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December 2005
library  >  Application Notes  >  General Articles

Future Challenges for Thermal Interface Materials


by ron hunadi, ph.d.
thermoset, lord chemical products


with the expected arrival of a one billion-transistor device, a near supercomputer on-a-chip, by the end of the decade, the question arises on how to maximize heat transfer across the interface. ron hunadi addresses the state-of-the-art of interface materials and the challenges ahead.

 

the challenge!

 

with the arrival of the near supercomputer-on-a-chip, transistor proximity will decrease to the point where the thermal management of the device will be an extremely daunting challenge. as pointed out by patrick gelsinger from intel in a recent article written by a. cataldo & p. kallender in electronic engineering times, if the status quo holds, the power required to drive a one billion transistor microprocessor in 2010 will be 600 watts.

 

obviously, we can expect some breakthroughs to occur in device design that will help alleviate some of the power requirements but nonetheless, the thermal management of such a device will require a superhuman effort. cryogenic cooling may be one obvious solution but this will necessitate the use of solutions beyond the capabilities of current commercially available thermal interface systems.

 

what is state-of-the-art?

 

thermal interface materials are available in a number of formats including gels, greases, adhesives, phase change materials, pads and tapes with thermal conductivities from 1 to 6 w/moc and thermal resistances of 0.2 to 0.5 oc/w. to get the most efficient heat transfer, it is critical to get good surface contact and ideally good surface wetting along with the complete elimination of air in the interface.

 

this poses a problem for tapes and pads and for this reason, gels, greases, adhesives and phase change materials are typically chosen for the most demanding applications. we, ourselves, have improved thermal interface materials to achieve thermal conductivities as high as 13 watts in liquid systems. as effective as these solutions are for today's devices, they will not meet the thermal needs of more powerful devices that will be introduced later in this decade.

 

today, gels, greases and phase change materials offer good surface wetting while minimizing stresses on the device especially with flip chip designs containing bare die. currently, silicone gels are replacing greases in many of the first level interface applications on flip chip devices, due to their superior power cycle and hast performance.

 

at this time, it is difficult to foresee the package design for a one billion transistor but with the number of expected i/o's, a flip chip design could be a serious contender. with continued geometry shrinks, it is almost certain that the power density will be significantly higher than today's designs and probably require interface materials with conductivities > 30 w and thermal resistances < 0.1oc/w.

 

greases, gels and phase change technology are being pushed to their performance limits. one can only add so much filler before a problem arises with the dispensing and printing performance. one can use different particle shapes, sizes and particle distributions/combinations but you still encounter a thermal performance wall.

 

as you reduce the thermal resistance closer to zero, the interfacial contact resistance starts to dominate the effective heat transfer equation. there are a number of ways to theoretically reduce the contact resistance and they are being explored. since gels, greases and phase change materials are either oligomeric or low-cross linked density systems, it is virtually impossible to achieve the filler particle contact achievable in many adhesive systems where shrinkage occurs during cure.

 

is there a solution?

 

a lot of work has been done with highly filled adhesive systems, containing silver and other metallic fillers, to achieve high thermal conductivities. unfortunately, a number of these products contain solvents and require tedious solvent removal steps that do not make their use amenable to high volume assembly.

 

we have been able to achieve 13 watt performance with a low volatile, solventless system. as we look forward to how we can improve its performance, we realize that we may not be able to improve the thermal conductivity beyond 20 w and keep it solvent-free.

 

how do we get the ultimate thermal performance from a thermal interface material ? certainly many of the fillers (whether they be metal oxide, metals, conductive fibers, etc.) that can be used, have thermal conductivities > 200 w.

 

the problem is there is virtually no direct particle contact since there is a layer of carrier or polymer between the particles. to get optimal thermal transfer, it is necessary to fuse the particles together or make direct particle-to-particle contact as well as direct particle to interface contact.

 

suppliers are examining ways to achieve this requirement by modifying metal alloy pastes or incorporating them into polymer based systems. of course, with the incorporation of metal alloys, one has to consider the stresses potentially incorporated into the bond line after processing. if these systems contain some level of solvent, then two temperature or ramped processing will likely be required to insure solvent removal and minimize void formation.

 

in addition, since these systems will be electrically conductive, there is always the concern that flaking off of electrically conductive particles can cause a short on nearby circuitry and this will need to be examined.

 

other solutions are possible such as the use of z-axis films or solutions incorporating particles, fibers or metal rods that will extend from the device interface to cooling system interface but it may be difficult to minimize the contact resistance. however the problem is solved, the challenge is daunting and will only help fuel new ideas and ultimately the development of new thermal interface products!

 

ron hunadi is market segment manager for microelectronic products at thermoset, lord chemical products.

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