Speeding up solder search for aerospace

Materials World magazine
1 Aug 2009

Nanoindentation is helping UK researchers to accelerate the search for reliable lead-free solders for use in aerospace.

Scientists at Oxford University are characterising SAC materials, which are alloys of tin, silver and copper (Sn-Ag-Cu). The results are being applied to finite element analysis software to identify promising new formulations.

Toxic lead-based solder has largely been replaced by SAC alloys in electronics, but the rigorous demands of the aerospace industry mean that no reliable alternatives have yet been developed.

‘When you put these solders into aerospace applications, the failure rates are too high,’ says Professor Patrick Grant of Oxford University’s Department of Materials.

Yet the aerospace industry is keen to replace lead (and other toxic metals such as cadmium and chromium), partly because suppliers of components and printed circuit boards have largely abandoned the material themselves.

A fourth element, usually bismuth or antimony, is incorporated into SAC alloys in an attempt to overcome the formation of intermetallic compounds (IMCs) between the solder and the component.

In operation, temperature fluctuations cause these materials to expand and contract at different rates, leading to strain. ‘Intermetallic compounds can concentrate this strain in certain regions, which leads to failure,’ says Grant.

Nanoindentation allows accurate measurement so that only tiny amounts of material need to be prepared and tested.

Grant’s team has used the technique to characterise creep up to 125ºC. This and other experimental data calibrates the computer model. ‘It could help us to screen potentially attractive compositions more quickly,’ explains Grant.

Project partners include several aerospace companies. Geoff Armstrong, Chief Engineer at one of the partners – Goodrich Actuation Systems – says, ‘Some electronics on aircraft have to be installed in locations subject to high levels of thermal cycling and vibration, for example, equipment in the wing or engine nacelle. That is one reason why aerospace has not adopted lead-free solders in the same timescale as other industries.

‘The work at Oxford contributes to our understanding of the basic mechanisms by which lead-free solder joints can fail, so it is an important step to finding an acceptable solder’.