Thermal Management Solutions Increase the Durability of Mil/Aero Electronics
Electronics for military applications are expected to operate first-time, every time in some of the most hostile and demanding environments. Military programmes are often designed to run over several decades, with hardware likely to be updated several times over the programme’s duration. Similarly, most aerospace electronics will be compromised by harsh operating environments; significant amounts of vibration, often extreme variations in operating temperature, as well as the risks of exposure to chemicals, de-icing fluids and the like, will all take their toll unless adequately protected. Commercial and military aerospace applications, too, will typically have multidecade programme lifetimes and the hardware will usually be upgraded several times during its service life. Long term protection of military and aerospace electronics is therefore very important.
Thermal management is key to addressing all concerns regarding the reliability and performance of electronic systems, not just about extending the lifetime in storage or in duty. The development of aerospace and defence electronic systems is driving significant increases in power densities, consequently thermal management has become a significant challenge for suppliers of hardware to this sector. Electrolube can provide a particularly versatile range of products that address these thermal management issues, providing effective heat dissipation and preventing device failure as a result of elevated operational temperatures.
Electrolube is headquartered in the UK and has been manufacturing high-specification, formulated chemical products, including thermal management solutions, conformal coatings and encapsulation resins, for electronics manufacturers since 1941. With in-house manufacturing plants and R&D departments based in the UK, India and China, Electrolube collaborates with each territory very closely, cross-exchanging ideas, to not only create products that meet the needs of local manufacturers, but also solve problems for customers around the world at a pace that is hard to match.
Most electrical components are low powered devices that produce negligible amounts of thermal energy; however, there are also many devices – such as CPUs, power diodes, power transistors and so on – that produce significant amounts of heat and it is important that this heat is removed from the device quickly and efficiently to ensure optimal performance, and to prevent premature component failure. This is somewhat exacerbated by the trend for product miniaturisation and the use of compact, higher powered devices, which place greater demands on thermal management systems.
Heatsinks are commonly deployed to conduct and radiate heat from high power electronic components, the device and heatsink – usually solid substrates – being mechanically bolted together. Ideally, the surfaces of these substrates should be perfectly smooth, but this is not always possible and, as a result, air gaps will be present at the interface. Air is an extremely poor thermal conductor (0.024W/mK), and so the interface between the heat-sink and device will compromise heat transfer efficiency.
To counteract this problem, these air-gaps need to be filled with a material that will produce a significantly improved thermal interface between the two substrates. This material can be a thermal paste, an adhesive, a room temperature vulcanized (RTV) silicone, thermal-pad, or some other thermally conductive medium. A good tip when introducing the interface material is to apply the minimum amount of product necessary to fill the air gaps, thus keeping the thermal resistance as low as possible. Applying too much material will increase the thermal resistance of the interface, resulting in a reduction in performance.
In most cases, due to the increased filler content, high thermal conductivity means that the material will generally have a high viscosity. As a result, the possibility of air entrapment needs to be considered when applying the product. A highly viscous material may increase the risk of air entrapment, reducing the thermal conductivity performance; in such cases, a lower viscosity material might be a more appropriate choice.
Before we take a look at the thermal management products available from Electrolube, it is worth considering the various methods used to measure the thermal conductivity when qualifying heat-transfer products. Some techniques only measure the sum of the material’s thermal resistance and the material/ instrument contact resistance. Electrolube uses a version of the heat-flow method that measures both of these values separately. Values quoted using this heat-flow method are closer to the materials ‘true’ thermal conductivity. Alternative methods that do not separate the material’s thermal resistance and the material/instrument contact resistance may look more impressive, but these higher readings are less accurate. Electrolube’s advanced in-house testing equipment ensures that all products reach the market with the reassuring capability to deliver reliability and performance.
Electrolube’s thermal product range
Electrolube provides thermal management interface solutions in the form of thermal pastes, gap fillers, RTV silicones, epoxy adhesives, epoxy/polyurethane-based potting compounds and thermal phase change materials.
Thermally conductive pastes consist of thermally conductive fillers in a carrier fluid, the former being a blend of one or more mineral fillers depending on the desired thermal properties, and the latter a silicone or non-silicone based medium. Thermal pastes do not cure and so they offer the best solution there is; the capacity for rework during the life of the component. A thin layer of paste should be applied between the device and the heat-sink to fill the air-gaps and improve the contact. Application can be via stencil printing, screen printing, automatic dispensing equipment, or even using an aerosol spray product such as Electrolube’s 100 percent ozone-friendly HTCA nonsilicone formulation.
Silicone based thermal paste products usually deliver lower oil bleed and evaporation weight loss compared with their non-silicone counterparts, as well as offering a higher upper temperature limit, which can be in excess of 200˚C. However, there are applications where silicones may be unsuitable – when devices are sensitive to silicone contamination, for example. Electrolube’s silicone thermal pastes include the HTS, HTSP and HTSX. HTS is the standard product, offering an operational temperature range of -50°C to +200°C and a thermal conductivity of 0.90W/mK, while HTSP is a higher conductivity version (3.0W/mK) for critical applications requiring a high level of thermal transfer performance. HTSX is a silicone thermal interface material (TIM), which has been developed to perform in far more extreme conditions than its sister product HTS. HTSX offers an exceptionally wide operating temperature range, has a thermal conductivity of (1.58W/mK) and has been developed to perform in the harshest and most extreme conditions. It also has a much reduced oil bleed than HTS.
Electrolube’s two non-silicone thermal pastes called HTCX and HTCPX – the ‘X’ suffix denoting ‘Xtra’ or higher performing versions of HTC and HTCP – provide increased thermal conductivity (1.35W/mK for HTCX and an impressive 3.40W/mK for HTCPX), lower oilbleed and lower evaporation weight loss. In fact, the ‘Xtra’ versions deliver oil-bleed and evaporation weight loss performances as good as Electrolube’s silicone-based pastes.
Other heat transfer methods include Electrolube’s silicone thermal gap pads GP300 and GP500, which are supplied in 200 x 200 mm, in thicknesses of 0.5 and 1 mm, and can be cut to size if required for ease of application. They are highly thermally conductive but have a higher thermal resistance when compared with thermal pastes due to the thickness of the gap pad versus the very low thickness achievable with a thermal paste. With a broad operating temperature range of -50°C – 150°C, GP500 features a thermal conductivity value of 5 W/mK and has a low thermal resistance value. GP300 has a value of 3 W/mK and a slightly higher operating temperature range of -50°C – 160°C. Both options are best suited to applications where the pressure of the contact surface is high, ensuring that the material fills all the air pockets in the interface.
The specially designed thermal interface material, Surface Cure Thermal Paste (SCTP), is an ideal choice for high performance applications such as those found in the military or aerospace industries. With extremely high thermal stability, even through and thermal shock, SCTP is an excellent choice in applications which undergo wide shifts in temperature. As a surface cure material, only the exposed surfaces cure creating a skin while the main bulk of material remains soft enabling rework of components if required.
Electrolube’s single part, dispensable thermally conductive putty, TCP400, is a unique formulation that can be used on vertical surfaces. With high heat transfer performance, TCP400 is very soft and exerts low stress on components. TCP400 is a low modulus elastomer with excellent pump out resistance and minimal oil bleed. It also requires no pre-mixing, so it can be used as supplied, making it ideal for automated applications for simple processing.
For certain applications it may be desirable to use a product that cures into a solid, which may or may not require bonding two surfaces together. Electrolube offers two types of product that fall into this category: TBS and TCOR.
TBS (Thermal Bonding System) is a two-part, high strength epoxy adhesive designed to bond a heat sink to the component. In addition to the mineral fillers, the adhesive contains small glass beads of controlled diameter: these allow for a set thickness (equal to the diameter of the beads) to be achieved in order to provide optimal performance. TCOR is Electrolube’s neutral cure, silicone RTV product with an oxime cure. This has a thermal conductivity of (1.80 W/mK) with improved bond-strength properties.
For certain types of heat generating circuitry such as power supplies, it may be beneficial to encapsulate the device in a heat-sink enclosure using a thermally conductive potting compound. Electrolube produces a variety of two-part technologies. The company’s flagship thermally conductive encapsulation product. ER2074, is a highly-filled epoxy resin possessing high thermal conductivity (1.26 W/mK), but as a result is high in viscosity (16,700cPs). ER2183 is a lower viscosity version of ER2074 (5,000cPs). The reduced filler content required to achieve this viscosity has little effect on the thermal conductivity performance: ER2183 is 70 percent lower in viscosity, but only exhibits a 13 percent decrease in thermal conductivity as a result (1.1 W/mK).
UR5097 is a polyurethane potting compound that possesses a similar viscosity to ER2183 (6,000cPs) and good thermal conductivity (0.65W/mK); however, it also has the added benefit of UL94V0 certification – particularly useful for applications requiring a thermally conductive, flame retardant resin.
Also, Electrolube’s primary and secondary conformal coating protection systems, include products based on acrylic, urethane, silicone, epoxy and 2K formulations, offering enhanced protection against certain specified environmental conditions (high humidity, condensation risk, high operating temperatures, etc.). Electrolube’s award winning coating, UVCL, is q highly effective ultra-fast drying conformal coating product. UVCL became an instant success due to its versatility in different application processes and excellent resistance to ‘thermal shock’ cycling, moisture vapour, salt-spray, solvent and fluid exposure, and condensing environments. UVCL meets IPCCC-830 requirements and is UL94 V-0 rated. The coating is highly flexible and cures in seconds with conveyorised UV lamps or within 3 days at ambient temperatures and humidity to ensure cure even in shadowed areas. With a 12-month shelf life, the single-component, low-viscosity UVCL provides superior ‘sharp-edge’ coverage than other materials with common selective coating methods.
Electrolube developed the 2K conformal coatings series by way of providing a solution to common problems faced by manufacturers with performance issues of current coatings in extremely harsh environments. The 2K series of coatings is significantly different in its protective capabilities due to its capacity for greater coating thickness and enhanced edge coverage. 2K coatings are VOC-free, fast-curing, high performance two-part conformal coatings that provide a solvent-free alternative to both UV and silicone materials, require less capital investment than UV materials and improve on the performance of most silicones in harsh environments. The majority of coatings in the 2K range are hydrophobic, giving excellent protection against water immersion, salt mist and humidity, making them ideally suited for mil/aero electronics. Electrolube’s 2K range of materials also provides sharp edge coverage and protective thickness and is unrivalled by other liquid applied conformal coatings. The materials have been developed specifically for thick film applications and engineered to optimise this performance advantage, whilst mitigating many of the issues associated with thick coatings, such as cracking during thermal shock testing and solder-joint lifetime degradation, especially for very sensitive devices such as BGA packages. The extreme protection provided by the increased thickness and superior coverage will be valued by many Military and Aerospace suppliers struggling with increased demands from OEMs for improved condensation resistance and even powered up sea-water immersion conditions.
Electrolube’s steadfast commitment to health and safety and reducing environmental impact has been widely recognised globally with accreditations from the Quality Management System – ISO 9001, the Environmental Management System – ISO 14001, the Occupational Health & Safety Management System – ISO 45001, the Automotive standard – IATF 16949 and the Aviation, Space and Defence standard – prEN 9100. Relevant systems and product formulations are fully compliant with industry standards, including MIL-STD 810 (environmental), US MIL-1-46058C, IPC-CC-830B, IEC61086, UL746 (conformal coating) and British Ministry of Defence DEFSTAN 59/47 Issue 4.
The recent revision and expansion of Electrolube’s thermal product range highlights the company’s dedication to providing high performance products of exceptional quality for mil/aero applications. And if no off-the-shelf product meets the needs of a particular application, depending upon volumes, the technical team will formulate one that does. This commitment ensures that customers have the products for today’s needs, as well as the technologies of tomorrow.
(The article is an original piece written by Electrolube)
