Whisker and hillock formation in thin films is well known as a highly local mechanism for stress relaxation, where in many cases, only a few whiskers form out of thousands of grains in a film. In this article, the microstructural characteristics for specific grains to form whiskers in β-Sn films are discussed in light of our recent whisker growth model, establishing a relationship among grain boundary sliding limited Coble creep, surface grain geometry, and film stress for different stress conditions, including for thermal cycling. Through our recent finite-element simulations of stresses induced by room-temperature aging and thermal cycling of textured microstructures, the role of elastic and thermoelastic anisotropy in creating preferred whisker formation sites and the general propensity of a film to form whiskers have been proposed for a range of β-Sn film textures. Taken together, these models suggest a strategy for identifying the effects of local microstructure and β-Sn anisotropy on whisker formation. If these predictions are accurate, then whisker growth risk may be effectively reduced by engineering film microstructures and textures for specific applications and stress conditions.

A whisker and hillock growth model based on local film microstructure, grain misorientation, and elastic strain energy density (ESED) as the driving force for growth was developed to predict preferred sites for growth. Local grain orientations and strains measured by synchrotron microdiffraction in nine regions containing whiskers or hillocks were compared with elastic finite element analysis simulations including Sn elastic anisotropy. Whisker and hillock grains were observed to have higher crystallographic misorientations with neighboring grains than generally observed in the microstructure. While elastic simulations predicted higher local out-of-plane elastic strains and ESEDs at those locations with high misorientations before growth, synchrotron measurements of out-of-plane strains of whisker and hillock grains after growth showed relaxation, with correspondingly low ESEDs calculated from measured strains. Hence, highly localized out-of-plane elastic strains and ESEDs of grains with high relative misorientations with their neighbors determined, at least in part, which grains became whiskers or hillocks.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

In this paper, the concept of a defect phase diagram is introduced which quantifies the effects of Cu and Pb additions to electrodeposited Sn films on surface defect formation, including but not limited to the formation of Sn whiskers. Transitions were observed in both the defect densities and the morphologies of hillocks and whiskers as Cu and Pb film compositions were systematically varied. Changes in crystallographic texture were also reported for a subset of the Sn-Cu-Pb alloys examined. The transitions between different defect types and the coexistence of certain defect types help to interpret the role of grain boundary pinning in hillock and whisker formation.

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.

The growth of surface defects on lead-free tin electroplated films is believed to be a stress relief phenomenon. These observed defects include hillocks and whiskers that grow spontaneously after deposition. Previous work has shown that it is possible to plate pure tin and observe only hillock growth. Whisker growth, however, is observed with the addition of copper contamination to the electrolyte. In this work, pure tin and tin-copper films of increasing copper content were electroplated on phosphor bronze substrates. The stresses associated with plating these films and the evolution of stress over time were measured using cantilever beam deflection. The density and morphologies of hillocks and whiskers were measured as a function of time and are related to cantilever beam stress measurements. Results show an increase in long-term stress with increasing copper content. An initial decrease in the plating stress over the first 24 to 48 hours and an increase in relaxation rate with copper amounts of less than 0.0018 mol / L in the electrolyte was also observed. A transition in film behavior observed in copper concentrations between 0.0018 and 0.0057 mol / L, as characterized by an increasing as- plated plating stress, decreasing relaxation rate, and the appearance of whiskers. A change in hillock appearance was also observed in these concentrations. Results support a growth model where a source feeds material into a defect while the ratio of incoming material to the rate of grain boundary movement determines the types of defects observed.