Electrochemical behavior of metal interconnects in electronic assemblies
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The electrochemical and corrosion behaviors of solder alloys—SnAgCu (SAC), SnZnBi, SnPb, and Sn—and printed circuit board finish materials Cu and AuNi were investigated in carboxylic acids (flux) and NaCl solutions using the potentiodynamic scanning technique. The results show that SAC and Sn are passivated in the diluted flux solution, but SnPb, SnZnBi, Cu, and AuNi are under active dissolution when anodically polarized. However, passivation of SAC alloy is not observed in concentrated flux solution. Although a passive film forms on SAC in a 2% NaCl solution, the film is less stable than in the flux solution. In addition, oxidation of the most commonly used lead-free and lead solders, SAC and SnPb, at high temperature was evaluated via sequential electrochemical reduction analysis (SERA). The SERA results revealed that the SAC alloy oxidized more significantly than SnPb under hot, dry conditions.
I. INTRODUCTION
The tin lead solder alloy has met the requirements of the electronics’ industry for many years; however the environmental impact of lead is becoming a major concern all over the world and European legislation requires that lead in solders be eliminated from mainstream electronics manufacture. Many of the new solder alloys are rich in tin with a variety of other elements added to enhance different characteristics. The current preferred solder replacements are based on the tin–silver-copper (SAC) system. Extensive work has been done on solderability, mechanical properties, and processing characteristics of lead-free alloys, but the corrosion behavior of the alloys and, consequently, the corrosion effect on lead-free solders and the impact on circuit reliability have not been studied in detail. With the move to lead-free soldering, a new material set has been introduced, and this can be expected to influence the corrosion processes on the circuit board assembly. Susceptibility to corrosion can be due to substances from the environment during use or from the original manufacturing process. Achieving high reliability is typically verified using the surface insulation resistance (SIR) technique.1,2 A circuit assembly can be contaminated with flux residues, which may contain ionic or hydrophilic materials, and from operating in harsh environments where other aggressive species may
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0333 2622
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 10, Oct 2008 Downloaded: 16 Mar 2015
be encountered. Contaminants when combined with moisture result in a lowering of SIR between conductors on the circuit and can also result in corrosion. On a circuit assembly, corrosion may take place with and without the assistance of an applied voltage. Without the voltage bias, corrosion is driven by the electrochemical force, which depends thermodynamically on the anodic and cathodic processes. With a voltage bias, an electrochemical force is provided to increase corrosion reactions that otherwise would not be possible without
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