A Double-side Cooled SiC MOSFET Power Module with Sintered-silver Interposers I. Design, Simulation, Fabrication, and Performance Characterization
Abstract : Planar, double-side cooled power modules are emerging in electric-drive inverters because of their low profile, better heat extraction, and lower package parasitic inductances. However, there is still concern about their reliability due to the rigid interconnection between the device chips and two substrates of the power module. In this paper, a porous interposer made of low-temperature sintered silver is introduced to reduce the thermo-mechanical stresses in the module. A double-side cooled half-bridge module consisting of two 1200 V, 149 A SiC MOSFETs was designed, fabricated, and characterized. By using the sintered-Ag instead of solid copper interposers, our simulation results showed that at a total power loss of 200 W, the thermo-mechanical stress at the most vulnerable interfaces (interposer-attach layer) was reduced by 42 % and in the SiC MOSFET by 50 % with a trade-off of only 3.6 % increase in junction temperature. The sintered-Ag interposers were readily fabricated into desired dimensions without post-machining and did not require any surface finishing for die-bonding and substrate interconnection by silver sintering. The porous interposers were also deformable under a low force or pressure, which helped to accommodate chip thickness and/or substrate-to-substrate gap variations in the planar module structure, thus simplifying module fabrication.
? The efforts presented in this section summarize admirable research to place SiC technology a cut above conventional silicon power modules in terms of performance.
? All the above approaches used existing packaging technologies and materials in novel ways, and employed good engineering practices to develop next generation power electronics modules.
? This approach sounds ideal and gives the sense of approaching virtually monolithic electronics modules. Concepts involving embedded systems with power die, passives, and interconnections within a single interposer block follow this approach.
? It is also possible, if desired, to integrate thermal management within the interposer, to compensate for the comparatively lower thermal conductivities of most existing interposer materials.
? Thermal cycling of the fabricated double-sided cooling power modules revealed some reliability issues attributed to thermal stresses of their multiple joints.
? This is possibly due to poor joints as the result of aluminum interconnection on these power devices. High thermal stress may also contribute to the failure of the power module.
? In order to eliminate the damage on the delicate die top surface when installing modules into the coolers, individual spring unit was used to contact the chip.
? These stresses cause detrimental damage to the power chips, substrates and other parts of the module if they are not properly managed.
• Some of the solutions that have been proposed to address these issues are using compliant mechanical materials like sintered silver as the die attach material.
• Most of the 3D packaging solutions proposed to date utilize Si devices. These propositions came at a time when Si technology was hitting a limit in terms of performance. Wire bondless integration offered hope from the standpoint of better signal integrity and reliability.
• There have been several attempts to provide better alternatives to wire bonding through 3D wire bondless technology. Although the industry has adopted a handful of these propositions, most of the proposed solutions exist only as proof-of-concept.
? The double-sided cooling is the trend for the power electronics packaging, with the potential ability to improve thermal performance, reduce parasitics of the power electronic modules.
? The base plate is removed from the module structure to reduce thermal stress induced by the bimetallic effect and the thermal resistance between junction and ambient.
? The double-sided DBC structure provides a mechanical balanced structure to enable double-sided cooling capability for a better thermal performance.
? In order to minimize the parasitic inductance and improve the thermal performance of the package, the top and bottom sides of the DBC are in direct contact with the power devices metal pads.
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