Nitin Jadhav

Whisker formation in Sn and Pb-Sn coatings: Role of intermetallic growth, stress evolution, and plastic deformation processes

We have simultaneously measured the evolution of intermetallic volume, stress, and whisker density in Sn and Pb–Sn alloy layers on Cu to study the fundamental mechanisms controlling whisker formation. For pure Sn, the stress becomes increasingly compressive and then saturates, corresponding to a plastically deformed region spreading away from the growing intermetallic particles. Whisker nucleation begins after the stress saturates. Pb–Sn layers have similar intermetallic growth kinetics but the resulting stress and whisker density are much less. Measurements after sputtering demonstrate the important role of the surface oxide in inhibiting stress relaxation.

Understanding the Correlation Between Intermetallic Growth, Stress Evolution, and Sn Whisker Nucleation

Stress due to intermetallic (IMC) growth is generally accepted as the driving force for Sn whisker formation, but there are still many unanswered questions regarding the development of stress and how it relates to the growth of whiskers. We have made simultaneous measurements of the evolution of stress, IMC volume, and whisker density on samples of different thicknesses to address the underlying mechanisms of whisker formation. Finite-element simulations are used to study the stress evolution due to IMC growth with various stress relaxation mechanisms: plastic deformation coupled with grain boundary diffusion is found to explain observed stress levels, even in the absence of whisker growth. A model of whisker growth suggests that the average steady-state stress is determined primarily by relaxation processes (dislocation- and diffusion-mediated) and that whisker growth is not the primary stress relaxation mechanism. Implications of our results for whisker mitigation strategies are discussed.

Altering the Mechanical Properties of Sn Films by Alloying with Bi: Mimicking the Effect of Pb to Suppress Whiskers

Stress is believed to be the main driving force for whisker formation in Sn coatings on Cu. This suggests that whiskering can be suppressed by enhancing stress relaxation in the Sn layer, which is believed to be the reason why Sn-Pb alloys do not form whiskers. However, Pb is no longer acceptable for use in electronics manufacturing. As an alternative, we used pulsed plating to create Sn-Bi coatings with an equiaxed microstructure similar to that of Sn-Pb alloys. An optical wafer curvature technique was used to measure stress relaxation kinetics in Sn, Sn-Pb and Sn-Bi alloy thin films during thermal cycles. The results show that Sn-Bi films have significantly enhanced stress relaxation relative to pure Sn films. Comparison between Sn-Bi samples with equiaxed and columnar microstructures shows that both microstructure and alloy composition play a role in enhancing the stress relaxation.

Stress Relaxation in Sn-Based Films: Effects of Pb Alloying, Grain Size, and Microstructure

Stress is believed to provide the driving force for growth of Sn whiskers, so stress relaxation in the Sn layer plays a key role in their formation. To understand and enhance stress relaxation in Sn-based films, the effects of Pb alloying and microstructure on their mechanical properties have been studied by observing the relaxation of thermal expansion-induced strain. The relaxation rate is found to increase with film thickness and grain size in pure Sn films, and it depends on the microstructure in Pb-alloyed Sn films. Measurements of multilayered structures (Sn on Pb-Sn and Pb-Sn on Sn) show that changing the surface layer alone is not sufficient to enhance the relaxation, indicating that the Pb enhances relaxation in the bulk of the film and not by surface modification. Implications of our results for whisker mitigation strategies are discussed.

Correlating whisker growth and grain structure on Sn-Cu samples by real-time scanning electron microscopy and backscattering diffraction characterization

Whiskers/hillocks grow out of Pb-free Sn coatings used in electronics manufacturing. To determine which grains form whiskers/hillocks, we use scanning electron microscopy and backscattering diffraction to simultaneously monitor the surface morphology and grain structure. To reduce surface roughness, we developed a “peel-off” method to prepare ultra-flat samples that were measured repeatedly while whiskers/hillocks formed. We find grains that form into whiskers/hillocks are present in the as-deposited film (i.e., not re-nucleated) and many have horizontal grain boundaries beneath them. Grain rotation during whisker/hillock formation means that measurements performed after the features grow do not indicate their initial grain orientations.

In-situ synchrotron micro-diffraction study of surface, interface, grain structure, and strain/stress evolution during Sn whisker/hillock formation

We have performed X-ray synchrotron micro-diffraction measurements to study the processes controlling the formation of hillocks and whiskers in Sn layers on Cu. The studies were done in real-time on Sn layers that were electro-deposited immediately before the X-raymeasurements were started. This enabled a region of the sample to be monitored from the as-deposited state until after a hillock feature formed. In addition to measuring the grain orientation and deviatoric strain (via Laue diffraction), the X-ray fluorescence was monitored to quantify the evolution of the Sn surface morphology and the formation of intermetallic compound (IMC) at the Sn-Cu interface. The results capture the simultaneous growth of the feature and the corresponding film stress, grain orientation, and IMC formation. The observations are compared with proposed mechanisms for whisker/hillock growth and nucleation.

In Situ Measurement of Voltage-Induced Stress in Conducting Polymers with Redox-Active Dopants

Minimization of stress-induced mechanical rupture and delamination of conducting polymer (CP) films is desirable to prevent failure of devices based on these materials. Thus, precise in situ measurement of voltage-induced stress within these films should provide insight into the cause of these failure mechanisms. The evolution of stress in films of polypyrrole (pPy), doped with indigo carmine (IC), was measured in different electrochemical environments using the multibeam optical stress sensor (MOSS) technique. The stress in these films gradually increases to a constant value during voltage cycling, revealing an initial break-in period for CP films. The nature of the ions involved in charge compensation of pPy[IC] during voltage cycling was determined from electrochemical quartz crystal microbalance (EQCM) data. The magnitude of the voltage-induced stress within pPy[IC] at neutral pH correlated with the radius of the hydrated mobile ion in the order Li(+) > Na(+) > K(+). At acidic pH, the IC dopant in pPy[IC] undergoes reversible oxidation and reduction within the range of potentials investigated, providing a secondary contribution to the observed voltage-induced stress. We report on the novel stress response of these polymers due to the presence of pH-dependent redox-active dopants and how it can affect material performance.

Degradation in Sn Films due to Whisker Formation

a) Brown University, Division of Engineering, Providence, RI 02912 b) EMC Corporation, Franklin, MA ABSTRACT Whisker formation in pure Sn coatings on Cu conductors is a serious impediment to the development of Pb-free electronics manufacturing. Understanding whisker formation is complicated by the fact that it is the result of multiple materials kinetic processes including interdiffusion, intermetallic formation and stress generation We report preliminary studies of whisker growth kinetics and stress evolution aimed at developing a fundamental understanding of the whisker growth process. A proposed model of point defect mediated stress generation provides a simple picture of how the different processes are connected. INTRODUCTION Thin films of pure Sn deposited on Cu can degrade by the formation of whiskers, i.e., thin filaments of Sn that can grow to lengths of 10’s – 100’s of microns. Commercially-used coatings on Cu conductors typically employ Pb-Sn alloys that do not develop whiskers. However, recent environmental regulations require the removal of Pb from electronics manufacturing . This has added a new urgency to understanding the creation of whiskers as the industry considers Pb-free alternatives to the current coating schemes. The phenomenon of whisker formation has been known for many years [1], but the mechanisms and driving forces for whisker formation are still not fully understood. A large body of data indicates that many factors contribute to the formation of whiskers. Understanding their growth therefore requires understanding the interaction of multiple kinetic processes including interdiffusion, intermetallic formation and stress generation. To understand how these processes interact, we are studying the coevolution of stress, intermetallic formation, concentration profile and whisker growth on well characterized thin film samples. In the current work, we report the results of preliminary experiments to study the driving forces and mechanisms controlling whisker formation. We present measurements of stress evolution (using wafer curvature) in vapor-deposited films of Sn on Cu and we also present measurements of whisker formation (using SEM) in films of Sn that have been partially covered with Cu. From these measurements (and others), we propose a picture of whisker formation in which the stress is mediated by the creation and diffusion of point defects. BACKGROUND Electroplated Sn and Sn-alloy finishes are used extensively in the electronic components industry to enhance solderability and corrosion resistance of electrical conductors made from metals such