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 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 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.     

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.     

Optimum Thickness of Sn Film for Whisker Growth

By depositing different thicknesses of Sn films over a silicon wafer precoated with Cr and Ni adhesion layers and then by bending the tinned wafer using a dead load applied at the center to introduce the same compressive stresses in the Sn films, the growth rate of whiskers appeared to have a maximum for a certain thickness. This is explained by assuming the Sn atoms to flow along the vertical grain boundaries (perpendicular to the interface) into the interface between Sn and Ni and then along the interface to the root of the whisker through some more vertical grain boundaries. The resistance along the vertical grain boundaries appeared to control the rate of whisker growth for thick films.     

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     

锡晶须及其抑制技术

晶须特别是锡须问题已成为印制板无铅焊接质量可靠性的核心问题之一.从研究外应力型、内应力型以及焊接过程中热循环等产生晶须的主要因素出发,分析了不同条件下晶须的产生机理,并给出了如何有效抑制晶须产生的措施.最后扼要指出了晶须研究的今后研究方向.     

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.     

Humidity Effects on Sn Whisker Formation

Due to legislative issues, Pb-containing metallizations on semiconductor components are rapidly converted to Pb-free alternatives. One of the most popular alternatives is Sn electroplating. The major problem of these platings is the formation of Sn whiskers. In earlier publications, two mechanisms were uncovered that are responsible for whisker growth. However, these mechanisms do not explain whisker growth in high humidity. Therefore, Freescale, Infineon, Philips, and STMicrolectronics (E4) joined forces and started a design of experiment (DoE) in order to resolve this mechanism. It is shown that in high humidities, whiskers grow due to oxidation and corrosion of the Sn plating, irrespective of the base material. It is also shown that board assembly mitigates the whisker growth by this mechanism but does not completely prevent it     

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     

Significance of Nucleation Kinetics in Sn Whisker Formation

Sn whiskers are believed to form in response to stress in layers used as protective coatings. However, what makes them form at specific sites on the surface is not known. We have used thermal expansion mismatch to induce stress and observe the resulting whisker formation. Cross-sectional measurements of the region around whiskers show that there are oblique grain boundaries under the whiskers that are not seen in the as-deposited columnar structure. The kinetics also suggest that the whiskering sites may be formed by a nucleation process. Based on these results, we propose a nucleation mechanism in which the boundaries of the surrounding grains migrate due to strain energy differences and create oblique boundaries at which whiskers can form. A simple model is developed to predict the stress-dependence of the nucleation rate.     

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.     

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.     

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     

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.     

The Effect of Annealing on Tin Whisker Growth

This paper presents a design-of-experiments study on the effect of annealing and simulated reflow on tin whisker growth. Copper, brass, and alloy 42 coupons plated with either bright or matte tin were subjected to one of three elevated temperature exposures. After the elevated temperature exposures, specimens along with a set of control specimens were then kept in room ambient conditions and monitored periodically using an environmentally scanning electron microscope. Surface observations up to 16 months of room ambient exposure revealed that tin whiskers formed on the surfaces of each specimen. However, various differences in whisker growth between the matte- and bright tin-plated specimens were observed. Columnar-type whiskers grown on the matte tin plated specimens were initiated from one grain at the surface, as opposed to the growth on bright tin which were independent from the surface morphology. Maximum length and length distribution data for matte and bright tin plating for the various exposures are presented. The result of this study shows annealing to be effective in reducing the maximum length of whiskers, particularly on bright finished coupons     

Length Distribution Analysis for Tin Whisker Growth

The global movement to lead-free electronics has led semiconductor device assemblers to switch terminals and finishes from lead-based to pure tin or high tin lead-free alloys. This transition has resulted in a reliability concern associated with the formation of conductive tin whiskers, which can grow from a device terminal or lead and cause current leakage or short circuits. This paper presents the results of an experimental study on tin whisker growth. Test specimens consisted of matte and bright tin finishes on copper, Alloy-42, and brass substrate materials. The heat treatments included annealing and two types of simulated reflow. Maximum whisker length and whisker density were measured on 24 different types of tin-plated specimens, after three, eight, and 16 months of room ambient storage after various heat treatments