Laser flash diffusivity and ASTM D5470 methods accurately measure conductivity and impedance, respectively. (See results for Indigo using these methods.) Another process named the IR camera method suffices for some solutions, but its low resolution and other limitations discourage assessment of thermal interface materials (TIMs) that are thin or wet surfaces. Laser flash, ASTM D5470, and IR camera methods are described below.

Laser Flash

The laser flash process irradiates a die-sized circular sample for about a millisecond or less from a xenon or laser flash source. Diffusivity is then directly calculated from the temperature rise versus time obtained from the back side of the material. Multiplying the specific heat, bulk density, and measured diffusivity of the sample produces the desired value of conductivity.

Short times, ease of sample preparation, and accuracy make the laser flash method a reasonable option. The system requires some ambient temperature control, but not so much as the IR camera method. This process also requires less skill to operate, and allows for a variety of realistic die sizes. Laser flash produces the best results with thermal materials in sample sizes similar to the actual applications.

ASTM D5470

For a thin thermally conductive material, the ASTM D5470 method is the standard for determining thermal impedance or conductivity. Given a known interface area and specific amount of heat, the temperature difference is measured and impedance calculated. A series of tests on multiple bond line thicknesses (BLTs) then allows the graphing of impedance versus thickness. Values at other BLTs are extrapolated from the graph.

As with the other methods, the data must be taken at operating temperatures above the liquidus (or melting) point. All heat applied must be well-insulated around the source in order to direct heat only through the sample. Additionally, the method requires careful control of pressure since pressure generally determines the BLT in the absence of sample shimming. The end state of constant heat differential must also be defined in advance, to the best time and temperature resolution possible by the set-up. Tests require close monitoring for that end state. See the official ASTM D5470 guidelines (purchasable ) for more details.

IR Camera

Another method uses an infrared (IR) camera to record the temperature difference across a junction. Copper and silicon sandwich the thermal interface material. A hot plate is affixed to copper, and a cold plate to silicon. Unlike the laser flash method, the IR camera gives a profile perpendicular to the interface surfaces; a side view of the sandwich.

Viewing heat differences along the edge of the interface would hardly be representative, however. To compensate, the interface is cut in half to access the center, which is polished and coated with paint to equalize the emissivity over the interface. Once heat flux becomes constant, an IR image displays temperature gradients. Given the known thermal conductivities of silicon and copper, it can provide the measured conductivity of the interface material.

The IR camera provides less accurate thermal performance measurements for two primary reasons. First, it has a maximum resolution of 4 mil, larger than the bond line thickness (BLT) of materials such as Indigo. Secondly, it cannot discern if conductivity has been reduced by contact resistance from voiding or poor wetting. Other disadvantages include the relatively high operation skill required, and an inherently slower procedure hindering multiple trials.