Sn Whisker Growth Research Articles

Ni Barrier-Induced Cracks in Matte Sn Films

A nickel barrier layer between a matte tin film and a C194 substrate has been used to prevent whisker formation on the tin surface by blocking the formation of Cu6Sn5 intermetallics, which is one of the root causes of whisker growth. However, the introduction of the Ni barrier to the matte Sn/C194 system greatly changes the mechanical properties of the bended leads. In this paper, the effect of a Ni barrier on the mechanical stability of matte tin films deposited onto C194 leads of an SOIC-8 package is investigated. Backscattered electron imaging and focused ion beam imaging of the cross-section samples indicate that, during the forming process, the surface cracks in the matte tin films often initiate and propagate from the Ni barrier, which lacks cooperative deformation ability in comparison with the ductile copper alloy substrate and the matte tin film. The thicker Ni barrier could induce considerable mechanical damage to the matte Sn films of the integrated circuit package after reflow treatment due to the coefficient of thermal expansion mismatch. The thermal-humidity testing of the Sn/Ni/C194 samples reveals that the surface cracks formed may be attributed to the built-in tensile stress arising from the formation of Ni3Sn4 intermetallic compounds (IMCs). A thinner Ni barrier layer can withstand the forming stress, thermal mismatch stress, and IMC-induced stress without causing cracks in the matte Sn films and does not promote Sn whisker growth.

Influence of current density on microstructure of pulse electrodeposited tin coatings

Abstract Pulse electrodeposited tin coatings on copper substrate have been synthesized from an aqueous solution containing sodium stannate (Na 2 SnO 3 .3H 2 O) and sodium hydroxide (NaOH). The effect of current density on surface morphology of the deposits has been investigated. As deposited coatings are characterized by X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, and line profile analysis. The X-ray diffraction analysis shows that the deposits consist of tetragonal (β-Sn) structure with microcrystalline grains. The deposits plated at lower current density exhibit (110) texture which decreases with increasing current densities. The effects of current density on Cu–Sn diffusion and whisker growth of the electrodeposited tin coatings are also reported here.

410 熱応力誘起によるSnウィスカの作製(材料力学II)

410 熱応力誘起によるSnウィスカの作製(材料力学II)

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.

Sn whisker growth during thermal cycling

Pure Sn plating on ceramic chip capacitors was tested by thermal cycling both in air and in vacuum for up to 3000 cycles and the whisker growth mechanism was clarified. A thin crystalline SnO layer is formed on the surface of Sn plating and whiskers, which exhibits a high level of cracking. Thermal cycling whiskers exhibit two distinct features: fine striation rings on the whisker side face vertical to the whisker growth axis; and deep grooves at the root of the whiskers. One ring of the fine striations corresponds to each thermal cycle. The formation of the grooves can be attributed to thermal cycle cracking along grain boundaries of Sn followed by oxidation and growth of whiskers from the root grains. The characteristic winding feature observed for thermal cycling whiskers can be attributed to the formation of root grooves with severe oxidation. Whisker growth in vacuum is faster than in air. Whiskers grown in vacuum are thinner and longer than whiskers grown in air, while the whisker density is not influenced by atmosphere. The interval of striation rings is wider for vacuum-grown whiskers as compared with air-grown whiskers.

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.

Investigation of Interfacial Reactions and Sn Whisker Formation in the Matte Sn Layer With NiP/Ni/Cu and Ni/Cu Multilayer Systems

This paper investigates the relationship between the interfacial reactions and Sn whisker formation in the matte Sn layer in NiP/Ni/Cu and Ni/Cu multilayer systems. All specimens were first reflowed at 250°C for 10 min and then aged at 150°C for 500 h. Sn whiskers were found only in the Sn/Ni/Cu multilayer specimen. Ni from the NiP layer was consumed to form the Ni3Sn4 phase in the Sn/NiP/Ni/Cu specimen. Thus, the phosphorous (P) atoms segregated at the interface and reacted with Ni and Sn atoms to form the Ni-P-Sn ternary phase between the phase and NiP layer. This Ni-P-Sn ternary phase acted as a diffusion barrier and decreased the diffusion rate and diffusion flux of the Ni atoms. The compressive stress induced by the flux of the Ni atoms was the driving force resulting in Sn whisker growth. The Ni-P-Sn ternary can inhibit this compressive stress. That is why no Sn whiskers were found in the Sn/NiP/Ni/Cu multilayer specimen.

Effect of intermetallic growth rate on spontaneous whisker growth from a tin coating on copper

Intermetallic compound (IMC) growth at the interface between a Sn coating and a Cu substrate with or without a Ni underlayer and its related stress state was evaluated by real-time measurements using the flexure beam method. For the Sn coating without a Ni underlayer, pyramid-shaped IMC grains of Cu6Sn5 grew along the grain boundaries of Sn from the Cu substrate, especially at the triple grain boundary junctions and the IMC grains rapidly increased their volume during the initial 3 days, During this time, the IMC growth rate was 2.3 μm3/day and the compressive stress in the Sn coating rapidly developed and became saturated at about −11 MPa. In contrast, platelet IMC grains of Ni3Sn4 formed at the surface of the Ni underlayer and the IMCs grew at a rate of 1.0 μm3/day for the Sn/Ni coating. In addition, the stress in the Sn coating with a Ni underlayer remained low, slightly tensile and no whiskers were formed on the Sn plating. Therefore, the IMC growth rate and shape are key factors that affect the mechanism of Sn whisker growth during room temperature storage.

Substrate Shape Effect on the Sn Whisker Growth in the Electroplating Matte Sn System

In this study, the substrate shape effect on the Cu substrates for Sn whisker growth has been investigated. A Cu foil, as a substrate, was bent to 90° by a universal testing machine. The matte Sn layers were electroplated on the Cu substrate under various current densities. Then, the samples were given heat treatment under various temperatures for 250 h. The results indicate that Sn whisker growth was promoted by the compression stress on the concave side and was restrained by the tension stress on the convex side. The increase of plating thickness in electroplating process offered the extensive residual stress to mitigate the Sn whisker growth. Increasing the aging temperatures also enhanced the thickness of the oxide layer. Thick oxide layers can prevent Sn whisker growth.

An assessment of Sn whiskers and depleted area formation in thin Sn films using quantitative image analysis

An experimental technique was developed to determine the extent of Sn whisker growth and depleted area formation on evaporated 1 μm tin (Sn) films. Deformation of the Si substrate placed a controlled magnitude of compressive or tensile stress across the films. Quantitative image analysis was used to monitor whisker growth and size of the depleted areas. The test conditions were: stresses 10–40 MPa; temperature, 180 °C; and time durations, 1–8 weeks. The whisker length increased with compressive stress. The whiskers appeared within the first week, but then did not grow significantly with additional time. Some whiskers were located in the centers of depleted areas. The depleted areas size was not sensitive to the applied stress, but did increase with annealing time. Both Sn whiskers and depleted areas were the result of potentially similar rapid, long-range diffusion processes. However, differing trends suggested that separate driving forces and/or rate kinetics controlled the two phenomena.

Whisker formation on a thin film tin lithium-ion battery anode

Tin (Sn) is a candidate material for anodes (negative electrodes) of lithium-ion batteries because of its high theoretical energy capacity. In this paper, we report an observation of Sn-whisker growth on Sn-thin films after lithiation and delithiation. The compressive stress generated by electrochemical lithiation of the Sn-thin films is likely the driving force for the growth of the Sn whiskers. Attention should therefore be paid to the issue of Sn-whisker growth for Sn-based electrodes since Sn whiskers may penetrate through the separator, and short-circuit the electrochemical cell.

Interfacial microstructure and bonding strength of Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce– xZn solder BGA packages with immersion Ag surface finish

Although it has been verified that tin whiskers can be prevented by the addition of 0.5 wt.% Zn into a Sn–3Ag–0.5Cu–0.5Ce solder, no detailed studies have been conducted on interfacial reactions and mechanical properties of Sn–3Ag–0.5Cu–0.5Ce– xZn solder joints with an immersion Ag surface finish. The intermetallic compounds formed during the reflow and aging of Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce– xZn solder ball grid array (BGA) packages were investigated. Because more heterogeneous nucleation sites, provided by CeSn 3 intermetallics and Zn atoms, formed in the Sn–3Ag–0.5Cu–0.5Ce– xZn solder matrix, and Cu and Zn have a stronger affinity than Cu and Sn, the Cu–Sn intermetallics growth in Sn–3Ag–0.5Cu–0.5Ce– xZn solder joints with Ag/Cu pads was suppressed. The 0.2% Zn addition for inhibiting rapid whisker growth in RE-doped Sn–Ag–Cu solder joints is more appropriate than 0.5 wt.% additions, as excess Zn addition causes poor oxidation resistance and inferior bonding strength.

Environmental Influence on Sn Whisker Growth

This paper considers the influence of 1) humidity and 2) acidic humidity on the growth of Sn whiskers. Sn whisker morphology was observed over a six-month period. The results show that the electroplated surfaces exposed to pure humidity are populated with Sn whiskers dimensionally smaller than surfaces exposed to acidic humidity. Variables analyzed include surface condition, Cu-Sn inter-metallic formation at the film/substrate interface by X-ray Diffraction (XRD), and film thickness.

The effect of electric current and surface oxidization on the growth of Sn whiskers

Electric current was applied on pure Sn-plated leadframes to evaluate the effects of current-induced stress on the growth of Sn whiskers. The samples were stored at room temperate and 55 °C/85% relative humidity (RH) conditions with an induced current range of 0.1 A to 0.5 A. The samples stored at the room temperature did not grow the whiskers at any of the current conditions until 3000 hrs. As the current flow increased, irregular intermetallic compounds (IMCs) grew at the interface between the Sn finish and Cu substrate. However, various lengths of columnar and bent whiskers were observed under all current conditions, after exposure to 55 °C/85% RH conditions for 1000 hours. At the same temperature, the higher current levels showed longer whiskers than lower current levels. The Sn oxide had the α-SnO 2 structure of the rutile phase which was non-uniformly formed on the surface of the Sn finish. The grain size of the SnO 2 was estimated to be several nanometers. The SnO 2 film was up to a thickness of ∼23 nm on the Sn whisker surface stored at 55 °C/85% RH conditions for 3000 hours.

(Invited) Whisker Formation in Pb-Free Surface Finishes

Tin-lead (Sn-Pb) alloys have been used extensively for surface finishing of electronic components in part because Pb was found to be effective in retarding Sn whisker growth in electrodeposits. Although whiskers appear to be a local response to the existence of residual compressive stress in the electrodeposited film, questions remain as to the dominant source of stress and the precise growth mechanism. Using both deflection measurements of plated cantilever beams and x-ray diffraction methods (sin2Ψ), we have determined the in-plane residual stress in Sn, Sn-Cu, and Sn-Pb alloys electrodeposited from both bright- and matte-finish methanesulfonate-based electrolytes. Stress measurements were made soon after deposition, and as a function of time. The appearance of hillocks and/or whiskers following plating is influenced by several factors such as intrinsic growth stress, stress induced by Cu6Sn5 formation at the Sn-Cu interface, and the deposit microstructure.

Effects of reflow atmosphere and flux on Sn whisker growth of Sn–Ag–Cu solders

We evaluated the Sn whisker growth behavior of Sn–Ag–Cu solder fillets on lead frames of quad flat packages (QFPs) upon OSP printed circuit boards that were exposed to 85 °C/85% relative humidity (RH) exposure. Three different concentrations of halogen flux for activated Sn-3.0wt%Ag–0.5wt%Cu were used to solder in air and in an inert N2 reflow atmosphere. The lead frames of the QFPs consisted of Sn plated Cu and Fe-42wt%Ni (alloy 42). Sn whiskers were observed on the surface of the QFP solder fillet joints that were reflowed with halogen containing flux in an air atmosphere. A substantial amount of Sn oxides were formed in those solder fillets while whisker growth and the amount of Sn oxides increased with the halogen content. Sn oxide formation apparently enhanced whisker formation. The combination of air reflow atmosphere and high halogen flux was the worst combination for solder fillet oxidation resulting in Sn whisker formation regardless of the electrode’s lead frame composition of Cu or alloy 42. In contrast, an inert N2 reflow atmosphere obviously prevented Sn whisker formation on Sn–Ag–Cu solder fillets under all conditions used in this work.

Creep and its effect on Sn whisker growth

A comprehensive quantitative whisker growth theory has been developed. In this theory whisker growth is controlled by the ratio of the long range grain boundary diffusion creep relaxation rate to the short range creep relaxation rate. The theory relies solely on solid state diffusion effects and does not require additional constraints or assumptions to explain whisker growth. It explains a number of heretofore relatively poorly explained and puzzling observations reported over the past 6 decades, including: (i) Sn films that are 1–10 μm thick are more susceptible to whisker growth than thinner or thicker films; (ii) Sn film contamination in the form of hydrocarbons, Cu, hydrogen, etc., tend to increase the whisker propensity; (iii) as-deposited Sn films tend to be more susceptible to whisker growth than fused films; (iv) postplate annealing tends to improve the whisker resistance; (v) whisker growth rate tends toward zero as the temperature exceeds 100 °C; (vi) whisker growth propensity depends upon the crystallographic orientation of the Sn film; and (vii) the large statistical variations reported in whisker density and incubation time. The theory predicts: (i) the magnitude of the as deposited film stress state is not fundamental to whisker growth; (ii) at high stresses where power law creep relaxation dominates, whiskers do not form.; (iii) whiskers occur in those regions of the film that are exposed to stresses lower than that required for power law creep; (iv) whiskers form when a quasiequilibrium stress state is achieved as the stress in the film tends toward zero; (v) anything that affects (limits or restrains) power law creep and its associated stress dissipation, such as grain boundary pinning, increases the propensity for whisker growth; (vi) pinning tends to increase the whisker propensity and increase the whisker growth rate; and (vii) films with certain crystallographic orientations and grain sizes are less prone to whisker growth, independent of the anisotropic thermomechanical properties, as a result of the anisotropic diffusion behavior of Sn.

Mitigation of Sn Whisker Growth by Composite Ni/Sn Plating

This paper considers the influence of composite pulse electroplated nickel/tin (Ni/Sn) layering on the mitigation of Sn whisker growth. The performance of the composite pulsed plating method in the mitigation of Sn whisker growth is also compared with two other plating procedures. The results indicate that, after a period of 6 months, the composite pulsed plating technique demonstrates much better resistance to Sn whisker growth than other plating techniques such as pure Sn plating and Sn plating with a Ni underlayer onto a brass substrate subjected to various environmental conditions. The primary conclusions are based on the analysis of microstructural characteristics, the average residual stress distribution in the film over different time periods computed by x-ray diffraction, the formation of intermetallic compounds, and the amount of Sn whisker growth in each case.

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.

Finite Element Modeling of Stress Evolution in Sn Films due to Growth of the Cu6Sn5 Intermetallic Compound

We use finite element simulations to quantitatively evaluate different mechanisms for the generation of stress in Sn films due to growth of the Cu6Sn5 intermetallic phase at the Cu-Sn interface. We find that elastic and plastic behavior alone are not sufficient to reproduce the experimentally measured stress evolution. However, when grain boundary diffusion is included, the model results agree well with experimental observations. Examination of conditions necessary to produce the observed stresses provides insight into potential strategies for minimizing stress generation and thus mitigating Sn whisker growth.