Cu-Sn Intermetallic Compound Growth in Hot-Air-Leveled Tinat and below 100°C - Tyco Electronics

Low-temperature intermetallic compound (lMC) growth in hot-air-leveled tin (HALT) at lower temperatures was investigated. A technique based on image analysis of backscatter electron scanning electron microscopy images was used to measure the average thickness of Cu-Sn IMC in HALT at 50°C, 75°C, and 100°C with a high degree of precision. The technique made it possible to observe new features of IMC growth in HALT at these temperatures. These measurements demonstrated that the parabolic growth law commonly used to describe IMC growth is not applicable to HALT, particularly at lower temperatures characteristic of storage and service: the conventional parabolic growth rate constant k is not a true constant but is a function of aging time. An alternative two-parameter equation for IMC growth has been proposed: the parameters, true parabolic growth rate K and equivalent preaging time t0, are true constants that do not vary with aging time. These parameters were determined for two kinds of HALT samples. The activation energy for lattice (bulk) diffusion derived from the true parabolic growth rates K is 11.8 kcal/mol. The growth parameters K and t0 can be used to estimate the solderability shelf life of HALT coatings. The activation energy can be used to calculate the IMC growth acceleration factors for temperatures within the applicable temperature range, and ultimately to develop relevant acceleration aging test treatments.