Lead free solders in aerospace applications

 

Nanoindentation, microstructure and modelling

Current environmental legislation restricting the use of toxic metals in manufacturing and service will make the continued use of metals such as lead, cadmium and hexavalent chromium increasingly problematic for the aerospace industry and the basic understanding and development of aerospace-relevant replacement technologies must begin now.

Pb containing materials are used extensively in Pb-based solders for electrical connections on printed circuit boards and printed wiring boards. Commercial Pb-free products are available for non-aerospace sectors but the limits to their use in the much harsher aerospace environment are unknown.

The objectives of this project are firstly to investigate the reliability of Pb-free solders under electrical/thermal regimes relevant to the harsh aerospace environment; and secondly to develop basic understanding and numerical analyses describing the electrical reliability of toxic metal replacement materials.

Nanoindentation at room and elevated temperature is being used to measure the spatial distribution of mechanical properties within lead free joints themselves, using joints typical of aerospace electronics, avionics and other configurations. The mechanical behaviour (constitutive laws) are then captured in finite element analyses of the evolution of thermal stresses in the joint region, and how these change and relax with time. Electron microscopy is being used extensively to relate mechanical data to the microstructure in various conditions of the joints: as manufactured (in-house), after isothermal ageing at elevated temperature; after thermal cycling in a pre-programmed aerospace standard thermal cycling rig; and after the combined effects of thermal cycling and electrical loading.

[Funded by EPSRC and an aerospace consortium]

Map of nanohardness of a lead free solder in the region of the Cu/Solder contact after thermal ageing
Map of Young's modulus in the same lead free slder region, obtained by nanoidentation