Quasi-ambient bonding to enable cost-effective high temperature Pb-free solder interconnects

Lead Research Organisation: University of Manchester
Department Name: Materials

Abstract

There is an increasing demand for electronics that can operate at temperatures in excess of 200 degrees C, well above the maximum operating temperature of traditional silicon microelectronics. Key application areas are in the power, automotive, aerospace and defence industries. Electronic devices capable of operating at such high temperatures are now available. However, new methods are also needed for integrating these devices into circuits and systems, and in particular for attaching them, both mechanically and electrically, to circuit boards and heatsinks.

At present high-temperature devices are typically attached by soldering using high-melting-point, lead-rich solders. However, there is a strong environmental imperative to reduce the use of lead in all electronics, so this cannot be accepted as a long-term solution. Alternative solutions employing gold-rich solders or sintered nano-silver pastes can be used, but these are expensive and can suffer from reliability issues. Low-cost, lead-free high-temperature solder alloys are also available; however, these tend to require significantly higher soldering temperatures and longer processing times, leading to slower production and higher thermal load on the devices during soldering.

This project will explore the use of quasi-ambient bonding (QAB) with reactive nanofoils as a route to lowering the process time and thermal load during packaging of high-temperature electronic devices. Reactive nanofoils are multilayer materials comprising alternating layers of two elements (typically nickel and aluminium) that react exothermically i.e. with the release of heat. Once the reaction is triggered, it is self-propagating and spreads throughout the foil. If the foil is sandwiched between two parts that are pre-coated with solder, the heat generated can be used to melt the adjacent solder layers momentarily and form a permanent bond. The heating is intense, but occurs over a short timescale, so that while the local temperature can reach up to 1500 degrees C, heating is confined to a narrow region around the foil, with negligible temperature rise occurring elsewhere.

Up to now, quasi-ambient bonding applications have used traditional lower-temperature solders. In this project we will extend the application of QAB to a range of low-cost, lead-free high-temperature alloys. The primary aim will be to develop bonding processes tailored for applications in high-temperature power electronics and optoelectronics. We will also explore the use of QAB for sealing of hermetic packages which is another key area where low cost and low thermal load can be an advantage. The processes developed will be evaluated in terms of bonding strength and in-service reliability, and benchmarked against alternative processes based on lead- and gold-based solders.

Alongside the process development and evaluation, we will carry out extensive modelling and characterisation aimed at gaining an improved understanding of the QAB process. Developments to date have been mainly empirical, and fundamental aspects of the process remain poorly understood. QAB is fundamentally different from traditional soldering because of the very short timescale over which the process takes place. In order for it to become established in mainstream electronics manufacturing, the potential detrimental effects of residual stresses and microstructural defects incorporated into QAB bonds need to be fully understood.

The proposed research has the potential to provide a low-cost, sustainable joining technology for electronics manufacturing that can continue to meet the operating temperature requirements of high-temperature electronics for many years to come. At the same time it will yield new fundamental insights into processes involving rapid solidification of complex alloys that will be of wide interest to the materials science and manufacturing research communities.
 
Description •Significant new knowledge generated: Understanding on the microstructural characteristics and the potential effects on the QAB joints aligned with the applications. •New or improved research methods or skills developed: Development of experimental set-up and numerical analysis of QAB process and necessary the surface modifications. •Important new research resources identified: An initial trials implemented at Diamond Light Source to in-situ observe the self-propagated exothermic reaction in QAB bonding with Ni-Al nano-foils. •Important new research questions opened up: The effects of the reacted nano-foils remaining in the bonded structures and internal stresses induced in the joints on the mechanical, thermal and electrical properties of the joints. •Particularly noteworthy new research networks/collaborations/partnerships, or combinations of these: new collaborations through ISCF-DER challenges of Innovative UK. •Increased research capability generated from training delivered in specialist skills: QAB system and process optimisation and advanced materials characterisation with P-FIB.

COVID related delays and restrictions to the planned Beamtime at Diamond have required us to reconsider specific experimental detail to move to more use of laboratory based characterisation (although the rate of progress with this has been slower than expected due to pandemic restrictions). Very pleased that the extension we have been awarded by EPSRC will enable staff to continue until the end of the project and thereby deliver on the objectives.
Exploitation Route In the context of your Pathways to Impact we envisage our research outcomes being taken forward by the power electronics industry, likely through the project industrial partners in due course for die-attach and baseplate bonding. Academic/research outcomes will be able to reveal the further details of the QAB bonding mechanism, effects of surface treatments as well as observation of the unique microstructural features.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Energy,Manufacturing, including Industrial Biotechology

 
Description The project industrial collaborators have expressed their great interest in exploitation of the potential research findings, and will to support from various angles to make use of the technologies to be developed. We have recently expanded the potential uses of the QAB process for bonding DBC substrate with baseplate of the power modules for minimising the CTE mismatch with the existing industrial collaborators.
First Year Of Impact 2018
Sector Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Electronics,Manufacturing, including Industrial Biotechology
Impact Types Economic

 
Title Advanced simulation tool for QAB bonding process analysis 
Description Phase field models has been established to underpin the fundamental understanding of relevant phenomena associated with QAB bonding and performance of bonded structures, e.g. void formation, incurred residual stress, and their effects on reliability of the bonds. 
Type Of Material Improvements to research infrastructure 
Year Produced 2020 
Provided To Others? No  
Impact The notable impacts include: i) provide detailed SPER bonding process in terms of liquid-solid interfacial interactions/reactions; ii) quantitively evaluation of internal voids and their formation, as well as resultant residual stresses; ii) the tool may be applicable to the potential applications to enable understanding the phenomena of multi-scale and multi-phase interactions in the manufacturing. 
 
Description ISCF-DER Challenge centre partnership 
Organisation Manufacturing Technology Centre (MTC)
Country United Kingdom 
Sector Private 
PI Contribution As an academic partner within the consortium the team has contributed invariably in the preparation of the relevant technical attributes to the DER centre biding team.
Collaborator Contribution Our partner had been working and co-ordinating the application and the relevant activities.
Impact The panel interview has be conducted by UKRI, and the result will be confirmed shortly by this week.
Start Year 2020
 
Description ISCF-DER Challenge centre partnership 
Organisation University of Nottingham
Department Nottingham Clinical Trials Unit (NCTU)
Country United Kingdom 
Sector Academic/University 
PI Contribution As an academic partner within the consortium the team has contributed invariably in the preparation of the relevant technical attributes to the DER centre biding team.
Collaborator Contribution Our partner had been working and co-ordinating the application and the relevant activities.
Impact The panel interview has be conducted by UKRI, and the result will be confirmed shortly by this week.
Start Year 2020
 
Description Teer coating limited 
Organisation Teer Coatings Ltd
Country United Kingdom 
Sector Private 
PI Contribution We provide a business need for potentially using the equipment, expertise and technology of Teer coatings limited.
Collaborator Contribution Teer coating offered some trials to develop novel surface coatings for bonding.
Impact Currently planning various experimental trials.
Start Year 2022
 
Description IMAPS-UK presence and engagement 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact IMPAS-UK 2020 February newsletter reported our current research activities which can be connected to this work.
Year(s) Of Engagement Activity 2018,2019,2020
 
Description Industrial demonstrations 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Working directly with project partners to demonstrate the uses of the technology using the benchmark components and materials.
Year(s) Of Engagement Activity 2018
 
Description Project Kick off meeting 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Kick off meeting to advertise the grant funding and help establish the collaboration. The process of signing confidentiality and collaboratory agreements was begun.
Year(s) Of Engagement Activity 2018