Battling heat: Thermal management

In response to global concerns about the impact on the environment, sustainability and the need to minimise energy usage, electronic equipment manufacturers are now more focused than ever on achieving high efficiency levels for their circuit designs. This is not only true for mains powered equipment, it is also the case for battery powered portable equipment, where designers are looking to squeeze more functionality into products and to get the maximum duty cycles from batteries. Increased efficiency means lower losses which, in turn, can result in less 'waste' heat being generated. Considered on its own, this is potentially good news for designers and would seem to reduce the thermal management challenges encountered in their new product designs. However, in reality, the thermal benefits of high efficiency circuit design are often offset by the heat generated due to the trend for more densely packed components. Additionally, factors such as limited ventilation in portable electronics equipment, due to the need to seal enclosures against moisture and dust ingress, also contribute to the overall thermal management problem. And so, while the cause of thermal management challenges may in many cases be changing, they are still a critical element in the creation of a reliable and robust end design. Because of the need to reduce energy consumption, the cooling fan represents the least desirable option for achieving heat dissipation in both mains and battery powered equipment. In addition to power consumption, fans also take up valuable space, present engineering challenges, are noisy and, as they have moving parts, can fail. As electronics have become more prevalent and components and product designs have shrunk, so the choice and diversity of thermal management materials has had to evolve to meet the new challenges. Some 25 years ago, there was little in the way of portable electronics equipment and fixed location product form factors were much larger. With significantly lower levels of functionality, the number of components required for a given product was also much lower. The nett result was enclosures with large, well spaced components that presented little in the way of thermal challenges. Where heat management issues did exist, fans, thermal greases and heatsinks proved more than adequate. Along with devices evolving in line with Moore's Law, many component manufacturers have integrated the functionality previously given by multiple discrete devices into small single package solutions. This has led to the stream of small, portable high functionality products that consumers crave and that have changed irreversibly the way we live and work. Inside small, sealed designs, closely packed integrated devices require careful thermal management. Many traditional thermal management materials are often not suitable for the task and so the choice available to the design engineer has expanded to include an array of new materials and approaches. Fully cured gels Highly conformable, one component pre cured silicones that can be dispensed are ideal for filling large and uneven gaps in electronics assemblies. The viscoelastic paste is a form stable, fully cured silicone material that takes little to no compressive force to deform during assembly. This characteristic helps avoid placing stress on component solder joints and leads that can result in either premature failure of the device or damage to the circuit board onto which it is attached. In some cases it is imperative that the device being thermally managed be electrically isolated from the chassis or heatsink to which it is coupled with the thermal gel. In this case the inclusion of small glass beads in the gel can provide an effective compression stop that ensures the two surfaces do not come into direct physical contact. Insulator pads Generally very thin materials (around 0.25mm) that comprise a silicone elastomer blended with a thermally conductive filler. Fibre glass cloth is commonly used to reinforce the material and provide some resistance to cut through that would negate the electrical isolating properties of the material. In applications with higher assembly pressures, or where the risk of cut through is greater, other more resilient carrier materials are available. There is a vast choice of insulator pad materials available that utilise different fillers to provide a range of thermal and electrical performance levels. Options such as low tack adhesive coatings that can aid assembly may also be specified. Adhesive tapes Thermally conductive tapes provide an effective alternative to mechanical fasteners such as screws, clips and rivets for bonding heatsinks to either ceramic or metal device packages. As well as speeding assembly and saving space they can also reduce bill of materials costs. Gap filling pads Perhaps the most successful of the 'new generation' of thermal interface materials, silicon based gap filler pads have supported the use of equipment enclosures or chassis as heat dissipaters in place of costly and heavy dedicated heatsinks. By fitting a piece of soft gap filling material between a device requiring thermal management and an enclosure, heat can be channelled away effectively. The typically large surface area of the equipment enclosure, coupled with the fact that it provides a direct thermal path to the lower temperature of the 'outside world', can negate the need for fans where they were previously required for a specific design. In some applications gap fillers can also allow the design to be completely sealed and therefore suitable for use in harsh environments where moisture may be present. Gap filling pad materials are available in a range of thicknesses that extends beyond 5mm, allowing large gaps to be bridged. Their soft nature (as low as 4 Shore 00) means large mechanical tolerances can be accommodated with only low assembly forces needed. Accurate blending of silicon based gap fillers using a range of materials with different thermal conductivities, results in a choice that allows designers to select a material that accurately meets the thermal requirements of their specific design. This is valuable as materials with better thermal characteristics – maximum values are around 4.0 W/mK – typically use more expensive ingredients or have more stringent requirements in terms of blending. Gap fillers are often available on a choice of aluminum foil or glass fibre carriers, the former having better thermal characteristics. A high strength acrylic pressure sensitive adhesive can aid assembly and allow permanent attachment of the gap filler pad to cold surfaces. Looking to the future, the choice and functionality of electronics equipment available to both business and consumers looks set to increase at a relentless pace. The focus, from the end user's perspective, is likely to be form, function and energy consumption. At the same time, equipment designers will have to make sure they continue to deal effectively with thermal management issues to give their products the reliability and robustness that is a prerequisite for end users. Author profile Mark Carter is an applications engineer with Chomerics Europe