Whisker Growth Behavior of Sn and Sn Alloy Lead-Free Finishes

Sn whisker growth behavior, over periods of time up to 10,080 h at room temperature, was examined for Sn and Sn-Cu, Sn-Ag, Sn-Bi, and Sn-Pb coatings electroplated on copper in 2 μm and 5 μm thicknesses to understand the effects of the alloying elements on whisker formation. Sn-Ag and Sn-Bi coatings were found to significantly suppress Sn whisker formation compared with the pure Sn coatings, whereas whisker growth was enhanced by Sn-Cu coatings. In addition, annealed Sn and Sn-Pb coatings were found to suppress Sn whisker formation, as is well known. Compared with the 2-μm-thick coatings, the 5-μm-thick coatings had high whisker resistance, except for the Sn-Cu coating. Whisker growth was correlated with coating crystal texture and its stability during storage, crystal grain microstructure, and the formation of intermetallic compounds at Sn grain boundaries and substrate–coating interfaces.     

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.     

Mitigation of Sn Whisker Growth by Composite Ni/Sn Plating

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.     

The Role of Silver in Mitigation of Whisker Formation on Thin Tin Films

The mitigating effect of alloying Sn thin films with Ag on the formation of Sn whiskers was investigated by time-resolved investigations employing x-ray diffraction for phase and stress analyses and focused ion beam microscopy for morphological characterization of the surfaces and cross-sections of the specimens. The investigated Sn-6 wt.%Ag thin films were prepared by galvanic co-deposition. The results are compared with those obtained from investigation of pure Sn films and discussed with regard to current whisker-growth models. The simultaneous deposition of Sn and Ag leads to a fine-grained microstructure consisting of columnar and equiaxed grains, i.e. an imperfect columnar Sn film microstructure. Isolated Ag3Sn grains are present at the Sn grain boundaries in the as-deposited films. Pronounced grain growth was observed during aging at room temperature, which provides a global stress relaxation mechanism that prevents Sn whisker growth.     

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.     

Effect of Cu additives on Sn whisker formation of Sn(Cu) finishes

Sn whisker formation on Sn(Cu) finishes has been studied. (1) With respect to the thickness effect, we found that Sn whisker density for pure Sn and Sn0.7Cu finishes has a linear relationship with the finish thickness. The safety thickness for Sn and Sn0.7Cu finishes is about 10 μm and 20 μm, respectively. (2) With respect to the alloying effect, we found that Sn whisker formation could be retarded by increasing Cu content in the Sn(Cu) finishes. We conclude that the Cu additives could reduce the two major driving forces of the Sn whisker formation, i.e., metal underlayer dissolution and thermal stress. The Cu additives self-formed a Cu-Sn compound barrier layer, which effectively prevents the reaction and dissolution with the metal underlayer. On the other hand, the Cu additives precipitated out as Cu-Sn compound in the Sn(Cu) finish layer, which is believed to be the reason for smaller values of the coefficient of thermal expansion (CTE) for Sn(Cu) alloys. The smaller CTE values results in a lower thermal stress level in the Sn(Cu) finishes.     

Tensile creep and microstructural characterization of bulk Sn3.9Ag0.6Cu lead-free solder

The microstructural and creep behavior of bulk 63SnPb37 and the Pb-free solder alloy Sn3.9Ag0.6Cu are reported and compared. The Sn3.9Ag0.6Cu alloy showed much lower absolute creep rates than 63SnPb37. The size and distribution of the intermetallic compound (IMC) coarsened with increasing creep temperature. A number of coarsened precipitates of Cu₆Sn₅ segregate around β-Sn grain boundaries. After creep at 80°C and 115°C. the β-Sn particles in the Sn3.9Ag0.6Cu alloy are strongly aligned at approximately 45° to the uniaxial tension, parallel to the maximum shear-stress planes. The powerlaw-defined stress exponent significantly increases with increasing stress in both the 63Sn37Pb and Sn3.9Ag0.6Cu alloys; therefore, the Dorn model is unsuitable for these materials over large stress and temperature ranges. Both sets of experimental data were successfully fit with the present power-law stress-dependent energy-barrier model and the Garofalo model. However, the application of the present power-law stress-dependent energy model resulted in a significantly lower estimated variance as compared to the Garofalo model.     

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.     

An integrated theory of whisker formation: the physical metallurgy of whisker formation and the role of internal stresses

Over 50 years of whisker research (cadmium, zinc, and tin) has not resulted in consensus about whisker formation fundamentals for metal films. New analytical tools have recently provided new insights into microstructural changes that occur during whisker formation. Integration of these newer observations with historical data led the authors to propose an Integrated Theory of Whisker Formation. The Integrated Theory incorporates physical attributes such as microstructure and internal stress states. Particular emphasis is placed on recrystallization, grain boundary diffusion, film-substrate interdiffusion (i.e., the Kirkendall effect), and stress gradients. The Integrated Theory does not require dislocation mechanisms for material transport to the whisker location. Material is transported to a whisker grain by the surrounding grain boundary network under the driving impetus of positive stress gradients. Transported atoms then move from the grain boundary network into the whisker grain. This movement into the whisker grain pushes the free surface of the whisker grain upward and, thereby, grows the whisker structure.     

Hillock and Whisker Growth on Sn and SnCu Electrodeposits on a Substrate Not Forming Interfacial Intermetallic Compounds

Intermetallic compound (IMC) formation at the interface between the tin (Sn) plating and the copper (Cu) substrate of electronic components has been thought to produce compressive stress in Sn electrodeposits and cause the growth of Sn whiskers. To determine if interfacial IMC is a requirement for whisker growth, bright Sn and a Sn-Cu alloy were electroplated on a tungsten (W) substrate that does not form interfacial IMC with the Sn or Cu. At room temperature, conical Sn hillocks grew on the pure Sn deposits and Sn whiskers grew from the Sn-Cu alloy electrodeposits. These results demonstrate that interfacial IMC is not required for initial whisker growth.     

Whisker mitigation measures for Sn-plated Cu for different stress tests

The effectiveness of the widely-used whisker mitigation measures for Sn-plated Cu base material (annealing at 150 °C for 1 h or a Ni interlayer) were investigated after temperature cycling and after storage at room temperature. It was found that these measures prevent whisker growth during isothermal storage, but not during temperature cycling. These mitigation measures do apparently not reduce the compressive stress that builds up during temperature cycling due to different coefficients of thermal expansion of Sn and Cu. A change of the Sn microstructure to globular grains is proposed and investigated as potential whisker mitigation measure for temperature cycling.     

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.     

Electromigration studies on Sn(Cu) alloy lines

The Cu alloying effect in the Sn(Cu) solder line has been studied. The Sn0.7Cu solder line has the most serious electromigration (EM) damage compared to pure Sn and Sn3.0Cu solder lines. The dominant factor for the fast EM rate in Sn0.7Cu could be attributed to the relatively small grain size and the low critical stress, i.e., the yielding stress of the Sn0.7Cu solder line. Also, we found that the shortest Sn0.7Cu solder line, 250 μm, has the most serious EM damage among three solder lines of different lengths. The back stress induced by EM might not play a significant role on the EM test of long solder lines. A new failure mode of EM test was observed; EM under an external tensile stress. The external stress is superimposed on the stress profile induced by EM. As a result, the hillock formation was retarded at the anode side, and void formation was enhanced at the cathode.     

Driving force for the formation of Sn whiskers: compressive stress-pathways for its generation and remedies for its elimination and minimization

Compressive stress is widely accepted as the driving force for tin whisker formation. There are several pathways for compressive stress buildup in Sn coatings, which include the following: residual stress generated during plating; stress formation due to interfacial reactions between tin and copper substrate; mechanical stress; and thermal mechanical stress due to coefficient of thermal expansion (CTE) mismatch between the tin layer and substrate during thermal cycling. In order to prevent or reduce whisker growth in tin deposits, compressive stress has to be eliminated or minimized. This paper discusses the pathways for compressive stress formation and various remedies for its elimination and minimization. Particularly, a novel approach for dealing with thermal mechanical stress due to CTE mismatch is discussed.     

Tin Whisker Nucleation and Growth on Sn-Pb Eutectic Coating Layer Inside Plated Through Holes With Press-Fit Pins

It has been long believed that residual stress is the root cause for tin whisker formation on pure tin-plated component leads. However, tin whisker formation could be observed on the surface of other tin-based alloys under certain conditions. In this study, the whisker formation was reported on a coating layer of Sn-Pb eutectic hot air solder leveling (HASL), which was under compression stress conditions due to the inserted compliant pins. In-Situ scanning electron microscopy was used to monitor the nucleation and growth of whiskers. In addition, a mechanical experiment and non-linear contact finite element analysis were used to estimate the magnitude of the stress in the HASL coating layer. It was found that the tin whisker formation with whisker size of more than 10 mum could occur on the surface of 60Sn-60Pb plating within less than 30 min at an ambient temperature under compressing stress conditions. The tin whisker initiation and growth were further studied at an elevated temperature of 70 degC to check if a higher temperature effects Tin whisker formation. It is believed that establishment of a quantitative relationship of whisker formation/growth under compressive stress and elevated temperature conditions could lead to better scientific methods for risk and reliability assessment and smooth transitions to lead-free assemblies.     

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.     

Whisker growth on surface treatment in the pure tin plating

Whisker behavior at various surface treatment conditions of pure Sn plating are presented. The temperature cycling test for 600 cycles and the ambient storage for 1 year was performed, respectively. From the temperature cycling test, bent-shaped whiskers were observed on matte and semibright Sn plating, and flower-shaped whisker on bright Sn plating. The bright Sn plating has smaller whiskers than the other types of Sn plating, and the whisker growth density per unit area is also lower than the others. After 1 year under ambient storage, nodule growth of FeNi42 lead frame (LF) was observed in some parts. The Cu LF showed about a 9.0 μm hillock-shaped whisker. This result demonstrated that the main determinant of whisker growth was the number of temperature cycling (TC) in the FeNi42 LF, whereas it was the time and temperature in the Cu LF. Also, whisker growth and shape varied with the type of surface treatment and grain size of plating.     

Microstructure-Based Stress Modeling of Tin Whisker Growth

A 3-D finite element method (FEM) model considering the elasticity anisotropy, thermal expansion anisotropy, and plasticity of beta-Sn is established. The Voronoi diagrams are used to generate the geometric patterns of grains of the Sn coating on Cu leadframes. The crystal orientations are assigned to the Sn grains in the model using the X-ray diffraction (XRD) measurement data of the samples. The model is applied to the Sn-plated package leads under thermal cycling tests. The strain energy density (SED) is calculated for each grain. It is observed that the samples with higher calculated SED are more likely to have longer Sn whiskers and higher whisker density. The FEM model, combined with the XRD measurement of the Sn finish, can be used as an effective indicator of the Sn whisker propensity. This may expedite the qualification process significantly     

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 physical model and analysis for whisker growth caused by chemical intermetallic reaction

The formation of intermetallic compound Cu₆Sn₅ gives rise to the internal stress in the lead-free coating, which causes the growth of Sn whiskers. This phenomenon is characterized with the expansion of inclusion in a plate perfectly bonded between two infinite solids. Based on the grain boundary diffusion mechanism, a model is established to evaluate the growth rate of Sn whiskers. The results show that the growth rate of whisker varies with the relative site between whisker and inclusion. When the distance between the whisker and inclusion exceeds a critical value, negative growth rate will appear, and it approaches zero as the distance increases. They explain some phenomena observed in experiments.