Hunt, C P; Di Maio, D (2009) Allotropic transformation in tin and lead free solder alloys: Measuring the effect and implications for industry. NPL Report. MAT 35

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Abstract

This jointly funded industry and UK government project, addressed concerns with the allotropic transformation in tin. ß-Sn is stable between 232 ºC and 13 ºC. Below this temperature ß-Sn becomes the equilibrium phase. This transformation has catastrophic consequences on the transforming material, mainly because of the 26% volume expansion that accompanies the change from the BCT to the diamond cubic structure. This phenomenon has recently become of interest in the field of electronic interconnections, due to the high tin content of the soldering alloys used for the manufacture of printed circuit boards.

Previous studies have shown that this transformation can occur on soldering alloys, if the right conditions are met. There are many variables that can influence the formation or suppression of the ß-Sn and in this study some of them will be investigated. In particular, this work will look at the effect of one thermal cycle to room temperature, the effect of cubic ice as a transformation seed, and the effect of thermal-cycling to generate susceptibility to subsequent transformation to ß-Sn. The incubation time (nucleation) plays an important role in determining the total time of the transformation.

Cubic ice is an allotropic form of ice that can naturally form; this type of crystal can enhance the nucleation speed of ß-Sn in pure tin. When a seed promotes the transformation, the nucleation time is greatly reduced. Findings indicate that this nucleation process can propagate from tin, which could be a tin plated termination, and into a tin alloy.

Stresses, such as the thermal stresses that occur on PCBs during service, can also accelerate the transformation. This was investigated here comparing the transformation time of samples that were thermally cycled with samples that were not.

An important part of this study will also look at the propensity to transformation of 3 commercial Sn alloys, the Sn / 0.5-0.7 Cu / < 0.1 Ni (Sn100C), the Sn / 3.0 Ag / 0.5 Cu (SAC305) and the Sn 0.3 Ag 0.7 Cu 0.1 Bi (SACX).

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MAT 35
Keywords:	Tin Pest, soldering, lead-free, electronics
Subjects:	Advanced Materials Advanced Materials > Electronics Interconnection
Last Modified:	02 Feb 2018 13:15
URI:	http://eprintspublications.npl.co.uk/id/eprint/4552

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