Corrosion behavior, whisker growth, and electrochemical migration of Sn-3.0Ag-0.5Cu solder doping with In and Zn in NaCl solution
Abstract Corrosion characteristics of Sn–3.0Ag–0.5Cu (SAC) solder doped with In and Zn in NaCl solutions were conducted by sweeping the voltage at a constant rate with a potentiostat. Whisker growth was completed by dipping in 3.5 wt.% NaCl salt solution. Electrochemical migration (ECM) experiment was carried out as a designed program in an electric field with a power supply. Surface morphology and elemental composition of SAC and its doped candidates were determined by SEM, EDAX, XRD techniques. Results showed that when the percent content of Zn was ⩽1%, corrosion current density (Icorr) increased with Zn% increasing, it was up to the highest value when %Zn was 1%. After that, the open circuit potential moved negatively quickly as a function of Zn percent, however Icorr increased with Zn percent increasing, also lower than that of doped solder with 1 wt.% Zn, higher than that of no doped solder. The same corrosive law was suitable for SAC candidate with In doping. SEM morphologies showed that whiskers existed in all cases of different In/Zn-concentration alloys. After exposure to severe conditions (3.5 wt.% NaCl solution) for 7 days, the longest whisker for 96.8(Sn–3.0Ag–0.5Cu)–0.2In–3Zn solder was about 300 μm, the average grown rate was evaluated to about 5 A/s which is higher than the reported result in the former literatures. The possible mechanism was that: after metals reacted with water/ions in water, products or oxidizer in solder expanded to induce compression stress, during the release of stress, Sn extruded out. ECM tests showed that dendrite growth was a result of system under far-equilibrium conditions in sorts of fields as electrical field, thermal field, concentration field, etc., the farer off the equilibrium, the easier that ECM process took place. Dendrite growth rate of SAC solder were faster than those of its candidates with In or Zn dopings, furthermore, rate with Zn doping was larger than that with In doing, which is due to differences on surfaces or different intermetallic compound formations (IMC) on surfaces. Doped with In, dendrites looked like emarcid petals, although they might not look like dendrites, contents on dendrites were mainly Sn. Whereas, dendrites looked like clew with only Zn doping, it was mainly Sn with little Zn. Different from the above, dendritic microstructures of SAC solder without doping entirely looked like branches, contents were mainly Cu and Sn. From the points of corrosion and whisker growth, Zn, In dopings in SAC solders may be not benefit to micro/nanoelectronic packaging, though other mechanical or soldering characteristics can be improved with their dopings.