Zou, L; Hunt, C (2007) Susceptibility of lead-free systems to electrochemical migration. NPL Report. MAT 1

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Abstract

The corrosion and electrochemical migration behaviour on circuit boards will change with the move to lead-free soldering systems, and may affect both circuit performance and long-term reliability. One failure mechanism of particular concern is that of the formation of metal dendrites in the presence of contamination (e.g. flux residues), which can lead to catastrophic failure. The propensity of the eutectic SnAgCu (SAC) lead-free solder alloy to form dendrites has been assessed in terms of the required critical flux concentration to form dendrites, and benchmarked against the performance of conventional SnPb alloy. Two PCB finish materials (Cu and AuNi) have been also investigated for comparison. The major investigatory tool was Surface Insulation Resistance (SIR) measurements, although compositional data were gathered from individual dendrites using SEM-EDX equipment. The work has shown that dendrite formation occurs more readily with the SAC alloy than with the SnPb alloy due to silver migration. Hence contamination of lead-free assemblies must be lower than that associated with a conventional SnPb assembly processes. Dendrites are more easily generated with AuNi finishes than with copper finishes due to nickel migration. A copper finish is least likely to form dendrites due to its low corrosion rate and low solubility of Cu(OH)2.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MAT 1
Keywords:	lead free alloy, dendrite, electrochemical migration
Subjects:	Advanced Materials Advanced Materials > Electronics Interconnection
Last Modified:	02 Feb 2018 13:15
URI:	http://eprintspublications.npl.co.uk/id/eprint/3924

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