engleski

Solderability Assessment: Reduction of Metallic Surface Oxides via Sequential Electrochemical Reduction Analysis

Removal of surface oxides is a key requirement for formation of a good bond between metals joined by soldering. Solderability loss caused by metal oxidation is an important problem that costs the electronic industry millions of dollars annually. Because they are semiconductros, surface oxides represent not only a mechanical barrier for soldering, but an energetic barrier to electron flow as well. Rosin fluxes activated by organic acids effectively dissolve oxides just before or during the bonding operation but leave tenacious residues that can cause corrosion problems if not fully removed. However, electrochemical reduction methods represent an alternative path to removal of surface oxides and enable a nondestructive determination of solderability. These methods can be applied for any geometry, including printed wiring leads, through-holes and surface-mount pads. In this work we applied chronopotentiometry as a base technique for solderability assessment. This method is actually named the Sequential Electrochemical Reduction Analysis (SERA) since it involves the reduction of surface oxides in sequence to provide a quantitative measure of both the type and the amount of oxides present at the surface. The purpose of this work was to investigate the mechanism and kinetics of reductive decomposition of surface oxides formed on Sn, Cu and Pb surfaces, which represent the main construction metals in electronic printed-board industry, using in-situ methods; chronopotentiometry, chronoamperometry, cyclic voltammetry and electrochemical impedance spectroscopy. Experiments were performed with both, anodically and naturally formed surface oxides. Oxidation of investigated metal electrodes resulted in formation of a duplex structure of the oxide film on their surfaces. These films showed extreme stability toward spontaneous decomposition. However, electrochemical techniques used in this work were very suitable for in-situ electroreduction of these oxides. It was established that electroreduction of these oxides down to pure metals, proceeds via solid-state mechanism, thus yielding oxide-free and ready-to-bond metal surfaces.

Sn; Cu; Pb; surface oxides; solderability assessment; electrochemical nondestructive reduction; Sequential Electrochemical Reduction Analysis

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