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Investigation methods of the beta to alpha tin allotropic transformation.

Di Maio, D; Hunt, C (2008) Investigation methods of the beta to alpha tin allotropic transformation. NPL Report. MAT 21

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It is well known that b tin is stable only down to 13°C. Below that temperature the thermodynamic stable phase is a tin, more commonly known as tin pest. This phase has gained its fame because of the catastrophic consequences to the transforming material. This subject has become of interest because of the transition from tin-lead to lead-free in the electronics interconnection environment. Lead-free alloys contain from 95 to 99% tin and might be subjected to the b/a transformation mentioned above. In studying this phenomenon it is important to establish a way of monitoring the transformation. Past studies were mostly based on optical observations or volume change observations. However none of these methods were very precise nor do they allow continuous measurements. In this study various techniques are developed, improving the precision of the measurements and allowing for continuous observation of the transformation. Three methods were developed in this research: strain measurements, optical observation by time-lapse photography, and electrical resistance measurements. Whilst the first method could detect the inception of the transformation, it has revealed of limited applicability. The large tin volume increase (26%) associated with the transformation could not be fully detected by the strain gauges used. Better results were obtained with the optical method. In fact it was possible to generate videos allowing a better understanding of the mechanics of the transformation. The electrical resistance method has been demonstrated to be the most useful because it allows a very accurate detection of the starting point of the transformation and in measuring the transformation rate. Special 'hybrid' samples were designed in order to use this technique not only with pure tin but also with tin alloys. This sample preparation has shown to shorten the incubation period. Hence this combination of sample preparation and monitoring method has proven to be a useful screening technique for verifying the propensity of tin alloys to transform.

Item Type: Report/Guide (NPL Report)
NPL Report No.: MAT 21
Subjects: Advanced Materials
Advanced Materials > Electronics Interconnection
Last Modified: 02 Feb 2018 13:15
URI: http://eprintspublications.npl.co.uk/id/eprint/4698

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