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Posted on 25 February 2019

New Approach to Thermal Management via Advanced Heat Transfer Solutions

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Much of modern life relies on the transporting, processing and storing of huge quantities of data through highly integrated semiconductor technology.

By Neill Ricketts, CEO, Versarien

Electronic products are being packed with a greater number of features and enhanced functionality, but their physical dimensions are being squeezed further and further, to fulfil consumers’ desires for sleeker form factors. Bandwidths are continuing to rise, yet the space into which complex digital systems must be housed is simultaneously decreasing. The removal of heat from the area around the central processing unit (CPU) at the heart of these systems is, as a result of this, becoming an ever more pressing challenge.

As each generation of microprocessor chips make use of smaller package formats while containing a larger numbers of transistors (in line with Moore’s Law), the higher levels of heat being generated potentially put system reliability at risk if this heat cannot subsequently be dissipated.

Industry now faces the issue of finding more effective ways to deal with the heat present within electronic designs so that operational longevity is not compromised. Conventional heat sink solutions are no longer proving to be adequate, as they do not offer the high levels of thermal performance necessary, plus they are in many cases too bulky to be implemented within space constrained system designs. New thermal management products which are now being called for will need to maximise heat transfer performance, while still offering a cost effective, robust implementation. In order to tackle this increasingly serious problem, for most of the last decade researchers have looked at how micro-porous metallic materials, which emulate the high surface area structures found in nature, might enable more effective transfer of heat to be realised.

Taking Inspiration from Nature

In both plant and animal physiology there is an overriding need to maximise all available surface area. This means that the various biochemical processes taking place can be carried out with the highest levels of efficiency. Sponges and corals, for example make use of their porous physical form to ensure that they can absorb enough nutrients from the external environment. Bone also has a porous nature so that it is lightweight while still having a high degree of mechanical strength.

The main obstacle holding back the use of micro-porous metallic structures in thermal management has been making the fabrication process simple and cost effective enough to be commercial viable. Several years ago, members of the University of Liverpool’s Department of Engineering started to investigate how this problem might be resolved. The Lost Carbonate Sintering (LCS) process that was subsequently developed there allows the production of a copper base material with a homogeneous distribution of micro-fine open cell pores throughout. The porous metal structure that this process is capable of creating has, as we will see, several key benefits.

LCS Process

The LCS process developed at the University of Liverpool consists of 4 main stages (Figures 1 to 4 illustrate these):

  1. Firstly the copper particles are mixed together with non-metal particles. The ratio of copper to non-metal particles and the particle size will affect the pore diameter and pore density of the material that is finally produced.
    Mixing of Copper and Non-Metallic Particles
  2. Next the mixture is compacted into net or near net shape forms.
    Compacting of Mixture
  3. Heat is then applied to the compacted mixture by placing it in a high temperature industrial oven. The copper particles within the mixture adhere to one another other without melting. Temperatures of around 1000 °C (within a vacuum) are needed for completion of this stage. The heat also causes the non-metal particles to be eliminated (or this can be done via dissolution after the material is cooled).
    Application of Heat to the Mixture
  4. Quality assurance and customisation activities (such as finishing) are then carried out.
    Removal of Non-Metallic Particles

The optimised morphology and designed surface area of the resulting open cell porous metal material allows far more efficient heat transfer than previous porous metal solutions. This means that larger quantities of thermal energy can be dissipated.

Commercialising LCS

Having partnered with the University of Liverpool and C-Tech Innovation Ltd, Gloucestershire- based start-up Versarien has taken the innovative IP developed there and been able to transform it into a fully marketable product. Employing the LCS process, the company can deliver an economically viable permeable metallic foam material in high unit volumes. Its VersarienCu advanced thermal interface material has the capacity to radically change how heat dissipation is executed in modern electronic designs. Highly versatile, it can be applied to a broad spectrum of different industry scenarios – with the cooling mechanisms in servers, workstations, automotive systems and power conditioning equipment all benefiting from its use.

VersarienCu

The VersarienCu offering is made from 99.7% pure, gas-atomised copper powder with a nominal 50 ìm particle size. It exhibits up to 10 times more effective transference of heat energy than conventional micro-channel heat sinks of equivalent size. A major reduction in heat sink weight and physical dimensions compared to competing products for a given level of heat transfer is thus possible, resulting in considerably smaller form factors. A heat transfer coefficient of approximately 150-200 kW/m2K can be achieved (with 64% porosity, 425 μm pore size and a flow rate of 2 l/min using de-ionised water).

Porosity Characteristics

The pore morphology used in VersarienCu can be set to fit specific application requirements. Diameters from 20 μm to 1.5 mm can be specified. Overall porosity levels can also be altered as necessary, with 50% right up to 80% possible. Smaller pore sizes will improve heat transfer, but will at the same time demand higher pumping power so that the fluid can pass through the material. For this reason, engineers will have to decide on whether their design needs the extra heat dissipation made possible by such pore sizes or whether trade-offs can be made so that bill of materials costs can be kept in check.

As semiconductor devices become ever more complex and their form factors shrink still further, the heat they generate will continue to rise, thereby putting system reliability at risk. By applying the biophysical principles that govern nature to electronic design it is now possible to implement advanced thermal management solutions based on micro-porous metallic materials. This has led to implementation of less cumbersome and more effective methods for getting rid of unwanted heat.

 

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