In microelectronic system, solder joints are constantly exposed to temperature, humidity, stress and other reactive environments during chip production and service period. Oxidation (and/or corrosion) of solder materials could be one of the primary yield and long-term reliability risk factors. Hence, an in-depth understanding on oxidation behaviors of solder interconnections becomes a critical issue in making solder interconnection technologies successful.
There are many techniques to characterize the surface oxides. Among those, electrochemical reduction analysis is known to be an inexpensive, simple and yet relatively precise technique to measure quantitatively both the type and the amount of oxides formed on metal surfaces. Before using the electrochemical reduction analysis for measuring surface oxides, an experimental assessment of this method was performed. An optimal current density for electrochemical reduction analysis which guarantees consistent results of oxide thickness was -30 ~ -60 μA/㎠. The reduction potentials of tin, lead and copper oxides were measured and used as standard values for determining the kinds of oxide reduced. Hydrogen evolution potentials of tin, lead and copper were low enough for oxides to be fully reduced on their surfaces.
63Sn-37Pb was oxidized at 85℃, 150℃and T/H conditions. Tin was enriched on the surface of eutectic 63Sn-37Pb and only tin oxide was found as a mixture of SnO and $SnO_2$. SnO was transformed into $SnO_2$ at the top surface as aging time goes on.
High Pb-Sn alloys (Pb-1Sn, Pb-2.5Sn and Pb-3.5Sn) were oxidized at room temperature and 150℃. Tin was enriched at the surface of high-lead tin alloys. SnO growth was saturated at low level of thickness (<100 Å) at room temperature and 150℃ except Pb-1Sn, because Pb-1Sn has some lead-rich area because the tin content is lower than the solubility limit. Addition of tin in lead, especially over solubility limit at room temperature, can effectively suppress the f...