor weather conditions, including high humidity and rain. Silicones allow water vapour to pass through quickly; however, it is not water vapour but liquid water that causes corrosion. Over time, TIMs containing epoxies absorb more water than silicones, increasing the risk of corrosion. When factoring in modulus and its
relationship to stress, silicones are the clear choice for thermal management. Consider the example of a stack with a substrate, chip and additional layers of electronics. Each layer has a different coefficient of thermal expansion (CTE) and, therefore, expands and contracts at a different rate. With their low modulus, silicones can absorb some of the resulting stress and convert it to movement. By contrast, epoxies have a higher modulus and are rigid and monolithic. Epoxies are strong, but their mechanical strength does not relieve stress.
Over time, stresses can cause epoxies
to crack and the cracks to propagate. Conversely, silicones are soſter and more crack resistant. Tey also help to prevent stresses from reaching sensitive wire bonds, which are relatively easy to break. All thermal management materials are designed to be reliable, but the long-term reliability of silicones is what truly sets them apart. Further, high-performance silicones are
superior to other thermal management materials in terms of bond line thickness and cure exotherm. Curing, a chemical process that converts materials like adhesives from a liquid to a solid, can generate heat, which, in turn, affects adhesive performance and properties. Cure exotherms such as those typical with epoxies generate more heat when bond lines are thicker. Silicones have no appreciable cure exotherm, so bond line thickness does not pose any issues.
Adhesives Termally-conductive silicone adhesives eliminate the need for mechanical fasteners, for instance, when bonding heat sinks and sensors to PCBs. Tey
www.electronicsworld.co.uk December/January 2021 27
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