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|Power cycle tests of high temperature Ag sinter die-attach on metalized ceramic substrate by using micro-heater SiC chip|
|Keywords: Power cycle test, SiC, Ag paste|
|Improvements of power conversion efficiency in electric systems has been developed by optimizing circuit and device designs with Si for decades. However, conventional Si devices and packaging materials including die- attach solders inherently limit their performances at high temperature or high voltage/current systems. Due to the operation temperature limitation, wide-bandgap (WBG) semiconductors like SiC or GaN emerge in industrial applications like trains and automobiles demanding higher temperature and power with a proper heat-resistant assembly structure. These next-generation semiconductors are expected to solve the electric energy conversion loss issues, and can widely be used in various applied electronic industries once a durable packaging system is suitably established to the advantageous characteristics of WBG semiconductors such as high electric density, voltage, frequency, and high junction temperature, as well. The packaging system of these power devices requires high reliability for these extreme conditions in various operation environments. Power cycle testing is one of the useful evaluation methods to prove the dependability of a device in a similar conditions to those in practical operation. In power cycling of switching devices, the electronic packaging materials experience stress from repetitive heat by the on-off electrical current switching because the module structure consists of layers of interconnection materials with different thermal and mechanical properties. The mismatching thermal and mechanical properties is hence critical to reliability degradation of a system, and cause accelerated failures by the power cycle tests with repetitive exposure to high temperature similarly to an actual operational environment. Furthermore, the heat of power modules is typically generated at the surface of a device chip and flows through various heat conduction paths, affecting reliability of the die-attach joint and other interconnections. Since the major heat path is through the die- attach of the device chip to the circuit substrate, the thermo- mechanical characteristics of an metalized circuit board with a cooling system need to be scrutinized under the stress power cycles. For the purpose described above, thermal resistance of the die-attached structure with metalized ceramic substrate is accurately measured by employing newly developed micro heater chip made of SiC single crystal, simulating effectively the heat generation in a SiC device. The optimized heater and temperature probe is formed with Pt thin-films deposited on an insulated SiC chip with dimension of 5 mm x 5 mm x 0.35 mm. The micro- heater chip can generate high heat exceeding 250 W capable to simulate the loss of very high power devices, with precise measurements of the chip surface temperature by using well- calibrated electric resistance of built-in Pt probe sensor. The high heat density of the heater chip creates a large temperature gradient simulating a power module with cooling system, and the high heat flux through the die- attach allows us to measure the heat resistance of the ceramic substrate in high resolution. In addition, the micro-heater SiC chip is applied to the power cycling tests of the various sample die-attach structures with different types of metalized ceramic substrate, e.g. Si3N4 or AlN, and die-attach materials; in particular, comparison is made between Ag sinter paste, Pb-Sn high-temperature solder, and Zn based Pb-free solder. The Ag sinter die-attach is processed at 250 C in air under a slight loading of 0.4 MPa. The Pb-Sn and Zn Pb-free solders are reflowed in a vacuum furnace using formic acid with appropriate temperatures over 300 C. These samples of die-attach are fixed on a water cooling system maintained at 25 C, and intermittent current maximum 2 A is injected to generate heat flux and high temperature at the chip surface. Thus the present study systematically evaluates power cycle reliability of a SiC device packaging aimed for high power and high temperature operations. Our method of power cycle testing with advanced micro-heater SiC chip may explore in evaluation and testing of future packaging systems with WBG semiconductors targeted for high- temperature/energy power modules.|
|Dongjin Kim*, Ph.D candidate
The Institute of Scientific and Industrial Research, Osaka University