Most electronic devices function more reliably at lower temperatures. However, as electronics develop smaller footprints and are packed closer together, higher heat densities will inevitably lead to elevated operating temperatures. Several issues are associated with higher operating temperatures. For example, the service life of the device or module is significantly decreased. Device interconnects and other components are more likely to experience fatigue from thermal stress. When accompanied by moisture, high temperatures can spur whisker or dendrite growth within a device. Whiskers and dendrites may form conducing lines that create shorts and eventually cause device failure. Thermal interface materials (TIMs) can help avoid failures associated with elevated device operating temperatures. Several options must be considered when choosing the best TIMs for a specific application.

Tip 1: Know the TIM properties.
When choosing a TIM for heat dissipation, one tendency is to select the material displaying the highest bulk conductivity. When considering bulk conductivities of different TIMs, note that the reported value applies only for the specific test method and test conditions employed to obtain that number. Therefore, similar test methods must be used for valid comparisons. Bulk conductivity is not the only property by which to choose a TIM. The heat removal capacity of a TIM is also determined by the TIM’s ability to create an intimate contact with the relevant surface. As a result, the contact resistance is an important property to note. Thermal resistance is another property to consider. Thermal resistance is measured by the temperature difference across the interface of a material per watt of energy moving across the said interface. Some data sheets may provide the thermal impedance, which is the thermal resistance normalized by a unit of area. Heat flows best across TIMs that have low thermal resistance.

Tip 2: Know the TIM test standards and caveats associated with these standards.
Most TIM data sheets will report the common industry or military standards to which the materials should comply, including ASTM D5470-01 and MIL-I-49456A. Unfortunately, many manufacturers are unaware of the challenges associated with building reliable test equipment. In fact, independent surveys have shown that repeatability errors within a given lab lie between 10% and 20%. As much as 40% error exists when comparing reproducibility between different labs. Because data reproducibility depends heavily on test equipment, TIM purchasers should conduct independent tests to ensure that materials comply with the given standards.

Tip 3: Know the common TIM options.
There are several different types of TIMs, each containing different advantages. Wet dispensed TIMs may be more appropriate for some applications, while pad or film TIMs may be better for others. Wet dispensed TIMs include adhesives, encapsulants, gels, and non-curing compounds. Adhesives may eliminate the need for mechanical connectors and create intimate surface contact, resulting in low contact resistance. Encapsulants can take on any thickness and offer higher mechanical strength than other wet dispensed TIMs. Gels provide stress relief and are therefore useful when protecting fragile components. Other TIM categories include pads, gap fillers, and phase change materials. Pads are applied without dispensing or curing and therefore can be easily reworked. Gap fillers readily accommodate irregular surfaces with minimal pressure. Phase change materials have low thermal resistance, require low mounting force, and can accommodate irregular surfaces as well.

Tip 4: Know the specialty TIM options.
Some applications have unique demands and hence require specialized TIMs. Some specialty options include flame resistant materials, controlled bond-line thickness materials, and controlled volatility materials. Flame resistant TIMs contain UL 94V or HB flammability classifications. Controlled bond-line thickness TIMs help avoid component misalignment in applications containing tight alignment specifications. Controlled volatility materials circumvent challenges associated with low molecular weight volatiles that may seep from TIMs and negatively affect a device in the future.