A synchrotron micro-diffraction investigation of crystallographic texture of high-Sn alloy films and its effects on whisker growth

For electroplated Sn and Sn alloy finishes, one of the reliability concerns remains the risk of whisker growth. Results from recent work have suggested that whiskers are most likely to form in regions of the films where high stress or a stress gradient exists. If strain/stress distribution information can be collected at a grain-by-grain level, correlations between such information and the propensity of whisker growth can be further understood. In this work, we utilized a highly focused X-ray beam from a synchrotron source to perform micro-diffraction on a series of Sn and Sn-containing finishes. The high brightness and small beam size of the X-ray enabled the generation of grain-by-grain orientation map as well as the strain/stress levels in individual grains. The electroplated finishes analyzed included pure Sn, Sn-Cu, and Sn-Cu-Pb finishes with various concentrations of Cu and Pb. Plating current density was also varied for each finish composition and the textures of these finishes were compared. After plating, these finishes were stored at ambient condition and examined regularly for surface defect formation. Once hillock or whisker growth was observed, the areas surrounding the growth were scanned with the X-ray. Additionally, these samples were also analyzed with standard X-ray diffraction and inverse pole figures were generated to compare the texture of the samples. A finite element model was also generated to simulate the texture of the finishes. By implementing the stiffness matrix of the finishes, we were able to explicitly implement the variation of finish texture on a grain-by-grain basis, and thus assess the strain/stress distribution in the finish. The analytical and simulation results from this study suggest that plating process parameters such as current density have a significant impact on the crystallographic texture of the plated finishes. Under similar strain conditions, certain textures would generate higher stresses in the finishes and result in higher levels of whisker growth.