In situ tensile testing of tin (Sn) whiskers in a focused ion beam (FIB)/scanning electron microscope (SEM)

Tin and tin-alloyed electroplated films are known to be susceptible to whisker growth under a range of conditions, many of which result in the generation of compressive stresses in the film. Compressive stress is considered to be one of the primary causes for whisker nucleation and growth. While extensive investigations have been performed on whisker growth, there have been few studies on the mechanical properties of tin whiskers themselves. We report on the tensile behavior of tin whiskers that were obtained by indentation and furnace aging of electroplated tin films on copper disks. Tensile tests of the whiskers were conducted in situ in a dual beam focused ion beam (FIB)-scanning electron microscope (SEM) system using a micro electro-mechanical systems (MEMS) based tensile testing stage. The strength of the whiskers was found to decrease with an increase in gage length and aged whiskers were found to be weaker than their indented counterparts. The observed gage length effect can be attributed to the probability of finding more defects as the whisker length increases. The effect of processing on the observed strength variation was investigated by analyzing the oxygen content in the whiskers via energy dispersive spectroscopy and the microstructure through transmission electron microscopy (TEM). The deformation mechanisms of whiskers were also inferred using post-mortem TEM. It was observed that the whiskers grown by indentation were dislocation free both before and after deformation. In contrast, whiskers grown by aging showed notable dislocation content (arranged in low energy configurations) even before deformation.     

Mechanism of Reaction Between Nd and Ga in Sn-Zn-0.5Ga-xNd Solder

The mechanism of reaction between Nd and Ga in Sn-Zn-0.5Ga-xNd solder was investigated in order to enhance the reliability of soldered joints. It was found that, after aging treatment at ambient temperature and 125°C for over 3000 h, no Sn whisker growth was observed in Sn-9Zn-0.5Ga-0.08Nd soldered joints. X-ray diffraction (XRD) analysis and thermodynamic calculations indicated that Ga reacted with Nd instead of Sn-Nd intermetallic compound (IMC), eliminating Sn whisker growth. Shear force testing was carried out, and the results indicated that Sn-9Zn-0.5Ga-0.08Nd solder still had excellent mechanical properties after aging treatment. This new discovery can provide a novel approach to develop high-reliability solder without risk of Sn whiskers.     

Mitigation of Sn Whisker Growth by Small Bi Additions

In this study, the morphological development of electroplated matte Sn and Sn-xBi (x = 0.5 wt.%, 1.0 wt.%, 2.0 wt.%) film surfaces was investigated under diverse testing conditions: 1-year room-temperature storage, high temperature and humidity (HTH), mechanical loading by indentation, and thermal cycling. These small Bi additions prevented Sn whisker formation; no whisker growth was observed on any Sn-xBi surface during either the room-temperature storage or HTH testing. In the indentation loading and thermal cycling tests, short (<5 μm) surface extrusions were occasionally observed, but only on x = 0.5 wt.% and 1.0 wt.% plated samples. In all test cases, Sn-2Bi plated samples exhibited excellent whisker mitigation, while pure Sn samples always generated many whiskers on the surface. We confirmed that the addition of Bi into Sn refined the grain size of the as-plated films and altered the columnar structure to form equiaxed grains. The storage conditions allowed the formation of intermetallic compounds between the plated layer and the substrate regardless of the Bi addition. However, the growth patterns became more uniform with increasing amounts of Bi. These microstructural improvements with Bi addition effectively released the internal stress from Sn plating, thus mitigating whisker formation on the surface under various environments.     

Mitigative Tin Whisker Growth Under Mechanically Applied Tensile Stress

Sn whisker/hillock growth is a result of the release of compressive stress in a Sn thin film. Filamentary Sn whiskers were formed on an electrodeposited Sn thin film aged at room temperature, while Sn hillocks were formed as the aging temperature was raised to 80°C and 150°C. By mechanically applying a tensile stress on the Sn thin film, the growth of the Sn whisker/hillock was significantly mitigated. This mitigation growth suggests that part of the compressive stress in the Sn thin film was neutralized by the mechanically applied tensile stress.     

Tin Whisker Growth on NdSn<Subscript>3</Subscript> Powder

Tin whiskers grew rapidly and spontaneously on NdSn₃ powder under atmospheric conditions. By in situ optical microscopy observation, the incubation period of whisker growth was found to be very short, only about 10 min to 30 min, and the whisker growth rate was very high (up to 73 Å/s). It is proposed that the strong tendency for whisker growth on NdSn₃ powder indicates that such growth is closely related to decomposition of NdSn₃ under atmospheric conditions. An electron beam irradiation effect on whisker growth was also observed, in which the whiskers cease to grow after observation by scanning electron microscopy (SEM).     

Sn Corrosion and Its Influence on Whisker Growth

The microstructure and crystal structure of condensation-induced corrosion products, vapor phase induced oxidation products, Cu-Sn intermetallics, and Sn whiskers that formed on electroplated matte Sn on Cu-alloy after exposure 2500 h in a 60 degC/93%RH ambient were characterized with scanning electron microscopy, (SEM), focused ion beam (FIB) microscopy, energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and selected area electron diffraction (SAD). The corrosion product was identified as crystalline SnO₂. The oxidation of Sn in condensed water was at least four orders of magnitude larger than that in moist vapor at 60 degC. All Sn whiskers were found to be within 125 mum of the corrosion product. Based on these observations, a theory was developed. The theory assumes that oxidation leads to the displacement of Sn atoms within the film. Because the grain boundaries and free surfaces of the film are pinned, the oxidation-induced excess Sn atoms are constrained within the original volume of the Sn-film. The trapped excess Sn atoms create localized stress, excess strain energy, in the Sn-film. If and when the pinning constraint is relaxed, as for example would occur when the surface oxide on the film cracks, then the Sn atoms can diffuse to lower energy configurations. When this occurs, whisker nucleation and growth begins. The theory was tested by detailed measurements and comparison of the corrosion volume and the whisker volume in two different samples. The volume comparisons were consistent with the theory     

Investigation of Sn Whisker Growth in Electroplated Sn and Sn-Ag as a Function of Plating Variables and Storage Conditions

Sn whiskers are becoming a serious reliability issue in Pb-free electronic packaging applications. Among the numerous Sn whisker mitigation strategies, minor alloying additions to Sn have been proven effective. In this study, several commercial Sn and Sn-Ag baths of low-whisker formulations are evaluated to develop optimum mitigation strategies for electroplated Sn and Sn-Ag. The effects of plating variables and storage conditions, including plating thickness and current density, on Sn whisker growth are investigated for matte Sn, matte Sn-Ag, and bright Sn-Ag electroplated on a Si substrate. Two different storage conditions are applied: an ambient condition (30°C, dry air) and a high-temperature/high-humidity condition (55°C, 85% relative humidity). Scanning electron microscopy is employed to record the Sn whisker growth history of each sample up to 4000 h. Transmission electron microscopy, x-ray diffraction, and focused ion beam techniques are used to understand the microstructure, the formation of intermetallic compounds (IMCs), oxidation, the Sn whisker growth mechanism, and other features. In this study, it is found that whiskers are observed only under ambient conditions for both thin and thick samples regardless of the current density variations for matte Sn. However, whiskers are not observed on Sn-Ag-plated surfaces due to the equiaxed grains and fine Ag₃Sn IMCs located at grain boundaries. In addition, Sn whiskers can be suppressed under the high-temperature/high-humidity conditions due to the random growth of IMCs and the formation of thick oxide layers.     

The Effect of Micro-Alloying of Sn Plating on Mitigation of Sn Whisker Growth

Tin (Sn) is a key industrial material in coatings on various components in the electronics industry. However, Sn is prone to the development of filament-like whiskers, which is the leading cause of many types of damage to electronics reported in the last several decades. Due to its properties, a tin-lead (Sn-Pb) alloy coating can mitigate Sn whisker growth. However, the demand for Pb-free surface finishes has rekindled interest in the Sn whisker phenomenon. In order to achieve properties similar to those naturally developed in a Sn-Pb alloy coating, we carried out a study on deposited films with other Sn alloys, such as tin-bismuth (Sn-Bi), tin-zinc (Sn-Zn), and tin-copper (Sn-Cu), electrodeposited onto a brass substrate by utilizing a pulse plating technique. The results indicated that the Sn alloy films modified the columnar grain structure of pure Sn into an equiaxed grain structure and increased the incubation period of Sn whisker growth. The primary conclusions were based on analysis of the topography and microstructural characteristics in each case, as well as the stress distribution in the plated films computed by x-ray diffraction, and the?amount of Sn whisker growth in each case, over 6 months under various environmental influences.     

Effect of Ag on the Microstructure of Sn-8.5Zn-xAg-0.01Al-0.1Ga Solders Under High-Temperature and High-Humidity Conditions

The effect of Ag on the microstructure and thermal behavior of Sn-Zn and Sn-8.5Zn-xAg-0.01Al-0.1Ga solders (x from 0.1 wt.% to 1 wt.%) under high-temperature/relative humidity conditions (85°C/85% RH) for various exposure times was investigated. Scanning electron microscopy (SEM) studies revealed that, in all the investigated solders, the primary α-Zn phases were surrounded by eutectic β-Sn/α-Zn phases, in which fine Zn platelets were dispersed in the β-Sn matrix. SEM micrographs revealed that increase of the Ag content to 1 wt.% resulted in coarsening of the dendritic plates and diminished the Sn-9Zn eutectic phase in the microstructure. Differential scanning calorimetry (DSC) studies revealed that the melting temperature of Sn-8.5Zn-xAg-0.01Al-0.1Ga solder decreased from 199.6°C to 199.2°C with increase of the Ag content in the solder alloy. Both ZnO and SnO₂ along with Ag-Zn intermetallic compound (IMC) were formed on the surface when Sn-8.5Zn-0.5Ag-0.01Al-0.1Ga solder was exposed to high-temperature/high-humidity conditions (85°C/85% RH) for 100 h. The thickness of the ZnO phase increased as the Ag content and exposure time were increased. Sn whiskers of various shapes and lengths varying from 2 μm to 5 μm were extruded from the surface when the investigated five-element solder with Ag content varying from 0.5 wt.% to 1 wt.% was exposed to similar temperature/humidity conditions for 250 h. The length and density of the whiskers increased with further increase of the exposure time to 500 h and the Ag content in the solder to 1 wt.%. The Sn whisker growth was driven by the compressive stress in the solder, which was generated due to the volume expansion caused by ZnO and Ag-Zn intermetallic compound formation at the grain boundaries of Sn.     

Corrosion Behavior of Pb-Free Sn-1Ag-0.5Cu-XNi Solder Alloys in 3.5% NaCl Solution

Potentiodynamic polarization techniques were employed in the present study to investigate the corrosion behavior of Pb-free Sn-1Ag-0.5Cu-XNi solder alloys in 3.5% NaCl solution. Polarization studies indicated that an increase in Ni content from 0.05 wt.% to 1 wt.% in the solder alloy shifted the corrosion potential (E _corr) towards more negative values and increased the linear polarization resistance. Increased addition of Ni to 1 wt.% resulted in significant increase in the concentration of both Sn and Ni oxides on the outer surface. Secondary-ion mass spectrometry and Auger depth profile analysis revealed that oxides of tin contributed primarily towards the formation of the passive film on the surface of the solder alloys containing 0.05 wt.% and 1 wt.% Ni. Scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX) established the formation of a Sn whisker near the passive region of the solder alloy obtained from the polarization curves. The formation of Sn whiskers was due to the buildup of compressive stress generated by the increase in the volume of the oxides of Sn and Ni formed on the outer surface. The presence of Cl^? was responsible for the breakdown of the passive film, and significant pitting corrosion in the form of distinct pits was noticed in Sn-1Ag-0.5Cu-0.5Ni solder alloy after the polarization experiment.     

Effects of continuously applied stress on tin whisker growth

A simple four-point bending technique in conjunction with scanning electron microscopy (SEM) was employed to investigate the relationship between continuously applied mechanical stress and tin whisker growth on bright tin-plated copper specimens at room temperature. Measurements of whisker length and density were periodically taken for each specimen during a 7-month exposure to applied stress. Tin whiskers were found on the tin-plated specimens with or without the presence of mechanically applied stresses. A continuously applied compressive stress resulted in formation of longer and more dense tin whiskers, in comparison with the cases of no applied stress and applied tensile stress. However, an increase in the applied compressive stress level caused an increase in the whisker density but a decrease in the whisker length. On the other hand, a continuously applied tensile stress was found to reduce both the whisker density and length, compared to the case of no applied stress. Apparently, application of a continuous tensile stress could provide an effective means in retarding tin whisker growth.     

Effects of Disordered Atoms and Nanopores on Mechanical Properties of β-Zn4Sb3: a Molecular Dynamics Study

Effects of disordered Zn atoms and nanopores on mechanical properties of β-Zn₄Sb₃ are studied by using the molecular dynamics (MD) method. Due to the influence of disordered Zn atoms in β-Zn₄Sb₃, the elastic modulus decreases from 90.85 GPa to 68.17 GPa, a decrease of 24.96%. The ultimate tensile stress decreases from 18.25 GPa to 9.96 GPa, a decrease of 45.42%. The fracture strain decreases from 32.7% to 20.8%, a decrease of 36.39%. Due to the influence of nanopores, the elastic modulus decreases with growing porosity, and the relationship between the elastic modulus and porosity leads to a scaling law. It seems that the porous radius and porous distribution are also important factors influencing the ultimate tensile stress and fracture strain, in addition to the porosity. However, our simulation results demonstrate that disordered Zn atoms and nanopores reduce the structural stability, dramatically decreasing the mechanical properties of β-Zn₄Sb₃.     

Effects of POSS-Silanol Addition on Whisker Formation in Sn-Based Pb-Free Electronic Solders

Previous studies have indicated that silanol in the form of polyhedral oligomeric silsesquioxane (POSS) trisilanol could form strong bonds with solder matrix without agglomeration, and inhibit diffusion of metal atoms when subjected to high ambient temperature and/or high current density. Addition of POSS-trisilanol has also been shown to improve the comprehensive performance of Sn-based Pb-free solders, such as shear strength, resistance to electromigration, as well as thermal fatigue. The current study investigated the whisker formation/growth behaviors of Sn-based Pb-free solders (eutectic Sn-Bi) modified with 3 wt.% POSS-trisilanol. Solder films on Cu substrates were aged at ambient temperature of 125°C to accelerate whisker growth. The microstructural evolution of the solder films’ central and edge areas was examined periodically using scanning electron microscopy. Bi whiskers were observed to extrude from the surface due to stress/strain relief during growth of Sn-Cu intermetallic compounds (IMCs). Addition of POSS-trisilanol was shown to retard the growth of Bi whiskers. The IMCs formed between POSS-modified solders and the Cu substrate showed smoother surface morphology and slower thickness growth rate during reflow and aging. It was indicated that POSS particles located at the phase boundaries inhibited diffusion of Sn atoms at elevated temperatures, and thus limited the formation and growth of IMCs, which resulted in the observed inhibition of Bi whisker growth in POSS-modified solders.     

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.     

Dynamic Recrystallization (DRX) as the Mechanism for Sn Whisker Development. Part II: Experimental Study

In Part I of this study, a dynamic recrystallization (DRX) model was proposed to describe the development of metal whiskers. A diffusion-assisted, dislocation-based mechanism would support the DRX steps of grain initiation (refinement) and grain growth. This, Part II, describes experiments investigating the time-dependent deformation (creep) of Sn under temperature conditions (0°C, 25°C, 50°C, 75°C, and 100°C) and stresses (1 MPa, 2 MPa, 5 MPa, and 10 MPa) that are commensurate with Sn whisker development, in order to parameterize the DRX process. The samples, which had columnar grains oriented perpendicular to the stress axis similar to their morphology in Sn coatings but of larger size, were tested in the as-fabricated condition as well as after 24 h annealing treatments at 150°C or 200°C. The steady-state creep behavior fell into two categories: low (<10⁻⁷ s⁻¹) and high strain rates (>10⁻⁷ s⁻¹). The apparent activation energy (ΔH) at low strain rates was 8 ± 9 kJ/mol for the as-fabricated condition, indicating that an anomalously or ultrafast diffusion mass transport mechanism assisted deformation. Under the high strain rates, the ΔH was 65 ± 6 kJ/mol (as-fabricated). The rate kinetics were not altered significantly by the annealing treatments. The critical strain (ε _c) and Zener–Hollomon parameter (Z) confirmed that these stresses and temperatures were nearly capable of causing cyclic DRX in the Sn creep samples, but would certainly do so in Sn coatings with the smaller grain size. The effects of the annealing treatments, coupled with the DRX model, indicate the need to maximize the creep strain rate during stress relaxation so as to avoid conditions that would favor whisker growth. This study provides a quantitative methodology for predicting the likelihood of whisker growth based upon the coating stress, grain size, temperature, and the similarity assumption of creep strain.     

A Statistical Study of Sn Whisker Population and Growth During Elevated Temperature and Humidity Tests

Storage tests at elevated temperature and humidity conditions have been widely adopted as one of the major acceleration tests for Sn whisker growth. However, the driving force associated and the nucleation and growth process of whiskers are yet to be fully understood. In this paper, Sn whisker growth on Cu leadframe material at two different test conditions is compared. Both loose and board-mounted components were used. At each read point, the length and location of every whisker observed was recorded. Statistical characteristics and growth rate of the whisker population will be presented for each of the tests conditions. On loose components, corrosion of the Sn finish was observed near the tip and the dam bar cut area of the leads with backscatter scanning electron microscopy (SEM) and optical microscopy. The entire population of whiskers was located in these corroded areas, and there were zero whiskers located in the noncorroded areas on the same leads. On board-mounted components, the corrosion level of the Sn finish, as well as the whisker population and length was greatly reduced compared to those on the loose components. These results suggest that the corrosion of Sn finish in high-temperature and high-humidity conditions is the major driving force for whisker growth. The cause for the difference between the loose and board-mounted components is also analyzed     

Sn whisker evaluations in 3D microbumped structures

Sn whiskering remains a reliability concern in electronic applications. Despite extensive research on growth rates and mitigation strategies, no predictive theory is in place. Literature data are available for Cu/Sn-based films and coatings as well as for board-level and flip-chip solder bumps but data are scarce for scaled-down solder volumes and for higher intermetallic-to-solder ratios. The current work investigates whiskers in “isolated geometries” for 3D solder-capped Cu microbumps with >2 orders of magnitude smaller solder volumes compared to state-of-the-art. To the best of the authors’ knowledge, this is the first time Sn whisker growth is reported in isolated solder volumes (e.g. <8 μm-side cube). Whiskers propensity was evaluated using JEDEC industrial specifications. The tested structures were: 5/3.5 μm-thick Cu/Sn films and 15 μm-diameter electroplated solder capping (Sn, SnAg, SnCu) on Cu microbumps (as-plated vs. reflowed). Selected Sn whiskers and “whisker-like” features were analysed and identified experimentally with SEM, EDX and FIB. In the absence of a predictive model, first-order and “what if” calculations based on IMC molar volume and oxide cracking hypotheses were carried out. This approach quantifies “figures of merit” for Sn whisker propensity with (1) different bump-limiting metallization (BLM) cases e.g. Cu, Ni, Co and (2) further microbump scaling. Future research recommendations are outlined to mitigate manufacturing risks by controlling “sit time” between bumping and stacking.     

In Situ Measurement of Stress and Whisker/Hillock Density During Thermal Cycling of Sn Layers

Compressive stress is believed to be the primary driving force that makes Sn whiskers/hillocks grow, but the mechanisms that create the stress (e.g., intermetallic compound growth) are difficult to control. As an alternative, the thermal expansion mismatch between the Sn layer and the substrate can be used to induce stress in a controlled way via heating and cooling. In this work, we describe real-time experiments which quantify the whiskering behavior and stress evolution during cyclic heating. The density of whiskers/hillocks is measured with an optical microscope, while the stress is measured simultaneously with a wafer-curvature-based multi-beam optical stress sensor. Results from three thermal cycles are described in which the samples are heated from room temperature to 65 °C at rates of 10, 30, and 240 °C/h. In each case, we find that the whisker/hillock formation is the primary source of stress relaxation. At fast heating rates, the relaxation is proportional to the number of hillocks, indicating that the stress is relaxed by the nucleation of many small surface features. At slower heating rates, the whisker/hillock density is lower, and continual growth of the features is suggested after nucleation. Long whiskers are found to be more likely to form in the slow heating cycle.     

Tin Whisker Growth Induced by High Electron Current Density

The effect of electric current on the tin whisker growth on Sn stripes was studied. The Sn stripes, 1 μm in thickness, were patterned on silicon wafers. The design of the Sn stripes allowed the simultaneous study of the effect of current crowding and current density. Current stressing was performed in ovens set at 30, 50, or 70°C, and the current density used ranged from 4.5 × 10⁴ A/cm² to 3.6 × 10⁵ A/cm². It was found that the stress induced by the electric current caused the formation of many Sn whiskers. A higher current density caused more Sn whiskers to form. Of the three temperatures studied, 50°C was the most favorable one for the formation of the Sn whiskers. In addition, the current-crowding effect also influenced whisker growth.     

Mitigation and Verification Methods for Sn Whisker Growth in Pb-Free Automotive Electronics

This work describes mitigation methods against Sn whisker growth in Pb-free automotive electronics using a conformal coating technique, with an additional focus on determining an effective whisker assessment method. We suggest effective whisker growth conditions that involve temperature cycling and two types of storage conditions (high-temperature/humidity storage and ambient storage), and analyze whisker growth mechanisms. In determining an efficient mitigation method against whisker growth, surface finish and conformal coating have been validated as effective means. In our experiments, the surface finish of components comprised Ni/Sn, Ni/SnBi, and Ni/Pd. The effects of acrylic silicone, and rubber coating of components were compared with uncoated performance under high-temperature/humidity storage conditions. An effective whisker assessment method during temperature cycling and under various storage conditions (high temperature/humidity and ambient) is indicated for evaluating whisker growth. Although components were finished with Ni/Pd, we found that whiskers were generated at solder joints and that conformal coating is a useful mitigation method in this regard. Although whiskers penetrated most conformal coating materials (acrylic, silicone, and rubber) after 3500 h of high-temperature/humidity storage, the whisker length was markedly reduced due to the conformal coatings, with silicone providing superior mitigation over acrylic and rubber.