Zou, L; Hunt, C (2008) Electrochemical impedance technique to predict circuit reliability with lead-free solders. NPL Report. MAT 15

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Abstract

The corrosion and electrochemical migration behaviour of metals will have changed with the move to lead-free soldering, and may consequently affect both circuit performance and long-term reliability. In a powered circuit a corrosion induced failure mechanism of particular concern is the formation of metal dendrites, which can lead to catastrophic failure. Dendrite formation is a complex electrochemical process, and metal corrosion is an initial cause. Electrochemical Impedance (EI) results of the SnAgCu (SAC) solder and Sn have been measured over a wide frequency, and benchmarked against results from the conventional SnPb solder. Two printed circuit board (PCB) finish materials (Cu and electroless nickel on immersion gold (ENIG)) have been also evaluated for comparison. Although the corrosion rate could not be interpreted from the EI results directly due to high ionic resistance of the sample system, the ionic resistance between two electrodes corresponding to metal corrosion can be separated from total impedance measured for conductor-insulator structure of the comb test pattern. The ranking from the lowest to the highest in terms of ionic resistance is ENIG < SnPb < SAC ¿ Sn <Cu. The low ionic resistance with SnPb solder compared with SAC and Sn solders suggests that the higher corrosion rate of Pb in the SnPb alloy, or, and high solubility of Pb(OH)2 are significant factors. In contrast the high ionic resistance for Cu finish board compared with ENIG finish board indicates its low corrosion rate and low solubility of Cu(OH)2.
The propensities of the five metal systems to from dendrites have also been assessed in terms of a required critical flux concentration, as measured using the traditional Surface Insulation Resistance (SIR) measurement. The dendrite composition formed on different finished bards were analysed using SEM-EDX. The work has shown that the propensity to form dendrites with the five metals correlates with ionic resistance measured in the EI results. This is consistent with the ionic resistance between two electrodes being a significant factor in the occurrence of dendrite formation, however the ionic resistance is a function of the metal anode corrosion rate and solubility of the metal hydroxide. EI measurements have been shown to be a potential non-destructive fast method to predict electronic circuit reliability.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MAT 15
Keywords:	Electrochemical migration, reliblility, lead-free solder, dendrites
Subjects:	Advanced Materials Advanced Materials > Electronics Interconnection
Last Modified:	02 Feb 2018 13:15
URI:	http://eprintspublications.npl.co.uk/id/eprint/4134

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