Thermal Stability of Al2O3/Silicone Composites as High-Temperature Encapsulants
MetadataShow full item record
Conventional microelectronic and power electronic packages based on Si devices usually work below 150C. The emergence of wide-bandgap devices, which potentially operate above a junction temperature of 250C, results in growing research interest in high-density and high-temperature packaging. There are high-temperature materials such as encapsulants on the market that are claimed for capability of continuous operation at or above 250C. With an objective of identifying encapsulants suitable for packaging wide-bandgap devices, some of commercial high-temperature encapsulants were obtained and evaluated at the beginning of this study. The evaluation revealed that silicone elastomers are processable for various types of package structure and exhibit excellent dielectric performance in a wide temperature range (25 - 250C) but are insufficiently stable against long-term aging (used by some manufacturers, e.g., P2SI, to evaluate polymer stability) at 250C. These materials cracked during aging, causing their dielectric strength to decrease quickly (as soon as 3 days) and significantly (60 - 70%) to approximately 5 kV/mm, which is below the value required by semiconductor packaging. The results of this evaluation clearly suggested that silicone needs higher thermal stability to reliably encapsulate wide-bandgap devices. Literature survey then investigated possible methods to improve silicone stability. Adding fillers is reported to be effective possibly due to the interaction between filler surface and polymer chains. However, the interaction mechanism is not clearly documented. In this study, the effect of Al2O3 filler on thermal stability was first investigated by comparing the performance of unfilled and Al2O3-filled silicones in weight-loss measurements and dielectric characterization. All test results on composites filed with Al2O3 micro-rods indicated that thermal stability increased with increasing filler loading. Thermogravimetric analysis (TGA) test demonstrated that the temperature of degradation onset increased from 330 to 379C with a 30 wt% loading of Al2O3 rods. In isothermal soak test, unfilled and 30-wt%-filled silicones lost 10% of polymer weight in 700 and 1800 hours, respectively. The dielectric characterization found that both Weibull parameters, characteristic dielectric strength (E0, representing the electric field at which 62.3% of samples are electrically broken down) and shape parameter (beta, representing the spread of data. The larger the beta, the narrower the distribution) can reflect the thermal stability of polymers. Both of them were influenced by microstructure evolution, to which beta was found to be more sensitive than E0. The characteristic dielectric strength of unfilled silicone decreased significantly after 240 hours of aging at 250C, whereas that of Al2O3/silicone composites exhibited no significant change within 560 hours. The shape parameter of Al2O3-filled silicone decreased slower than that of unfilled silicone, also indicating the positive effect of Al2O3 micro-rods on thermal stability. Improved thermal stability can be explained by restrained chain mobility caused by interfacial hydrogen bonds, which are formed between hydroxyl groups on Al2O3 surface and silicone backbone. In this study, the effect of hydrogen bonds was investigated by dehydrating Al2O3 micro-rods at high temperature in N2 to partially destroy the bonds. Removal of hydrogen bonds impaired thermal stability by increasing the initial weight-loss rate from 0.025 to 0.036 wt%/hour. The results explained the importance of interfacial hydrogen bond, which effectively reduced the average chain mobility, hindered the formation of degradation products, and led to higher thermal stability. The main discoveries of this study are listed below: 1. Al2O3 micro-rods were found to efficiently improve the thermal stability of silicone elastomer used for high-temperature encapsulation. 2. Characteristic dielectric strength and shape parameter obtained from Weibull distribution reflected the change of material microstructure caused by thermal aging. The shape parameter was found to be more sensitive to microscale defects, which were responsible for dielectric breakdown at low electric field. 3. Hydrogen bonds existing at filler/matrix interface were proven to be responsible for the improvement of thermal stability because they effectively restrained the average chain mobility of the silicone matrix.
- Doctoral Dissertations