Advanced Pb-free interconnect materials and manufacture processes to enable high-temperature electronics packaging

The past decade has witnessed the rapid development of wide-bandgap (WBG) semiconductors (e.g. GaN and SiC). These devices can operate under harsher environments compared to traditional Si semiconductors, presently being exploited in the integrations into wider range power systems in electric vehicles (EV), rail, aerospace industry. However, to maximise their potentials and full capacity, high-temperature interconnection materials and processes that can meet the stringent requirements achieving high reliability with WBG devices have become the bottlenecks in electronics integration of these devices. The packaging of WBG devices demands not only electrical, mechanical robustness, but also optimal efficient thermal managements, where currently no interconnection materials can fully satisfy the operating conditions of SiC devices. Therefore, it is imperative to develop advanced interconnection materials tailored with the effective assembly methods suitable for bonding the related components for reliable high-temperature operation. 

With an attempt to address the technical challenges, this PhD thesis is intended to push the current soft soldering boundaries towards the high-temperature regimes close to the range of metal brazing. In this research, nano Ag-Al, Zn-5Al based alloys, Cu-15Ag-5P and Incusil brazing alloys have been investigated with an intention of exploring their potentials for high-temperature electronics packaging. It is also of prime importance to develop and optimise joining processes to demonstrate their suitability as high-temperature interconnection materials. Therefore, the main emphasise are placed on the capability of the developed processes under a benign condition such as a relatively low bonding temperature and pressure without uses of protective environment and flux. To assess and verify the materials and developed bonding processes, the characterisation and evaluation on the resultant joints and interfaces have also been performed to address the microstructural and mechanical integrity of the bonded structures.

Two strands of research practices in this work are in line with the development of a cost-effective and simplified nano-Ag sintering process.  Binary Ag-Al joints are formed by sintering of a nano Ag-Al paste self-made by adding 10 wt% nano Al powders into a commercial nano-Ag paste to reduce cost. The Binary Ag-Al joints can suppress voids evolution and maintain mechanical stability which promises an advantage over the conventional sintered nano-Ag. Self-assembled monolayers (SAMs) coatings which can prevent Cu from oxidation are also applied to enable nano-Ag sintering on bare Cu under the ambient atmosphere without flux, as a tangible and cost-effective method for high-temperature electronics interconnects without needs of metallization on the Cu substrate.

Zn-Al alloys as potential high-temperature interconnection materials have also been explored through two potential manufacturing routes: i) the transient liquid phase soldering (TLPS) assisted by ultrasonic vibration (USV) under the ambient conditions without flux has been performed to enhance and accelerate interfacial reactions between Zn-5Al and Cu or Ni substrate; ii) micro-scale Zn-Al based paste is designed and synthesised as the potential high-temperature interconnection materials, where the mechanical alloying process is performed to produce micro-scale Zn-5Al based powders. Electroless Ag plating has been applied onto Zn-5Al powders to suppress the oxidation during the sintering process, which has yielded a nano-Ag dendritic structure due to the replacement of Zn-Al by Ag, which can be subsequently almost voidlessly sintered onto Cu substrate.

Cu-15Ag-5P and Incusil brazing alloys are applied in high-temperature electronics packaging assisted by self-propagating exothermic reaction (SPER). The SPER provides intense localised heat in the adjacent of the bondline or bonding interfaces to achieve the interconnects on a millisecond scale with insignificant thermal impacts on the components to be connected. Such formed joints are expected to be effective under high-temperature operation due to the excellent high-temperature properties that can be offered by the brazing alloys. Apart from the externally attached nanofoil to allow the heat to penetrate through the metals (e.g. substrate), an attempt has also been made to include nanofoil as part of the bondline to evaluate the effects of nanofoil on the bonded structures. The study offers certain insights into an effective assembly route viable for high-power electronics packaging.