Ibaad Gandikota, Aniket Bharamgonda, Abhijit Dasgupta, and Patrick McCluskey
Mechanical Engineering Department, University of Maryland, College Park, MD, USA
For more information about this article and related research, please contact Prof. Patrick McCluskey.
Abstract:
A single-stage dual-active-bridge DC-AC microinverter prototype board with Gallium Nitride (GaN) primary switching devices was designed and assembled for environmental reliability testing. An unpowered temperature cycling test was conducted with an amplitude ranging from −40 to 105°C at a ramp rate of 5°C/min. The electrical testing revealed a failure in the board between 800 and 1000 hours, which was located at the primary devices. Non-destructive failure analysis was performed on the board to determine the failure modes and mechanisms. Using a Confocal Scanning Acoustic Microscope (C-SAM) with a 75MHz transducer, it was determined that failure was located at the solder bumps, and the scans revealed that two of four primary devices, EPC2001C, experienced a solder bump delamination at the gate, source, and drain pads. Hence, it is concluded that the delamination occurred due to coefficient of thermal expansion (CTE) mismatch between substrate and die.
To understand the failure mechanism and predict the fatigue life of the assembled GaNFET, Finite Element Analysis (FEA) simulation was performed using ANSYS structural models, where the sub-modeling feature was used to track the location of the GaN device on the board, and a total of five temperature cycles were simulated to determine the average strain energy of the critical solder joint. The Garofalo creep model was used to capture the creep deformation during thermal cycling. Syed’s Energy-based fatigue model for SAC305 solder was used to determine the life of the land grid array package assembly of the EPC2001C, and the model constants were calibrated using the experimental failure time. Furthermore, this study was extended to examine the effects of encapsulation materials on the life of the primary switching devices. The encapsulation materials were studied in three distinct configurations: solely potting, solely underfill, and a hybrid combination of potting and underfill. The results reveal that the encapsulation materials significantly increase the fatigue life of the GaNFET.
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