Interview: Thermal Interface Materials Perfectly Dosed - Wacker Chemie AG

Interview: Thermal Interface Materials Perfectly Dosed

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 does the dosing technology face with these materials? A couple of questions answered by Dr. Markus Jandke, Technical Manager Electronics at WACKER.

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

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. Lifetime of essential electronic components crucially depends on efficient thermal management. Our SEMICOSIL® TC products meet our customers’ needs in high thermal conductivity and lowest thermal resistance – including outstanding processability.

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

With respect to the chemical structure: our heat conductive silicones typically represent dielectric materials. In general the formulation contains specific fillers and a silicone formulation which is tailor-made to optimize product and processing properties. While pastes generally do not crosslink, adhesives and gap fillers do. Thermally conductive adhesives are used to maintain intimate thermal contact by building up adhesion between thermal substrates at elevated temperature or at moderated temperatures. For tolerance compensation of larger areas substrates gap fillers are used as thermal interface materials. 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 accomplished not until a significant filler loading (of e.g. oxidic particles in the 1-200 µm range) that allows contact between filler surfaces.
Sufficient particle contacts and related with that notable bulk thermal conductivity are only reached at very high filler loadings. This is referred to the term “percolation”.

Which further requirements are set on thermal interface materials?

Particular emphasis has also to be put on the stabilization of the interaction of filler and matrix.
Due to the different thermal expansion coefficients and behavior of the involved substrates the interface area is also mechanically stressed during long-term operation of the electronic devices (load cycling, temperature changes). In that 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 minimum and stable thermal resistance of the TIM material is targeted. Since – even at 3-4 W/mK- TIM materials represent the bottleneck in thermal conductivity in the device configuration (involved substrates exhibit much higher thermal conductivities) the thermal interface layer should be selected as thin as possible by design.

To transfer heat, the filler particles must be in contact. For instance, today’s 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.
There are a vast numbers of chemical parameters that have to be ideally aligned (filler shape, particles size, surface chemistry and type, molecular design and functionality of silicone polymers) to allow for a thermally conductive formulation that offers e.g. bulk conductivities beyond 3 W/mK and concomitantly still the suitable rheology for low abrasivity, high feeding and dispensing performance as well as low bonding forces.

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

An increased level of interaction between 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 decisive for feeding the material from the packing and for feeding into the dispensing head. Also the dispensing performance (ml/sec) of a given equipment and the bonding forces of mechanically sensitive substrates are affected.

Due to these particularities, the specific focus of thermally conductive materials has to be set on 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 collaborations with dosing equipment manufacturers offers key solutions for customers perfectly aligned combination of material and mass production processability?

This is absolutely a must!

Customers run mass production processes for their electronic parts.
Since TC material formulation has an impact on process and vice versa, close collaboration between material and dosing equipment manufacturers are needed. 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.