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

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.

A primary aim of this proposal is to expand the range of industrial applications for quasi-ambient bonding (QAB) technology. This will be of direct benefit to a range of stakeholders:

1) UK Industry and Economy:
The potential industrial importance of this technology is demonstrated by the wide range of industrial partners who attended our grant application planning meeting in March 2017 and who have provided letters of support. The new technology that is at the heart of this proposal is urgently needed to address the increasingly stringent regulations on the use of toxic metals such as Pb. Currently available lead-free technologies are expensive and/or liable to failure (with associated cost implications). Also, they will not be able to support increasing operating temperatures in the longer term. The QAB process that is at the heart of this proposal has the potential to overcome these challenges, allowing UK industry to stay at the forefront of technological progress in a wide range of applications. The industrial partners on this project cover the whole microelectronics supply chain including materials suppliers, technology providers, testing & qualification and end users in the optoelectronics and high-temperature/harsh environment electronics sectors.

2) Environment and Society:
The elimination of heavy metals such as Pb from electronics is highly desirable from an environmental perspective, reducing the potential for toxic pollutants going into landfill. This proposal will enable the cost effective substitution of Pb-solders with non-toxic alternatives in a wide range of applications. In addition, QAB has the potential to provide shorter production times for microelectronic components and hence improved efficiency and reduced energy consumption. Streamlining of the production process and reducing raw material costs (compared to Au and Ag based solder materials) has the advantage of substantial cost saving. Such cost savings allow UK industry to sustain a highly skilled workforce. In addition, the expansion of this to a wider range of industrial applications provides opportunities for job creation.

3) Project Staff:
This project is only achievable due to the collaboration of three academic groups with different scientific expertise (materials, manufacturing and microsystem engineering). The academics and postdoctoral researchers working on this project will benefit from regular meetings with the other academic partners and opportunities to travel to other groups to perform experiments. They will also interact with the industrial partners so gain directly relevant industrial knowledge and experience. This combined expertise will help to facilitate greater academic-industrial integration to the benefit of both parties. The project will also train a number of highly employable and skilled postdoctoral workers with the necessary expertise to work across disciplines or to transition to industry as they desire.

4) Academia:
This proposal will improve our fundamental understanding of QAB through detailed characterisation and innovative in situ materials characterisation. For example, the new test rigs for synchrotron X-ray diffraction measurements have potential for academic application in any high speed phase transformation process, and as such will be of interest to those working on cutting edge materials characterisation, synchrotron beamline development or high speed imaging. The work on ignition by electromagnetic induction heating will also yield new results relevant to a wide range of other applications.