By RANALD PRINGLE
Versarien Technologies
www.versarien.com
Whether it is being viewed from a technical or logistical perspective, the role that thermal management plays in electronics design cannot be ignored. Generally, the operational lifespan of semiconductor devices will halve for every 10°C rise in their junction temperature. Therefore, the efficient removal of heat from the system is paramount. But, given that complexity levels are increasing all the time, engineers can see sizable challenges on the horizon.
With a greater number of digital and analog components being placed onto PCBs, every effort must be made to manage the temperature in the immediate vicinity of such devices. Furthermore, the transition to smaller-format enclosures means there is much less space in which this would take place.
There are several routes that engineers can take to accomplish the objective of keeping heat in check for these components. They could look at active cooling mechanisms, such as fans, but these have several drawbacks. First, they occupy a lot of room (which is far from ideal). Their operation also adds noise (something that is unlikely to be acceptable if superior signal integrity is required). The biggest issue with this approach, however, is reliability; fans are prone to failure. They are the most likely piece of hardware in an electronics system to break down. In any situation, this is likely to be problematic, but in some circumstances, it simply cannot be tolerated. In the best-case scenario, unwanted expenses will be accrued to replace the fan unit (and these can be increased significantly if the system is in a remote location). However, in many applications (such as mobile backhaul towers or factory production lines), the downtime represents the biggest hit. The commercial damage can be serious if productivity is impacted or a service is not accessible, even for a relatively short time.
The alternative is to use passive cooling, based on items such as heatsinks. These are not dependent on any moving parts and, therefore, are significantly more reliable. This, factored in with their cost and size advantages, is making them increasingly commonplace in modern electronics designs. Just one final aspect needs to be addressed: performance.
Combining strong heat dissipation capabilities with a compact form factor is difficult to accomplish. The thermal resistance figures that heatsinks can deliver are not low enough. Too often, trade-offs are required.
Through the validity of passive cooling in modern electronics is clear, it is under attack simultaneously from two angles. It has to cope with heavier thermal loads and has less board real estate in which to do so. Conventional passive heatsink options are simply too cumbersome to be effective in the extremely crowded circuitry of modern data/telecommunication infrastructure, consumer electronics goods, household appliances, LED lighting systems, or the diagnostic equipment/instrumentation needed for healthcare.
Applying basic mechanics to the problem is not going to help — this simply dictates that, to reduce thermal resistance, the heatsink will need to be larger, with a great number of fins and increased fin height. If the space in which the heatsink is positioned is already in limited supply, anything that increases its dimensions would result in other problems (such as redesigning the enclosure or leaving out some of the functionality). The core technology must be tackled.
Thermal management principles
Quite often, engineers do not give enough consideration to the thermal management aspect of their design until the later stages. This will usually compound the problem, because any space left by then is certain to be restricted. Thinking about heat removal earlier is definitely advisable. By referring to the thermal resistance from junction to case of the constituent semiconductor devices, as cited on the manufacturers’ datasheets, it is possible to assess which thermal management option is the best fit for the system’s design. For heatsinks, thermal resistance is usually given in °C/W. The lower this value is, the better the heatsink’s ability to dissipate heat to the surrounding air will be.
New heatsink materials
The shortcomings of passive cooling have prompted certain forward-thinking manufacturers to investigate new potential heatsink materials that possess morphologies that are more effective at transferring heat. One way that has proved particularly interesting is to create microporous metallic structures that maximize the surface areas through which heat energy can be dissipated. This mimics the structures seen within the natural world (sponge, coral, and bone).
Inspired by nature and exploiting the high thermal conductivity of copper, VersarienCu is an advanced thermal interface material made up of a homogeneous distribution of micro-fine, open-cell, interconnected pores. The material is a direct result of collaboration between industry and academia. Research conducted at the University of Liverpool led to the development of a unique process for producing materials that is highly suited for thermal management.
Fig. 1: a) Mixing of copper and non-metallic particles. b) Compacting of the mixture.
Fig. 2: a) Heat is applied to the mixture. b) Non-metallic particles are eliminated.
This process consists of the following stages (See Figs. 1 and 2 ): First, copper particles are mixed with non-metal particles. The proportion of non-metal particles to copper particles, as well as the particle size, directly affects the pore diameter and pore density of the material produced. Then the copper/non-metal mixture is compacted into net or near-net shape forms. These are then subjected, over a protracted period, to elevated temperatures (approximately 1,000°C) while being held within a vacuum. This causes the copper particles within the mixture to adhere to one another without melting. It also eliminates the non-metal particles. Finally, a high-temperature copper oxide coating is applied to the material, which boosts its radiant properties. Use of this material allows major reductions in heatsink for a given footprint.
By employing VersarienCu’s intricate microporous metallic structure, it has been possible to produce a series of high-performance heatsink products. The optimized morphology of this material permits heatsink size and weight to be dramatically reduced. Because of this, the Versarien LPH00xx series of low-profile metallic foam heatsinks can be deployed in extremely confined spaces. Despite their low profiles, these heatsinks do not compromise on thermal performance. When compared to microporous ceramic heatsinks of equivalent size, they will be able to maintain a thermal resistance that is several °C/W lower.
Fig. 3: Low-profile heatsinks based on VersarienCu material.
Thermal management is becoming a core issue that must be addressed. In many cases, it can be the biggest challenge that engineers face in bringing their products to market. There are a growing number of applications in which passive cooling is the best (possibly the only) option. On a frequent basis, the heatsinks currently available simply cannot offer the levels of performance required by contemporary component-packed electronic circuitry. The thermal management sector has to explore and experiment with disruptive technology if it is to keep pace with progress being made in other aspects of electronic design; otherwise, the gap will keep widening.
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