By Kent Larson, Principal Scientist, Dow
Electronic devices inherently generate heat, but heat is the enemy of reliable electronics since higher temperatures reduce the life of printed circuit boards (PCBs), solder joints and components. Faster data speeds in today’s electronic devices and the growing number and density of electronic components, are further increasing the amount of heat, requiring even higher levels of thermal protection. For engineers designing 5G products for consumer, transportation, telecommunications and industrial applications, reliable thermal protection is a top priority to avoid device failure.
In communications applications, 5G base stations and optical interconnects that require greater data transfers at faster speeds are challenging experts in electromagnetic compatibility (EMC). Meanwhile, consumer electronics with high-density packaging and smart architectures are susceptible to electronic pollution that can disrupt or disable circuits. Electricall-conductive adhesives provide reliable shielding against electromagnetic interference (EMI) whilst maintaining their other properties. They are tuned specifically to provide electrical conductivity but also provide some thermal conductivity.
Thermally-conductive silicones are helping engineers improve reliability and extend service life. These thermal interface materials (TIMs) draw significant amounts of heat away from electronic components, are stable at high temperatures and retain their properties over time. In addition to adhesives, thermally-conductive silicones include gap fillers, thermal greases and encapsulants and gels.
All of these products consist of a silicone base and a filler. Aluminum oxide (AL2O3), an electrical insulator with a high thermal conductivity for a ceramic, is often used as a filler material. Thermal conductivity is the heat flow through a material. Thermal 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.
Thermally-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. They 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 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.
The 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, and bond line thickness and cure exotherm. Advanced silicone technologies also address several challenges that are 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, which can increase the risk of electronic 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 softer and more crack resistant. They 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, then 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.
Thermally-conductive silicone adhesives eliminate the need for mechanical fasteners, say, when bonding heat sinks and sensors to PCBs. They also decouple the stresses caused by the expansion and contraction of layers with different CTEs. These adhesives are available in one-part (1-part, 1K) formulations that require cold storage and two-part (2-part, 2K, A&B) formulations that require mixing.
To support different dispensing requirements, thermally-conductive adhesives come in a range of rheologies. Flowable adhesives support the fast filling of channels and cover larger surface areas. Low-flow products allow a controlled flow-out after dispensing to fill the desired area. Highly thixotropic products allow for quick dispensing with exact placement and without overflow or sagging.
Thermally-conductive silicone adhesives are available with three different curing different mechanisms: condensation, addition and Thermal Radical Cure (TRC), a proprietary curing system from Dow. All provide long-lasting performance and bond lines that resist temperature and humidity. Fast-curing adhesives with short cycle times are available, but engineers need to consider all their requirements, such as demand for greater energy efficiency, before selecting a cure system.
One-part condensation cure adhesives are room temperature vulcanising (RTV) silicones that use the moisture in the air to cure. They are the simplest of adhesives because, once they are dispensed, they cure without additional manufacturing action. Condensation cure adhesives maintain their properties with environmental ageing and are suitable for a range of electronic applications. For lighting, they can be optically clear and have UL 94 V-0 recognitions.
Cure time for one-part condensation cure adhesives depends on many factors, including ambient temperature, relative humidity and the depth that moisture from the air needs to penetrate. Bond line thickness has very little impact on cure speed. Condensation cure adhesives may take longer to cure if they have a limited surface area in contact with the air. Advanced formulations have a wide adhesion profile and provide reliable bonding to most substrates.
Thermally-conductive silicone adhesives that use addition curing require heat from an oven to cure. They can be either one- or two-part materials. TRC adhesives have mild heat requirements for reduced energy costs and a controllable flow immediately after dispensing to eliminate over-flow. They adhere to a wide variety of substrates and are not sensitive to traditional heat cure trace contaminations such as from sulfur species and amines.
Thermally-conductive silicone gap fillers provide a highly conformable thermal path between a heat source and heat spreader. They are replacing traditional elastomeric pads that can fail to protect solder joints or leads. Thermally-conductive silicone gap fillers also conform to and completely fill uneven spacing or higher part-to-part tolerances that create challenges for dimensionally-constrained pads.
Because thermally-conductive gap fillers are soft and compressible, they dampen vibrational energy and oscillations. For enhanced reliability, they provide excellent mechanical and elastomeric stability during temperature ageing.
Advanced formulations of these materials are optimised for dispensability and rheology. They have a controlled/engineered flow that can range from flowable-after-dispensing to low-slump to non-slump/flow. Slump resistance is important during vertical, pre-curing assembly. Thicker bond lines need more thixotropic gap fillers while thinner bond lines need materials that flow easily and thin out.
Thermally conductive silicone gap fillers adhere with tack adhesion instead of chemical bonding, a consideration for repairability. They can dissipate heat from critical automotive components, such as engine or transmission control units, braking and stability controls and sensors. In applications where electrical isolation is required, products may include glass beads for a compression stop.
Thermal greases are one-part materials that provide an interface between a heat sink and a heat source. They eliminate air gaps to maximise heat transfer and dissipation. Thermal greases are applied in thin layers, typically less than 100 microns. Given this limited thickness, thermal resistance (rather than thermal conductivity) is the dominant heat flow property. Notably, thermal greases have an especially low thermal resistance compared to other types of thermally-conductive silicones.
Historically, thermal greases have been used in microprocessors with flat chips and over a relatively limited temperature range. Automotive applications have broadened the temperature range that is required and positioned electronics vertically. In turn, gravity has contributed to a problem called bleeding or bleed-out where the liquid polymer separates from the conductive filler particles. Compared to other grease chemistries, advanced silicones exhibit less bleed-out for greater assembly reliability.
During PCB assembly, bleed-out can happen when the liquid polymer migrates beyond the bond line and interferes with soldering or wire bonding. Bleed-out can also interfere with other manufacturing operations and contact-adjacent PCB components. After PCB assembly is complete, bleed-out is generally a cosmetic issue.
Dry-out, a separate but related issue, refers to the thermally-conductive material that is left behind on the bond line after the liquid polymer’s migration. The enhanced dry-out resistance that silicone greases provide is important because leftover filler materials cannot wet surfaces properly. Because thermal greases are no-cure pastes, grease chemistry rather than curing must address dry-out. Thermally-conductive silicones in advanced formulations do this reliably and are available with reduced volatiles for sustainability and environmental health and safety.
Along with bleed-out and dry-out, traditional thermal greases may suffer from pump-out, the non-recovering loss of thermally-conductive materials from in-between substrates. Pump-out happens when there are repeated bond line compressions from shape changes where different layers have different rates of expansion and contraction.
Applications for thermally conductive silicone greases include lighting assemblies, telecommunications equipment, consumer electronics, power supplies and power components for transportation.
Encapsulants and gels
Encapsulants and gels are thermally-conductive silicones that combine thermal protection with physical protection. Encapsulants can protect against mechanical and thermal shock, chemical attack, dust and humidity. Gels have a remarkably low modulus and can protect the most sensitive and delicate components against mechanical stresses, including those from thermal cycling.
Encapsulants and gels have high flowability for improved processing. They support fast curing with moderate heat. These materials come in a range of viscosities and have a high thermal conductivity relative to viscosity. Historically, low viscosity silicones achieved very fast dispensing at the expense of filler level and, in turn, thermal conductivity. To achieve an ultra-low modulus with maximum stress relief, older compounds also used less conductive filler.
The advanced silicones used in today’s encapsulants and gels eliminate these unwanted tradeoffs. These materials also can achieve good adhesion without the use of a primer, which helps to reduce cycle times. They require addition curing with moderate heat, offer fast cure times and harden into rubber-like elastomers for reliable protection. They also provide excellent dielectric properties, adhere to many substrates and are a good choice for automotive and power module applications.
Advanced silicone solutions
Engineers need the reliable, long-lasting protection that advanced formulations of thermally-conductive silicones provide. It is important to choose materials that meet demanding requirements and to work with an experienced supplier that can provide technical support whenever and wherever it is needed, around the clock and around the world. Thermal protection is critical not just for today’s electronics devices but for technology trends that promise faster data speeds with more heat-generating electronic content.