03-06-2016 | | By Roland Hofmann
Effective heat management is a must for practically any electronic device or application. Numerous solutions are available that meet the requirements of today's innovative applications. By Roland Hofmann, Product Manager for Thermal Management at Rutronik.
Because components are becoming ever more efficient, because applications are becoming ever smaller and increasingly mobile, and in light of the desire for solutions for automated production, requirements relating to heat management have changed. Numerous new solutions address these requirements, but to select the right strategy from this diverse range, a case-specific analysis must be performed as each application is subject to different circumstances. CFD analysis (computational fluid dynamics) provides a convenient option. It can be used to illustrate how products and prototypes handle heat as early as the concept phase. It takes into account thermal resistance, the heat conductivity of various materials, convection and radiation, and also CAD data and real installation and ambient conditions. On this basis, the CFD analysis provides a three-dimensional illustration of the causes and effects of heat as well as the temperature and flow distribution in an assembly, thereby providing a solid basis to make decisions on which housing design to use and finding a suitable heatsink.
Small electronic devices, especially portable devices such as smartphones, tablets, and cameras, require ever thinner and lighter solutions to effectively dissipate or distribute heat. This affects not only consumer electronics, but also data communication infrastructure, which contains ever more complex electronics in an increasingly compact space. Electric and hybrid vehicles require long-life and lightweight batteries, while Industry 4.0 demands more monitoring and control options. Solar panels need to be able to withstand greater heat generation, and in modern medical engineering, mobile devices are increasingly being used that need to be convenient to carry. For applications of this type, Panasonic's PGS (Pyrolytic Graphite Sheet) offers an ideal solution for both transporting heat away from one hotspot (Figure 1) and distributing it over a horizontal plane (right hand side of Figure 1). The sheet serves as a heat conductor between the hotspot and the heatsink, combining horizontal and vertical dissipation (Figure 2). PGS provides an effective and rapid means of dissipating heat towards the cooling mechanism, especially when the heatsink or heat spreader is not located directly adjacently to the heat source. If it is fitted directly on the heat source, e.g. the CPU, GPU, batteries or ASIC, it transmits heat to the external housing.
PGS consists of a light and flexible layered material called pyrolytic graphite, and with a material thickness of between 10µm and 200µm, it also fits into small devices. The material is flexible, enabling it to be cut to shape as needed. With a bending radius of 2mm, the layers can be bent more than 3,000 times by up to 190° (Figure 3). PGS offers a thermal conductivity of 700 to 1950 W/mK, depending on the layer thickness. This is between two and five times the conductivity of copper and seven times that of aluminum.
Figure 4 illustrates the effectiveness of PGS in lowering temperatures at hot spots. It shows the temperatures on the ABS layer (acrylonitrile butadiene styrene) where the IC's contact with the surface is buffered by a silicone thermal pad (type A / type B), as well as in cases where a large (type A-1 / type B-1) or small (type A-2 / type B-2) PGS with a thickness of 70µm is positioned between the silicone layer and PCB. Figure 4 clearly shows that even if just a small sheet is used, the PGS reduces the temperature at the hot spot considerably—even when no silicone thermal pad is used (thereby creating an air gap). PGS also provides a limited level of shielding from electromagnetic interference, allowing it to also be used as a solution for EMC in many applications. This stable material is not sensitive to environmental influences and does not show any signs of aging.
Pin-fin heat sinks like the one in Figure 5 provide a very effective means of heat dissipation in all areas where processors are used. The pins are arranged to optimise flow, enabling them to achieve a high degree of efficiency and optimum air through-flow. Their low weight and ease of installation are also arguments in favor of pin-fin heatsinks. They are available in many forms from Assmann WSW. This manufacturer offers not only standard products but also special-purpose solutions in which the pressed aluminum profiles are cut in accordance with specific customer requirements in state-of-the-art CNC machining centers. Custom solutions include various materials, profile cross-sections and lengths, designs incorporating hollow-fin profiles, perforations, bore holes, welded heatsinks, anodised visible and decorative surfaces, and special packaging for manual, partially automated or fully automated assembly. For this, the manufacturer works very closely with us here at Rutronik to achieve a technical and cost optimum for the customer. The sooner that heat management is included in the development process and the more information that is available, the more easily this can be implemented. Dimensioned profile drawings as well as drawings for machining purposes (including tolerances) are essential. If the required surface is also specified at this point (perhaps anti-scratch or smoothed), this can be accommodated in the selection. Additional 3D data is also helpful for complex, customised profiles.
Heat also plays a major role in LED designs — inadequate heat management not only affects the life of the LED, but also its colour spectrum and light output. Because the bulk of the heat is transmitted in the opposite direction to the light, towards the PCB, it can be dissipated by the PCB itself, the housing or an additional heatsink on the board. With output voltages measured in just milliwatts, ambient lighting applications can usually forgo the use of a heatsink, but high-powered LEDs such as those in floodlights or office, shop and street lighting would destroy the crystal in a matter of seconds without additional cooling mechanisms. A wide selection of heatsinks are available to prevent this (Figure 6). At Assmann WSW, for example, they range from small finger-shaped models with a heat resistance of more than 16K/W to large extruded profiles with Rth < 1K/W. Alongside its standard products, this specialist for heat management also develops customized solutions for LED applications.
Air gaps between components and heatsinks must always be avoided to provide effective heat dissipation, because air acts as a thermal insulator. Thermally conductive pads are suitable for compensating for unevenness and different component heights. The supplier 3M offers stackable pads that fill air gaps of 0.5 to 2mm, enabling heat output to be dissipated away from the source (Figure 7). These pads stand out with their high thermal conductivity and their ability to efficiently fill gaps. They also offer excellent compressive strain relief and adapt to the application superbly. The pads are made of acrylic, meaning that they are free of silicone, so they can be used in automotive applications. The acrylic material also offers a number of other benefits, among them the absence of oil leakage, which can occur when using excessive amounts of thermal paste, and the pads also form a singular mass when stacked. They can be cut more easily and attached with great ease using adhesive films. Because they stick easily, the assembly can be disassembled more comfortably.
Where the aim is to reduce the heat resistance, thermal pastes remain a popular solution, but applying these is a time-consuming and complex process that still often produces unclean results. Adhesive films such as those offered by 3M are increasingly establishing themselves as a less troublesome alternative. They are available with single-sided and double-sided adhesion and are easy to apply. Thanks to their ceramic filling, they combine moderate heat conductivity with strong adhesive force. They also offer superb surface wetting properties and impact strength, and still provide consistent performance throughout their service life.
Fans are not always a preferred or suitable solution for some applications due to their unreliability. They consume energy in a way that is not conducive to achieving energy efficiency targets, even though manufacturers have been able to reduce energy consumption somewhat. Space requirements, noise output and service life are other critical aspects. But for applications such as drive engineering, frequency converters, control cabinets, power supplies and welding equipment, fans remain the solution of choice, because their use massively increases the fresh air throughput, forcing air convection.
To increase the life of fans, some manufacturers have developed new and improved bearings. Jamicon, for example, offers a brushless DC fan which, thanks to HTLS (High operating Temperature and Long Life Sleeve bearing system), can achieve operating lifespans of up to 100,000 hours. The special design of the optimised “Superflow” bearing from Delta Electronics reinforces the bearing's seal. The bearing also has a recycling function and offers a longer life than conventional sliding bearings.
Delta Electronics has also added the FTA 0102AA fan system to its product range. With an easily readable temperature readout, low noise output and uninterruptable operation enabled by system redundancy, it is perfectly suited to 19” computer cabinets. As a further benefit, because the fan system is operated via the normal mains power supply, it consumes around 30 percent less energy than comparable devices with AC motors.
As this article has discussed, there is no single answer to the question of what the best strategy is for heat dissipation, because there are numerous factors that influence the choice. But one thing is certain in all cases—the earlier that heat management is considered in the development cycle, the greater the range of options and the easier it is to implement them. Rutronik supports its customers in every phase of development with extensive advice provided by application engineers and product managers. You can make use of a broad range of solutions from leading suppliers with whom Rutronik is in close partnership.