Date of Graduation

12-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

David Huitink

Committee Member

Meng, Xiangbo

Second Committee Member

Ware, Morgan

Third Committee Member

Hu, Han

Keywords

accelerated testing, high temperature, packaging, power electronics, reliability

Abstract

With electrification progressing across many sectors including industry, automotive and aerospace, the power density requirements are changing. The increased power density results in higher and higher ambient temperatures that electronics are exposed to. The response has been to move towards wide bandgap (WBG) semiconductor devices that can withstand much greater temperatures and can operate at much higher voltages than silicon. Additionally, these WBG devices deliver low drain-source on resistance (RDS_on) capabilities, enabling high current power modules that increase power density even further. This also requires the packaging to evolve in order to withstand the new requirements. As a result, researchers are looking beyond the traditional solder materials for die attach towards options more suited for high temperatures. These reliability-critical die attach applications provide an excellent opportunity to use silver, which has traditionally been held back by its high cost. Silver has a high melting point of 961 ℃, but fine particles of it, in sub-micro or -nano range, can be sintered at a temperature significantly below the melting point. This sintered silver forms a continuous network of silver that has a melting temperature approaching that of bulk silver. While this high melting temperature implies great potential, its actual reliability at those elevated temperatures is not entirely understood. This study aims to advance the implementation of sintered silver in electronic packages by investigating the reliability of sintered silver die attach at elevated temperature, under static and dynamic stressing conditions. First, the degradation at high temperatures is tested by high temperature storage tests. Two different bonded substrates result in different degradation rates in air. CuMo, which is tested in addition to copper as a CTE-matched packaging material to SiC, show slightly greater degradation in air than Cu, especially above 175 ℃ temperature, but in both cases the degradation was significantly reduced by removing oxygen particles from the air. In order to test the reliability under dynamic conditions, a four-point bend setup was built that allowed for time-efficient reliability testing. This allowed for testing at repeated mechanical loading conditions, as well as allowing for a time-efficient method of fatigue testing than traditional thermal cycling. The setup was used to test board-level sintered silver die attach samples for multiple applied mechanical strains and thermal conditions, allowing for an empirical relationship to be developed. The correlation can provide a critical design tool for design engineers to predict reliability depending on mechanical and thermal conditions. To demonstrate the translatability of the mechanical cycling test to a thermal cycling scenario, a finite element model was used to relate the experimentally obtained failure times to the strain energy density for each of the test conditions. A correlation was then developed that predicts the reliability based on the strain energy density, regardless of the type of loading, assuming that interfacial crack propagation is the failure mechanism. It was demonstrated that the mechanical test bench can run 900 cycles during the same time period that one cycle can be run in a typical thermal cycling test.

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