Ceramic Enabled Advanced Packaging for Wide Bandgap Power Devices

: Design, Implementation, and Integration

Abstract

Silicon carbide (SiC) devices are increasingly being utilized in a variety of applications today.
While they offer notable advantages over silicon (Si) devices, such as reduced switching losses
and fast switching frequencies, the traditional methods of power module packaging present
obstacles in further enhancing power density and reliability. Additionally, power density, reliability, and electromagnetic interface (EMI) are critical design considerations for both power
modules and power converters. Therefore, it is crucial to focus on improving these aspects
through advanced power module packaging.
Traditional power module packaging contains many layers, including power chips, solder, conductive traces, an insulating substrate, a baseplate, thermal interface material (TIM), and a
heatsink. This construction is widely used in the production of power modules. However, there
are still some drawbacks to this structure. Firstly, the thermal conductivity of TIM is relatively
low. Therefore, the existence of the TIM layer reduces the thermal conduction efficiency of the
package. In addition, each layer within this architecture has its own coefficient of thermal expansion (CTE). The different CTEs across these layer stacks significantly influence the device
reliability, since the mismatch can induce high thermal stresses and resultant device failures.
Besides that, using metal heatsinks in this traditional packaging structure can also influence the
EMI performance.
The primary goal of this PhD thesis is to explore a non-TIM power module architecture, specifically a chip-on-ceramic heatsink design, to improve thermal, mechanical, and EMI performance in power modules. This innovative non-TIM packaging technique bonds conductive
copper traces directly onto a ceramic heatsink, eliminating the need for TIM and simplifying
the manufacturing process by reducing the number of layer stacks between the chips and the
heatsink. Both air-cooled and liquid-cooled non-TIM power modules are designed and fabricated to characterize, evaluate, and compare their properties. Various manufacturing methods,
including machining, 3D printing, and isostatic pressing, are used to create different types of
alumina (Al2O3) and aluminum nitride (AlN) ceramic heatsinks. The air-cooled non-TIM module design, using AlN ceramic heatsinks, demonstrates superior thermal performance compared
to conventional modules with standard 6063 aluminum alloy heatsinks, with measured junctionto-ambient thermal resistance values of 1.81 ◦C/W and 1.84 ◦C/W, respectively. Additionally,
the air-cooled non-TIM power module packaging is successfully tested for continuous operation
at 650 V with 2 kW, indicating its potential for high-power converter applications. An extensive
analysis, including electrical-thermal-mechanical finite element analysis (FEA) and digital image correlation (DIC) tests, is conducted to assess the in-plane deformation of each MOSFET.
The results show a significant mechanical stress reduction in the non-TIM packaging, indicating
enhanced reliability.
The liquid-cooled power modules, designed with the chip-on-ceramic heatsink structure, demonstrate that the non-TIM package achieves a 23%-26% reduction in thermal resistance compared to modules mounted on commercial aluminum alloy liquid-cooled heatsinks. EMI testing of
the liquid-cooled modules shows that the chip-on-ceramic heatsink package reduces commonmode (CM) current by more than 5 dB in the 5–20 MHz range compared with conventional
aluminum-alloy heatsink modules. A holistic design process is proposed to create system-onpackage power converters with ultra-high power density based on the non-TIM power module
architecture. A high-density 800 V, 10 kW three-phase power converter is developed, featuring integrated gate drivers and expandable dc-link capacitors. This converter, measuring just
48.6 mm ×50.8 mm ×67.5 mm, achieves an impressive power density of 60 kW/L at the rated
power. This power density meets the 2035 targets set by the Aerospace Technology Institute
(ATI), which are 45 kW/L, demonstrating the potential of the chip-on-ceramic heatsink packaging in high-power-density converter applications.

Date of Award	4 Feb 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Xibo Yuan (Supervisor), Phil H Mellor (Supervisor) & Wenzhi Zhou (Supervisor)