Study on Whisker Growth on Solder Joints—Part I: Study on Acceleration Test Method

The shift to lead-free tin-based solder in electronics has created a frequent observation of whisker growth from the plated tin surface. With an expectation that the tin whisker growth is not limited to the tin-plated surface, a three-year project of the Japan Electronics and Information Technology Industries Association (JEITA) for a study of whiskers on lead-free electronics was initiated in 2006, as one of the Strategic Core Technology Advancement Programs funded by Organization for Small and Medium Enterprises and Regional Innovation Japan. This paper reports one of the results of this JEITA project, relating to whisker formation from solder joints and its acceleration test methods. When samples were submitted to soldering process and then exposed to a high-temperature, high-humidity environment, a portion of the solder surface became unevenly corroded (oxidized). Tin in the area between corrosion received the stress and formed tin whiskers. Since soldering flux with bromine content exhibited a tendency to create whisker relatively fast, the influence of temperature and relative humidity was studied with this type of flux. The results from this test were analyzed for acceleration factors and found to have a good linearity in Arrhenius plotting with varied temperatures and a constant humidity, with a calculated activation energy of 0.84 eV. Combined with the test results with constant temperature and varied humidity, the same characteristic formulas for whisker growth from the plated tin surface under high temperature/humidity in the reports of the International Electronics Manufacturing Initiative were found to be also applicable to the cases of solder joints. The constants in the formulas, however, were found to be approximately equal to those for the humidity behavior of the conductive adhesive/tin plating interface or those found for electrochemical migration.