Perfect Dosing of Thermal Interface Materials

What are the benefits of WACKER thermal interface materials (TIM or thermally conductive materials)? In what way is abrasiveness related to functionality? And what challenges do these materials pose for dosing technology? These questions are answered by Dr. Markus Jandke, Technical Manager Electronics at WACKER.

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.

"Efficient thermal management is crucial to prolonging the lifetime of essential electronic components."

Dr. Markus Jandke, Technical Manager Electronics at Wacker Chemie AG

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.

"With regard to new products, it is essential, to test dosability thoroughly on industrial-scale equipment. This offers key solutions for customers, a perfectly between material and mass production processability."

When it comes to application, what are the dosing limits in your experience? Why can’t we simply add more filler, which would be necessary in order to increase thermal conductivity?

Greater interaction between the fillers and matrix lead to higher viscosity and has a detrimental effect on processability. This only plays a minor role if filler loading is small. In high-TC materials these effects become critical for feeding the material from the packaging and into the dispensing head. They also affect the dispensing performance (ml/sec) of a given equipment and the bonding forces of mechanically sensitive substrates.

Due to these particularities, for thermally conductive materials, the focus must be specifically on their processing properties. SEMICOSIL® TC is the result of years of intensive research and product development. But the material alone is not sufficient.

Do you think close collaboration with dosing equipment manufacturers can provide solutions for customers by providing a perfect alignment between the material and its processability in mass production?

This is an absolute must! Customers run mass production processes for their electronic parts. Since the TC material formulation has an impact on the process and vice versa, it is essential for the material and dosing equipment manufacturersto work together closely. With regard to new products, it is essential, that the dosability has been tested thoroughly on industrial-sized equipment. This offers key solutions for customers, a perfectly aligned combination of material and mass production processability.

Interested in SEMICOSIL® Products?

WACKER offers a broad range of SEMICOSIL® grades for different application areas. You will find more details about the products outlined above and other SEMICOSIL® grades in our product portal.

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