Chanyang Choe

Heat-resistant die-attach with cold-rolled Ag sheet

This work introduces a bonding technology that employs a cold-rolled Ag sheet, which provides a unique reaction in air, to achieve low-pressure and low-temperature die-attachment for wide-bandgap semiconductors. The cold rolling of the Ag sheet induces grain refinement down to a sub-micron level with high residual stress, which is the driving force for hillock growth on the Ag sheet, resulting in intimate bonding. The detailed mechanism underlying the hillock joining was investigated via transmission electron microscopy (TEM). TEM images and electron-diffraction patterns showed that a large quantity of self-generated Ag nanoparticles formed in the contact area between two Ag substances.

Influence of thermal exposure upon mechanical/electrical properties and microstructure of sintered micro-porous silver

Abstract Silver sinter paste has been considered as promising joint materials for SiC and GaN power modules due to its excellent thermo-stability and high electrical/thermal conductivity. However, sintered Ag has experienced mechanical/electrical properties deterioration and microstructural variations under high temperature exposure. It is strongly related with reliability of power modules. Thus, we evaluated thermal exposure effects on the mechanical properties, electrical resistivity and microstructure of sintered micro-porous silver specimens under high temperature exposure of 250u202f°C. Silver paste was printed in a specimen shape of a tension test, and was sintered at 250u202f°C for 1u202fh in air with no pressure. The sintered silver was exposed for 0, 50, 200, 500, and 1000u202fh at 250u202f°C. Tensile strength and electrical resistivity were obtained at each aging stage. The tensile strength decreased and again recovered during high temperature exposure, while the electrical resistivity decreased. For microstructure evolution, sintered silver grains, porous structure and fracture surface were characterized by SEM and EBSD. The relationship between microstructural variations and mechanical/electrical properties was discussed in detail.

Development of thermal shock-resistant of GaN/DBC die-attached module by using Ag sinter paste and thermal stress relaxation structure

Abstract GaN die-attach/DBC substrate joint structure bonded by Ag sinter paste/W thin film/Ag sinter paste sandwich die-attached layer was designed and its high-temperature reliability was investigated. GaN chips were bonded to DBC substrate using Ag sinter paste with thin W film at 220u202f°C and at 0.4u202fMPa for 60u202fmin. The joints structure was subjected to thermal shock testing in temperature range of −50 to 250u202f°C for duration up to 1000u202fcycles. The initial average die shear strength of the GaN/DBC joint structure was about 30u202fMPa, and was maintained up to 1000u202fcycles. These results reveal that Ag sinter paste/W thin film/Ag sinter paste sandwich die-attached layer can provide a superior thermal shock resistant. The microstructure evolution of GaN/DBC joint structure during the thermal shock testing was observed by FE-SEM and EDX analysis. In addition, FEM numerical simulation has also been implemented in this study, which revealed that the GaN/DBC joint structure with the sandwich die-attached layer shows a stress-shielding effect from the external force, and thus achieves a reliable power module structure for wide-bandgap semiconductors for its application in high-temperatures.

Printed wire interconnection using Ag sinter paste for wide band gap power semiconductors

This study reports a novel high-temperature and high-current wire interconnection by printing Ag sinter paste for WBG semiconductor power devices. The Ag sinter paste wire interconnection (APW) was successfully fabricated connecting two Cu electrodes coated by Ag. Both the high-temperature reliability and the high-current reliability of the APW were evaluated by aging test at 250xa0°C for 1000xa0h and by electro-migration (EM) test at 2.2u2009×u2009104 A/cm2 for 2000xa0h. In the aging, a resistivity of about 4.1u2009×u200910−6xa0Ωxa0cm was achieved, which further decreased to 3.4u2009×u200910−6xa0Ωxa0cm after aging for 1000xa0h. This reduction can be attributed to further sintering of the sintered Ag network. The initial shear strength was 17xa0MPa, which, while high enough, was maintained even after an aging test of 1000xa0h. In the EM test, the resistance of APW was not changed significantly up to 1000xa0h and then only slightly increased at 2000xa0h. This only slight change in resistance demonstrates greater stability than that of conventional wire interconnection material such as Al and Cu under high temperature and high current density. The influence of EM on the interface between APW and a Cu substrate was observed by scanning electron microscopy with EDX. The change of resistance of the APW was induced by the formation of Cu oxide at a cathode side, which leads to a crack extension along the interface between APW and a Cu substrate. Thus, the results obtained in the present work show the APW as a promising new alternative to wire bonding technology for WBG power electronic devices.

Low-stress design of bonding structure and its thermal shock performance (− 50 to 250 °C) in SiC/DBC power die-attached modules

Low-stress design of bonding technology with a sandwich structure of sintered Ag and tungsten (W) thin film was developed for SiC power die-attached modules. The die-attached bonding layer was designed as sintered Ag/W/sintered Ag structure. Experiment results show that the initial bonding strength was larger than 65xa0MPa for this die-attached structure and larger than 35xa0MPa with a thermal shock test from −u200950 to 250xa0°C for 1000 cycles. These results are largely better than that almost all sintering Ag technology reported in previous studies. Furthermore, the sandwich structure also compared with the sintered Ag structure which just using sintered Ag paste as bonding layer. The thickness of Ag paste is set as 100, 200 and 500xa0µm in the sintered Ag structure. The results show that the initial bonding strength of sintered Ag structure was about 60–70% of the value of W sandwich structure and about one-third of that after 1000 cycles. X-ray and SEM observation revealed that sandwich structure significantly decreased the size of crack extension in the sintered Ag layer during the thermal shock test. Finite element analysis reveal that the shear stress at the pore location of sandwich structure decreased to almost half values of the sintered Ag structure with the thickness of sintered Ag of 500xa0µm, and decreased almost 20% compared with the thickness of sintered Ag of 100xa0µm. The bonding technology with the W sandwich structure should be an attractive for low stress design in SiC power die-attached modules, which significantly increased its function for long-term high temperature applications.

Thermal effect on material properties of sintered porous silver during high temperature ageing

Thermal ageing of micro-porous sintered silver was examined under high temperature exposure. Silver sinter paste was printed in a shape of a tensile specimen on a Cu substrate, and was sintered at 250 °C for 1 h in air with no pressure. Specimens were exposed for 0, 50, 500, and 1000 h at 250 °C. Tensile strength was obtained at each ageing stage at a strain rate of 1.0×10−5/s. The evolution of microstructure was characterized by SEM/EBSD. The relationship between ageing microstructure and tensile strength was discussed. TEM observation was performed to explain the porosity reduction process of micro-porous structure.