A mechanics-induced complication of impression creep and its solution: application to Sn–3.5Ag solder

Impression creep, wherein a flat-tipped cylindrical punch is used to load a small area of the specimen in nominally uniaxial compression, enables testing of very small material volumes and miniaturized components with minimal sample preparation. One of the attractive features of the impression creep test is its ability to establish steady-state within very short times for rapidly creeping materials. However, for relatively slowly creeping materials, a considerable time is necessary for the evolution of a steady-state plastic zone under the punch, resulting in a prolonged period of decreasing creep rate even when the constitutive creep behavior is strain-independent. This test-dependent transient behavior, where the creep rate decreases slowly even at very long test times, complicates the determination of true material creep parameters. Here, we discuss the source of this problem, and prescribe a methodology to substantially shorten the required test times in order to make the test technologically attractive. Because of the emerging importance of lead-free solders in microelectronics packaging, the analysis presented here is based on finite element simulations of eutectic Sn–3.5Ag solder, deforming via power-law creep.