Thermal News

Interview with Amanda Hartnett, applications engineer with Indium Corp.

Indium Corp. applications engineer Amanda Hartnett recently discussed with Thermal News her knowledge of metal thermal interface materials and their potential applications and benefits.

What differentiates metal thermal interface materials from other interface materials?

I spend an extensive amount of time studying and testing the properties of metal thermal interface materials as well as other thermal interface materials, primarily polymer-based. They each have applications which they are best suited for, but the metal thermal interface materials stand in a category of their own with regard to their thermal performance.

Generally, I don’t have to be much of a salesperson for these materials. Thermally and physically, they speak for themselves. As soon as a thermal engineer begins perusing the properties of metal thermal interface materials, they are interested. The thermal conductivity of these metal TIMs are very attractive. Indium and indium alloys have thermal conductivities as high as 86W/mK.

Is thermal conductivity alone enough to justify the use of a metal thermal interface?

Absolutely not! If it were, everyone would have already moved on to implementing diamond or carbon nanotubes as their interfaces.

When I measure the performance of thermal interface materials, I characterize them based on their thermal resistance. This value is typically more valuable than bulk thermal conductivity. For a compressible TIM, the thermal resistance assumes the actual contact which will be made between the interface material and it’s mating surfaces. This provides a measurement of thermal performance which is as close to real-world per Watt or per cm2 as I can provide without being application-specific.

The thermal resistance value is incredibly valuable, but even that is not the deciding factor for whether to implement a specific thermal interface material. In addition to an application’s unique requirements, there are other material properties which are commonly investigated to justify the selection of a thermal interface material.

For instance, there is the compatibility between the TIM and the device in which it will reside. Some interface materials are affected by the specific surface finish of the device. Other interface materials require a cure cycle, which may exceed the device’s specs.

Also, it is important to consider the reworkability of an interface material. TIMs such as our compressible metal Heat-Spring are very simple to rework. Others, such as conductive epoxies, can be quite difficult.

If thermal resistance is the primary measure of performance for a thermal interface material, what variables affect this value?

Thermal resistance data is presented in a chart of thermal resistance vs. pressure. In general, higher applied pressure produces a drop in resistance. This is because, under higher loads, thermal interface materials fill any surface imperfections, effectively lowering the contact resistance and thinning the interface bondline.

Thermal resistance is also affected by the thermal conductivity of the TIM material and its bond-line thickness. Logically, the higher the thermal conductivity the less sensitive the resistance is to bond-line thickness.

Since metal thermal interface materials already have a high thermal conductivity, is the key to optimizing their performance maximizing applied pressure?

For compressible metal thermal interface materials, yes. These are metal foils intended to be compressed at room temperature. The more pressure the better. For instance, if you place a 0.004-inch-thick pure indium compressible TIM and apply 50PSI to it, you will achieve a thermal resistance of approximately 0.12 cm2-ºC/W. If the same TIM receives a load of 100PSI, the thermal resistance drops to approximately 0.06 cm2-ºC/W.

Is this high pressure and thermal performance relationship true for all metal thermal interface materials?

The rule of compression applies to most metal thermal interface materials primarily because pressure thins the interface bondline.

Other metal thermal interface materials are less affected by pressure. Solder TIMs, for example, provide significantly lower thermal resistance then a compressible metal thermal interface, and only light pressure is ever applied to these. Solder TIMs have the benefit of being reflowed and melted for attachment. In its molten stage, solder TIMs flow as a liquid, filling in surface irregularities without a notable applied pressure. Pure indium, used as a solder TIM delivers a thermal resistance to 0.03 cm2-ºC/W.

Are there any thermal interface materials available that outperform solder TIMs?

When extreme thermal performance is required, liquid metals or phase change metals outperform solder TIMs.

One issue with solder TIMs is that, when they re-solidify after they are reflowed, they may trap air or flux in the bond. These voids degrade thermal resistance. Liquid metal TIMs never re-solidify, so voiding isn’t an issue. The thermal resistance of a liquid metal TIM, with a 0.002” bondline, can be as low as 0.015 cm2-ºC/W. Another benefit is that they are not reflowed; therefore reflow equipment is not necessary.

Good things do come at a cost, however. When liquid metals are used, the assembly must be designed to seal in the liquid metal.

Amanda Hartnett is an Applications Engineer supporting Indium Corp.’s engineered solder and thermal interface materials. She is experienced in addressing thermal issues in many high power applications from industries including medical, military and photonics. She is a Certified IPC specialist and has authored and presented technical papers on thermal management and microelectronics packaging materials at various industry technical seminars, including SMTAI, IMAPS, MEPTEC, and Semi-Therm. Hartnett holds a BS in Chemistry from Utica College and has performed graduate studies in business administration at the State University of New York Institute of Technology.

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