The use of silicon has skyrocketed along with the development of advanced electronic devices over the past few decades and it is in these types of devices where silicon has excelled for its use in semiconductors. Six silicone materials were prepared by Dow Corning, consisting of a polydimethylsiloxane (PDMS) matrix, either filled or unfilled. The Hot Disk TPS technique was used to measure the thermal conductivity and thermal diffusivity of these silicone materials.
Thermal conductivity testing is important for materials that go into electronic devices, as each component of a device must be tuned so that the overall product works properly. A commonly used silicone in electronic devices is the previously mentioned polydimethylsiloxane (PDMS). PDMS is a versatile polymer, as it can be modified through the addition of different fillers which alter the thermal characteristics of the material. Through this method, different PDMS composites can be prepared, each with differing thermal properties.
Figure 1. Chemical structure of silicone polydimethylsiloxane (PDMS).
Research performed by Dixon, in conjunction with Dow Corning shows the versatility of PDMS in action. Dixon et al. examined six different silicone materials, each made up of PDMS and either some kind of filler, or no filler. Each of these six materials are representative of broad families of silicone materials used for different applications. The main outcome of the research was to determine the thermal conductivity and thermal diffusivity of the six silicone materials. This was accomplished through the use of the Hot Disk transient plane source (TPS) technique for the measurement of thermal conductivity, thermal diffusivity and specific heat.
In addition to the actual measurement of thermal properties, Dixon and Dow Corning wanted to show the ease with which the TPS technique can be used for the measurement of thermal conductivity and thermal diffusivity. With this in mind, the report investigates parameter optimization, such as selection of sensor size, measurement time and power output.
The six materials investigated include:
- Unfilled PDMS, partially cross-linked
- Unfilled PDMS, cross-linked to a soft elastomer
- Silica-filled PDMS
- Alumina-filled PDMS
- Proprietary filler in PDMS
- 2nd Proprietary filler in PDMS
Results were obtained by using the Hot Disk TPS method, which involves placing the sensor between two samples of the same material. See Figure 2 and 3 for TPS standard setup. Results for the measurements are presented in Table 1.
Figure 2. Paste is loaded into both sides of the sample cell.
Figure 3. Once the paste has been loaded into the sample cell, a TPS sensor is placed in between the two cell halves.
Table 1. Experimental results from transient plane source technique for the measurement of thermal conductivity and thermal diffusivity. λ is thermal conductivity, α is thermal diffusivity. The values are presented, as well as the standard deviation associated with the measurements.
This report both shows the high quality and precision of the transient plane source technique, as well as the versatility of PDMS as a silicone material. Through the use of different fillers, the thermal properties of PDMS were drastically altered. The measured value for the 2nd proprietary filler in PDMS resulted in a thermal conductivity of 4.52 W/m•K, which is over 25 times higher than the unfilled, partially cross-linked PDMS material.
Since each of the materials measured have different thermal, electrical and physical properties, they each have their own applications for industry. For example, the first two materials examined in this report (unfilled PDMS) find uses in the food and beverage industry as antifoam agents and surfactants. The silica-filled material finds use in electronic devices, specifically as a pottant, which is used to hold crystalline solar cells in place in their modules. The alumina-filled material is also commonly used in electronics, as a thermally conductive adhesive. Overall, the six PDMS materials investigated in this report find uses as components of automotive or general electronics, and also in Chip Scale Packaging.
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