Understanding the interaction between silver and lead-free solders in order to develop new solder alloys

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The printing of electrically conductive tracks on glass is an important technology in areas such as the automotive industry, where silver pastes are used to create the tracks for heated windscreens. The printing of conductive tracks on glass is also a key part of the manufacture of some photovoltaic systems (solar panels). After firing, such conductive networks need an electrical connection in order to operate, which is achieved by soldering. In former times, the solder alloys used contained lead, but realisation of the health and environmental concerns with this metal have led to the formulation of lead-free solders (such as SAC solders, based on the tin-silver-copper system). While these avoid the risks of using lead, the joint strength achieved is not reliably high enough, and it is critical for continued use of this technology that the problem is better understood, and that a solution is found.In this project we will explore the fundamental behaviour of the existing technology further to understand the key origin of the problems seen; these include microcrack formation in the joint region, and residual stresses after soldering, both of which contribute to reduced joint strength. We will fabricate and microscopically examine the joints produced between different solders and silver powders, to fully understand for the first time the interactions occurring and how they impact on the joint strength. Understanding the problem is just the first step however. Using the knowledge we gain, we will design new silver paste compositions to mitigate the problems. These novel alloys will be produced and processed at laboratory and trial scales, leading to validation of the alloy produced in accordance with industry standard methods. The work will involve close collaboration with the industrial partner, Johnson Matthey plc (a global science and chemicals company, with unique expertise in precious metals technology) through their technology centre in Sonning Common, Oxfordshire, UK and other sites, to trial the technology industrially.