Perfect Dosing of Thermal Interface Materials

WACKER thermal interface materials are used in a variety of applications. What industry is the main driver?

WACKER provides thermally conductive adhesives, gap fillers and pastes as well as silicone-based potting compounds. Among other industries, automotive and electronic applications play a significant role for us. Efficient thermal management.is crucial to prolonging the lifetime of essential electronic components Our SEMICOSIL^® TC products meet our customers’ needs for high thermal conductivity and extremely low thermal resistance – as well as outstanding processability.

Tell us more about the typical structure of WACKER thermal interface materials. For example, how high is the solids content?

In terms of their chemical structure: our heat conductive silicones are typically dielectric materials. In general the formulation contains specific fillers and a silicone compound that is tailored to optimizing product and processing properties. While pastes generally do not crosslink, adhesives and gap fillers do. Thermally conductive adhesives maintain intimate thermal contact by building up adhesion between thermal substrates at elevated or moderate temperatures. Gap fillers are used as thermal interface materials for compensating the tolerances of large-area substrates.. Thermal contact is established by mechanical fixing of the substrates. Gap fillers cure at room temperature.

Unfilled silicones and silicone elastomers similarly to organics exhibit a bulk thermal conductivity of approximately 0.2 W/mK. Thermal conductivity is only achieved when the filler loading (of e.g. oxidic particles in the 1-200 µm range) is high enough to allow contact between the filler particle surfaces. Sufficient particle-to-particle contact points and, consequently, appreciable bulk thermal conductivity, are only obtained at very high filler loadings. This is referred to by the term “percolation”.

What are the other requirements for thermal interface materials?

It is also important to stress the stabilization of the interaction between filler and matrix. Due to the different thermal expansion coefficients and behaviors of the substrates involved, the interface area is also mechanically stressed during long-term operation of the electronic devices (load cycling, temperature changes). In this respect the materials have to avoid delamination or cracks in TC adhesives / gapfillers or dry out / bleeding out of filler and polymer in TC pastes.

Overall, the TIM materialshould show minimum, stable thermal resistance. Even at 3-4 W/mK- TIM materials represent the bottleneck in thermal conductivity within the device configuration (the other substrates exhibit much higher thermal conductivities); consequently the thermal interface layer should be made as thin as possible by design.

To transfer heat, the filler particles must be in contact with one another. For instance, modern technically and economically interesting filler systems are based on oxidic structures which partially exhibit high hardness. Resulting from the broad range of different particle geometries – from spherical to plate-shaped with sharp edges – there are differences in abrasiveness.

To produce a thermally conductive formulation that offers e.g. bulk conductivities beyond 3 W/mK while still having a rheology favoring low abrasion, good feeding and dispensing properties, as well as low bonding forces, it is necessary to balance vast numbers of chemical parameters. These include filler shape, particle size, surface chemistry and type, molecular design and the functional groups on the silicone polymers.