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PhD Defence Lisheng Wang | Advanced packaging for SiC power modules

Advanced packaging for SiC power modules

The PhD defence of Lisheng Wang will take place in the Waaier building of the University of Twente and can be followed by a live stream.
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Lisheng Wang is a PhD student in the Department of Power Electronics. Promotors are dr.ir. R.J.E. Hueting and prof.dr.ir. G. Rietveld from the Faculty of Electrical Engineering, Mathematics and Computer Science.

This dissertation investigates innovative packaging techniques to enhance the performance and reliability of silicon carbide (SiC) power modules. Recognising the superior electrical properties of SiC, such as higher operation temperature, higher breakdown voltage, and high-frequency capabilities, the research addresses significant challenges in conventional packaging, including thermal management, parasitic inductances, and mechanical robustness. The focus is on exploring solutions to interconnection challenges that hinder the realisation of the full potential of SiC semiconductor power modules, aiming to enable these devices to operate at higher power densities, switching speeds, efficiency, and reliability.

The specific research objectives of this dissertation are:

(a) To perform a comprehensive review of topside interconnections for wide bandgap (WBG) power semiconductor packaging, examining materials, failure modes, advantages, challenges of existing technologies, and future trends.

(b) To develop a new embedded concept with improved thermal management and reduced thermo-mechanical interfacial stress, and to address the limitations of the narrow laser drilling process window compared to conventional embedded technologies.

(c) To introduce a cost-effective alternative bonding scheme and process for large-size substrate attachment coupled with reliable metal-plated metallisation for aluminium (Al) surfaces.

A comprehensive literature review of topside interconnection technologies for WBG semiconductor power devices and modules has been performed. First, the challenges and driving factors for the interconnection of WBG semiconductor dies are discussed. Then, the technical details and innovative features of state-of-the-art interconnection techniques are summarised for commercially-used WBG semiconductors, namely SiC and gallium nitride (GaN). The review categorises and compares existing topside interconnection materials, highlighting their advantages, challenges, and failure modes. Based on this discussion, potential future directions for interconnection technology development are suggested, concluding that superior performance of WBG semiconductors can be achieved by combining novel materials with innovative designs for topside interconnections.

Subsequently, a ceramic substrate-embedded design with a planar copper topside interconnection has been developed to improve the laser drilling process window, and to enhance both reliability and thermal conductance. First, a detailed study was performed of the interaction between ceramic substrates, FR-4 resin (i.e., a woven glass fibre-reinforced epoxy resin laminated composite material, which is Flame Retardant-4), and the lamination and curing processes. It is found that aluminium nitride (AlN), despite its high thermal conductivity, is unsuitable as a ceramic substrate for this application due to its lower flexural strength. Instead, a 0.32 mm thick silicon nitride (Si3N4) ceramic substrate has been selected given its superior combination of thermal conductivity and flexural strength, offering a 30% safety margin against ceramic breakage as verified by warpage testing of the fabricated proposed module. The proposed module incorporates a pre-sintered die top system (DTS) layer on the source, gate, and Kelvin source pads, which improves the laser drilling process and reduces interfacial stress in the topside interconnection. Thermal and thermo-mechanical simulations demonstrated that the novel power module achieves significantly lower thermal resistance and von Mises stress, which are 5.8% and 29%, respectively, of those in conventional embedded modules. Electrical testing confirmed that the SiC dies in the embedded module exhibit comparable static electrical performance, and a buck converter using this module showed higher efficiency due to reduced parasitics. Manufacturing results further validated the design's reproducibility and process stability, achieving a 100% production yield for 8 samples, attributed to the effective DTS layer protection, clean laser-drilled micro vias, and excellent plating quality.

Subsequent exploration focuses on the reliability of plated Nickel (Ni)/Silver (Ag) metallisation on Al for sintered Ag joints and development of a new plated Ni/Copper (Cu)/Ag metallisation stack for enhanced reliability. Shear strength, thermal shock (TS), and high-temperature storage (HTS) reliability of sintered Ag joints across different metallisation layers have been analysed, alongside microstructural and elemental analyses to understand failure modes. Traditional Ni/Ag metallisation shows limited reliability due to poor adhesion between Ni and Ag, while the Ni/Cu/Ag metallisation stack exhibited superior performance, with shear strengths consistently above 40 MPa for up to 500 TS cycles, and failure modes predominantly involving Al/Ni delamination and cohesive failure. Longer sintering times further enhance shear strength of both metallisation stacks, reaching values between 100 MPa and 130 MPa. Additionally, a new failure mode has been observed in the Ni/Cu/Ag joints, suggesting a reduced impact of Al/Ni metallisation weakness. This confirms that the new metallisation stack effectively addresses delamination issues in sintered Ag joints with Al heatsinks and direct bonded aluminium (DBA) substrates, thereby maximising the potential of sintered Ag joints.

Further investigations involved shear strength tests on different solder, sintered Ag and transient liquid phase (TLP) joint samples as a function of aging via thermal cycling and high-temperature storage tests. The joint microstructure and failure modes have been analysed using a scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDS). The findings show that sintered Ag has the maximum shear strength of around 70 MPa both before and after aging, while solder joints have the lowest shear strength that decreases from 50 MPa to about 35 MPa after thermal cycling and aging. TLP bonding demonstrates a stable shear strength of approximately 50 MPa after aging due to intermetallic compound growth. TLP bonding thus has a higher reliability than solder joints and is a cost-effective bonding alternative to Ag sintering.

Lastly, a novel wet placement process for Ag sintering of large-size power devices utilising a newly-developed vacuum-assisted fixture has been developed. Experiments of this new placement process show that the pre-heating temperature plays a critical role in determining the shear strength and bonding quality of large-size sintered Ag joints. Elevated pre-heating temperatures (105 °C, 130 °C) led to violent volatilisation processes, resulting in joints with higher void fractions (5.7%, 10.2%) and reduced shear strength (around 70 MPa). Conversely, low preheating temperatures like 60 °C lead to insufficient volatilisation, compromising production efficiency due to the very long heating times needed. Optimal shear strength (77 MPa) and minimal void fraction (0.1%) have been achieved at a pre-heating temperature of 80 °C. The STPak sub-module power devices realised with this process demonstrate full functionality after sintering at 230 °C, 15 MPa, and 10 mins.

In summary, this research fills a crucial gap in the research and application of WBG semiconductors by providing a comprehensive overview of topside interconnection for WBG semiconductors, serving as a valuable reference for packaging engineers. Additionally, it outlines potential future directions, guiding further research in this area. Moreover, this study contributes significantly to the field of power electronics by addressing critical interconnection packaging challenges for SiC power modules. The proposed and validated innovative solutions enhance the processability, reliability, cost-effectiveness, and thermal performance of SiC-based power modules, potentially expanding their application in high-power and high-frequency applications. This advancement not only benefits traditional power modules but also shows huge potential for embedded and system-level integrated power modules, further enriching the field.