Electrochemical Behaviour of Solder Alloys
For a powered circuit, a metal corrosion–induced failure mechanism of particular concern is that of the formation of metal dendrites in the presence of contamination and moisture residues, which can lead to catastrophic failure. The propensities of two lead-free solders, SnAgCu (SAC) and Sn, to form dendrites have been assessed using electrochemical impedance (EI) technique and surface Insulation resistance (SIR) measurement, and benchmarked against the performance of conventional SnPb alloy. Two PCB finish materials (Cu and ENIG) have been also investigated for comparison. The dendrites formed from different metals were analysed using SEM-EDX equipment. The work has shown that dendrite formation occurs more readily with the SnPb solder than with the SAC and Sn solders due to the high corrosion rate of Pb in the Pb in SbPb alloy, and/or high solubility of Pb(OH)2. In contrast, the low propensity to form dendrite for Cu finish board compared with ENIG finish suggests its low corrosion rate and low solubility of Cu(OH)2. The good correlation of ionic resistance measured in the EI results and the propensities of metal to form dendrite indicates that metal corrosion rate and the solubility of metal play a major role in dendrite formation.
KeywordsHigh Corrosion Rate Metal Corrosion Dendrite Formation Flux Concentration Ionic Resistance
The work was carried out as part of a project, Measure Electrochemical Corrosion of Lead-Free Process Residues in Electronic Assemblies, in the Processing Programme of the UK Department of Trade and Industry.
- 3.Zou CL, Hunt C (1999) The effect of test voltage, test pattern and board finish on surface insulation resistance (SIR) measurements for various fluxes. NPL Report CMMT(A)222Google Scholar
- 5.Zou CL, Hunt C (2006) Electrochemical behavior of solder alloy. NPL Report DEPC-MPR (PAPER) 040 SeptemberGoogle Scholar
- 7.Zamanzadeh M (1990) Electrochemical examination of dendrite growth on electronic devices in HCl electrolytes. Corrosion 46(8):665–671Google Scholar
- 9.Brusic V, Dimilia DD, Macinnes R (1991) Corrosion of led, tin, and their alloys. Corrosion 47(7):509–518Google Scholar
- 11.Beccaria AM, Mor ED, Bruno G, Poggi G (1982) Corrosion of lead in sea water. Br Corros J 17(2):87–91Google Scholar
- 14.Zou CL, Hunt C (2007) Susceptibility of lead-free system to electrochemical migration. NPL Report MAT1Google Scholar