Abstract
Insulated gate bipolar transistor (IGBT) power module is a key component of numerous electronic devices functioning in several critical systems in electric vehicles (EVs), photovoltaic (PV) modules, and more/all electric aircrafts (MEAs/AEAs). They function as a key component of actuator devices in many of these systems. A key constituent of the module is solder joints which are IGBT-attach, Diode-attach, and Substrate solder joints. These solder joints, which are composed of 96.5% tin, 3.0% silver, and 0.5% copper (SAC305), are the weakest component of the device. During manufacturing of the module, process voids are formed in the joints. The joints also experience accelerated degradation caused by both ambient and operational temperature loads. This investigation studies the thermo-mechanical reliability of IGBT power module to advise on qualification techniques and the module’s response to the manufacturing, operational and ambient challenges. Specifically, this work reports on: effective number of accelerated thermal cycles (ATCs) for accurate damage and fatigue life prediction, critical solder joint in IGBT power module, impact of voids on the integrity of solder joints and fatigue, and effect of ambient and operating temperature loads on degradation of solder joints in IGBT power module.SolidWorks software is used to create adequate number of realistic 3-D computer aided design (CAD) models of a typical IGBT module for each research objective. The IEC 60068-2-14 thermal cycle test, in-service operating power, and Anand’s time independent visco-plastic constitutive model are implemented as appropriate in static structural package in ANSYS mechanical software to simulate the response of the SAC305 solder material in the models to ambient and operational temperature loads amidst the challenges. Four ATCs namely 6 ATCs, 12 ATCs, 18 ATCs and 24 ATCs are implemented as loads to investigate effective number of ATCs for accurate damage and fatigue life prediction. Python programming algorithm, deployed in Monte-Carlo technique, is used to generate realist distributions of spatial random voids on three representative volume elements (RVEs) of the critical solder joints in three IGBT modules employed to understand the impact of voids on the integrity of the solder joints. For the investigation on effect of ambient and operating temperature loads, eight 3-D CAD models are used. The temperature loads specific to aeronautic, automotive passenger compartment, automotive under-the-hood and railway applications are deployed.
It is found that thirty (30) ATCs are adequate to predict damage and degradation of solder joints in IGBT power module. Deployment of four fatigue life constitutive equations from Morrow, Coffin-Manson, and Syed generated polynomial model which produced minimum and consistent lives of the IGBT modules at about 30 ATCs. More analysis of results shows that the key degradation mechanism of solder joints in IGBT module are stress, plastic strain, and strain energy density. Accumulated plastic strain in the joints is found the predominant damage factor. Critical solder joint in the module depends on the load cycle the device experiences. IGBT-attach solder joint is critical in active power load cycle. Substrate solder joint degrades most in passive thermal cum combined passive thermal and active power load cycles. It was observed that combined elliptical and spherical voids induced the highest damage in the solder joints in the IGBT modules. Ten percent elliptical voids in the joints reduced the fatigue life of the module by 59.46%.
Further findings demonstrate that the module has shortest average fatigue life of 7.5 years and longest average life of 16.2 years in operations in aeronautic and railway ambient temperatures, respectively. In comparison with operation in aeronautic condition, the module operates 36.0% and 85.3% longer life in auto-under-hood and auto-passenger ambient temperatures, respectively. Stress damage is found to concentrate at the periphery of solder joints which is the area to strengthen to improve device reliability.
This study provides new knowledge on thermo-mechanical reliability of solder joints in IGBT power module for implementation in improved design, manufacture, qualification and deployment of the device in several applications.
| Date of Award | 18 Jun 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | David Hughes (Supervisor) & Emeka Amalu (Supervisor) |