Wickham, M; Jennett, N M; Nunn, J; Gohil, D D; Hunt, C (2009) Measuring tin whiskering propensity:measuring internal stress in tin coatings by nanoindentation. NPL Report. MAT 29

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Abstract

The propensity of tin platings to whisker has been investigated using nano-indenting. Typical platings and components were conditioned using damp heat or thermal cycling and subsequently inspected for whiskering and measured for hardness as an indicator of internal stress using a nano-indenter. Conditioning samples using damp heat or thermal cycling resulted in a high percentage of components exhibiting a variety of whisker types and denisities. Using damp heat conditioning of components with Sn platings on Cu, over 60% of samples exhibited whiskering. The whiskers generated were usually straight, thin, and of high aspect ratio. Samples with Sn platings on Ni barrier layers were less affected by thermal cycling. The whiskers generated were usually shorter and thicker in style. Whiskers up to 30mm were generated with whisker thicknesses generally in the region of 5mm in diameter.
The coatings tested were supplied on a variety of substrates with a range of surface shape and roughness, making trends in the nano-indenting results of some samples difficult to observe and resulting in a large spread in indentation response. In other cases, where the measurement spread was low, the differences between specimens of the same type and age was larger than the differences between the types or between the ages. The effect of the severe surface roughness is to increase dispersion of the results and is very unlikely to have caused the systematic shifts observed. Variations in local coating thickness would cause shifts in hardness and were investigated by direct thickness measurement of the indented samples. Thickness was shown to vary significantly but, although a contributing factor, it was not the main source of the indentation response variations.

Evidence for ageing effects other than the growth of an intermetallic layer was provided by the difference in response of some coatings. Here there was a dramatic increase in the creep behaviour of the coating with age. This indicates that the dominant effect in this system is the rapid annealing of the coating by the ageing process.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MAT 29
Keywords:	tin whisker, nanoindenting, lead-free
Subjects:	Advanced Materials Advanced Materials > Electronics Interconnection
Last Modified:	02 Feb 2018 13:15
URI:	http://eprintspublications.npl.co.uk/id/eprint/4348

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