Why Your Thermal Interface Material May Not Be as Conductive as You Think: Exploring Thermal Conductivity & Measurement Techniques

If you’re an electronics designer or assembler addressing thermal management issues, safety is likely your number one concern. When selecting a thermal interface material (TIM) to dissipate heat, you probably started by comparing the thermal conductivity (K) of your options. But did you know that manufacturers’ advertised values of thermal conductivity can be widely inaccurate?

We found that out for ourselves when evaluating a range of TIM samples from various manufacturers. In our own tests, we found significant discrepancies in the thermal conductivity of the TIM samples that we tested versus the values stated in their literature. It turns out that the manner in which the materials are evaluated can make a big difference in the resulting conductivity values. The potential impact on end use applications could be a real safety hazard.

Before we drill down on the details, let’s quickly review some basics.

What Kind of Thermal Interface Materials (TIM) Are We Talking About?

Thermal Interface Materials, or TIM, are any materials that can be inserted between two parts in order to enhance the thermal coupling between the two components, typically for heat dissipation. There are many kinds of TIM, including thermal grease, thermal adhesive, thermal gap fillers, thermally-conductive pads, thermal tape, phase-change materials and metal TIMs. 

For our testing, we evaluated thermal conductivity pads similar to, and including, Rogers’ own ARLON Secure® Silicone Thermal Transfer Adhesives, composites of silicone polymer and functional additives formulated to deliver a unique set of thermal, physical and electrical properties.

What is Thermal Conductivity? 

Thermal conductivity is the most recognized and referenced thermal property of a TIM material because it expresses the intrinsic ability of a material to conduct heat. As a bulk or absolute property, thermal conductivity (K) doesn’t change with the size or shape of the material. Thermal conductivity is measured in watts per meter-kelvin (W/(m·K)) and is defined as the time rate of heat flow, under steady state conditions, through unit area, per unit temperature gradient in the direction perpendicular to the area.

The K of materials plays a significant role in the cooling of electronics equipment. From the die where the heat is generated to the cabinet where the electronics are housed, conduction heat transfer and, subsequently, thermal conductivity are the integral components of the overall thermal management process. The question of accuracy of thermal conductivity measurement, however, can be an issue as you will see as we continue our exploration.

Measuring Thermal Conductivity of TIM Materials

The industry standard for characterizing thermal interface materials is ASTM D5470 (updated to ASTM D5470-12 in 2012), which defines devices that use two metal bars (meter bars) and are placed between a heater and colder source separated by the specimen to be tested. The test method permits latitude in the basic design while defining the smoothness of the meter bars in an attempt to minimize interfacial impedance between the meter and the specimen, and improve machine-to-machine agreement. Limitations and modifications to ASTM D5470 have emerged to improve the accuracy of TIM measurements and to increase measurement precision.

In our lab, we tested thermal conductivity using two state-of-the-art processes: C-Therm Tci and a TIM tester. Both testing methods are influenced by testing parameter of pressure and thickness.

  • C-Therm employs the most up-to-date technique of Modified Transient Plane Source. The test is accurate, fast and easy. 
  • The TIM Tester process is well-defined and well-suited for measurement of thermal interface materials used in electronic packaging. It measures bulk thermal conductivity in materials having moderate-to-high thermal conductivity.

Our Surprise Findings

We tested 5 competitive high thermal conductivity pads. Our results yielded thermal conductivities ranging from 44% to 74% lower than the thermal conductivity reported on manufacturers’ data sheets. 

We have no way of knowing whether the differences were due to the latitude allowed in the ASTM standard, or variations in vendor equipment. We do know that test results for thermal conductivity varied widely. Also disconcerting, vendors of the materials we evaluated did not typically cite what method they used to measure conductivity, so this leaves designers without any information to help determine accuracy or to compare against other materials.

What Does This Mean for You?

We know that safety is a major concern for designers. At Rogers Corporation, we are developing and evaluating our testing processes to ensure you the highest accuracy of the data we publish. Based on our testing and evaluation, we see that is not always the case in the market. We encourage designers to ask tough questions, understand testing techniques and work with suppliers they feel they can rely on for accurate and consistent data to back up their product claims.

Next in our Series…

In our next blog in this series, we will look at two thermal properties that can be better “real-world” measures for designers than thermal conductivity when comparing TIMs for a specific application. Subscribe to Rogers’ blog to make sure you don’t miss future topics of interest.

To learn more aboutARLON Secure Silicone Thermal Transfer Adhesives, visit our website.

 

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