Reliability Analysis of a Microinverter Prototype: A Case Study on Sn-Ag-Cu Solder Under Thermal Fatigue for Encapsulated GaN Packages

Abstract

Abstract. A single-stage dual-active-bridge direct current-alternating current (DC-AC) micro-inverter prototype with Gallium Nitride (GaN) primary switching devices was assembled and subjected to environmental reliability testing. Testing consisted of an unpowered temperature cycling with an amplitude ranging from −40 °C to 105 °C. Electrical performance characterization was performed during the accelerated testing at regular intervals until a failure was observed. Nondestructive failure analysis was performed on the board to determine the failure modes and mechanisms, and it revealed that the failure occurred due to solder bump delamination at the gate, source, and drain pads. To better understand the failure mechanism and predict the fatigue life of the assembled GaN field effective transistors (FET), thermo-mechanical simulation was performed using finite element analysis (FEA), where the submodeling feature was used to track the location of the GaN device on the board, and the average strain energy accumulation at the critical solder joint for each set of temperature cycles was simulated. Garofalo creep model was used to capture the creep deformation during thermal cycling, and Syed's Energy-based fatigue model for SAC305 solder was used to determine the life of the land grid array (LGA) package. The model constants were calibrated using the experimental failure time. This study was then extended to examine the effects of encapsulation materials on the life of the primary devices. The results reveal that encapsulation significantly increases the fatigue life of the Gallium Nitride field effective transistors (GaN FET) for all encapsulation materials.