Fabrication And Characterization Of Ag-Al Die Attach Material For Sic-Based High Temperature Device

An Ag-Al nanopaste for high temperature die attach applications on SiC power devices has been developed. The Ag-Al nanopaste was studied by varying the Al weight percent in the Ag matrix as well as the organic additives content. The Ag-Al nanopaste was sintered in open air at 380°C for 30 minutes to burn off the organic additives, causing Ag and Al nanoparticles to undergo solid-state fusion. The sintered Ag-Al die attach material’s physical, thermal, electrical and mechanical attributes were examined. X-ray diffraction studies revealed the formation of Ag2Al and Ag3Al compounds in the post-sintered nanopaste. The sample with 80% Ag and 20% Al weight percent content having a total nanoparticle content of 87.0% demonstrated the best electrical and thermal characteristics. Its melting point was 518 ± 1°C. Based on homologue temperature ratios of 0.67-0.85, the Ag80-Al20 die attach material can be used between 258.59°C to 400.18°C. It’s electrical and thermal conductivities were higher than those of solder alloys and conductive epoxies at 1.01 x 105 (ohm-cm)-1 and 123 W/m-K, respectively. The coefficient of thermal expansion was 7.74 x 10-6/°C, which is close to that of SiC and can help minimize thermal mismatch. The Ag80-Al20 sample also had the lowest porosity percentage at 19% amongst all samples and a density value of 6.42 g/cm3. The organic additives used in the nanopaste affected the creation of a dense die attach material as well as the mechanical attributes of the die attach material, i.e. the modulus of elasticity, hardness and stiffness. SiC die back metallization tests concluded that Ag and Au coatings gave the best joint adhesion strength between 28.9 – 38.1 MPa for high temperature power device applications. In essence lower organic additives content improved the attributes of the die attach nanopaste

Power Module Packaging in Automotive Applications

In this study, nano silver paste was used as die attach material with the aim of increasing reliability of joints in power modules in automotive applications. Prior to joining, nano silver paste was spread on the interface between silver coated copper substrates and dummy chips by screen printing method. 5 groups of samples were produced using three different joining techniques based on different combinations of ultrasonic force and persistent pressure in air and vacuum atmospheres. The bonding quality of the interface region was evaluated by microstructural examination and quasi-static shear tests. On the other hand, electrical properties of sintered nano silver particles within the joints were characterized through resistivity measurements. Sintered nano silver regions in all samples exhibited two types of porosity, namely, macro and micro porosity. Macro pores formed during the evaporation and removal of organics present in the starting paste, while micro pores were left in the structure because of insufficient sintering of silver nano powders. Although the sintered silver interface in samples produced using 5 MPa persistent pressure in air displayed a minimum amount of porosity, pores as large as 5 m in diameter were observed in joints produced in air by a preload of 0.01 MPa with or without ultrasonic force. In addition, vacuum sintering yielded relatively porous interfaces compared to samples manufactured in air even though the same compaction pressure was applied during sintering. Accordingly, in the samples produced either in air by the application of low preloads of 0.01 MPa or in vacuum at 5MPa, additional microcracks were formed, particularly in the interface region between silver coating and sintered nano silver particles. Stress-strain curves of the joints exhibited linear elastic, small strain hardening and fracture regions similar to wrought alloys. The strengths of the joints increased proportionally to the degree of sintering as expected. The shear strength reached to 32 MPa in samples sintered in air at 5 MPa constant pressure, whereas shear strength decreased to 4 MPa in highly porous joints produced by ultrasonic force and preloading with 0.01 MPa. All samples revealed shear-type dimples in the direction of mechanical testing indicating ductile behavior of joints. The electrical resistivity of the sintered nano silver layer showed the same trend as the mechanical properties. The weakest or most porous joint had the highest electrical resistivity of approximately 125.5 μΩ-cm). On the other hand, the least porous silver joint, manufactured at 5 MPa constant pressure in air exhibited the lowest electrical resistivity (7.8 μΩ-cm); however, it was five times higher than that of bulk silver. The results have presented that the nano silver paste is the most promising die attach material to replace conventional solder and conductive epoxies

High power gallium nitride micro-electronics: thermal management using microfluidics

Last four decades have seen unprecedented development in communication, defence, electronics, and computing technologies. The increased power density plus the miniaturisation of the device present challenges in managing the high heat flux in the microchip level. Besides, the highly heterogeneous heat flux in electronic devices presents more challenges to thermal management (TM). This calls for the development of more efficient cooling technologies for these high-power microelectronic devices. This PhD study aims to address this challenge by developing the high performance of heat transfer fluids (HTFs) and compact cooling devices. Gallium Nitride (GaN) based transistors which acted as inhomogeneous high heat flux output were targeted in this work. The work involves formulating, characterisation and performance measurements of various heat transfer fluids (including base fluids and nanofluids), design, fabrication and assemble, and package and experiments of microfluidics including foam metal, micro-jet impingement. Both experimental work and modelling were performed and the following main conclusions were obtained. • Heat transfer fluids study Two types of nanofluids were formulated and investigated for the application in room temperature and the low temperature. The BN/DI water nanofluids used in the room temperature shows 5.2 % enhancement in the thermal conductivity compared to the base fluid for the 0.5 wt.%. The other material rGO/EG+DW nanofluids used for the temperature as low as -50 ℃ has 17 % thermal conductivity increase with the concentration of 2.0 wt.%. This suggests that the nanofluids can have a better thermal performance for the microfluidic channel than the base fluids. • Performance of the microfluidics With the experimental comparison of the copper-foam based microfluidic channel and the micro-jet channel, the micro-jet channel was chosen due to a higher heat transfer coefficient. Both base fluids and nanofluids were experimental tests and the numerical simulation was validated with the micro-jet channel. The test showed that the BN/DI water nanofluids with a concentration of 0.5 wt.% can increase the heat transfer coefficient 5 % compared to the DI water. Meanwhile, the 2.0 wt.% rGO/EG+DW nanofluids showed a similar trend with an 11% increase in the heat transfer coefficient compared to EG+DW base fluid. The direct measurement of the temperature with Raman thermography was used to measure the temperature in the finger of the die. The experiment test suggests that with the target power density of 5 W/mm in the finger (1×10⁷ W/mm² in the finger), the peak temperature in the devices was 120 °C far below 200 °C. The thermal resistance for the jetting channel was 19.76 °C/W. The device used in the experiment was GaN-on-SiC. For the GaN-on-Diamond, a higher power density can be obtained. Thus, for the thermal management of the GaN devices, the nanofluids, material selection for the devices thermal package and micro-jet channel play important roles once the specific GaN transistors are selected