Sn Whisker Research Articles

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.

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.

Whisker growth in Sn and SnPb thin films under electromigration

The growth of whiskers under electromigration in Sn and SnPb thin films deposited by magnetron sputtering were systematically investigated in this work. Fast whisker growth was observed in the Sn thin film which results from the small size of grain. And with decreasing the sample length or current density, both the mean length and density of the whiskers decrease in the Sn thin films. The determined critical sample length for Sn whisker growth is about 82.2 μm, which is larger than the value obtained in stripe samples. When Pb is added, three different layers form in the SnPb thin film. And due to the high sample temperature Pb becomes the dominant diffusion element. The addition of Pb is thought to inhibit the growth of need-like whisker effectively, which is benefit to reduce the failure of interconnection.

Inhibiting the growth of Sn whisker in Sn-9Zn lead-free solder by Nd and Ga

The effect of Nd and Ga additions on Sn whisker growth inhibition in Sn–9Zn solder is newly reported in this work. After ageing treatment at room temperature and 125 °C for over 4 months, no Sn whisker growth was observed in the joints soldered with Sn–9Zn–0.5 Ga–0.08Nd and Sn–9Zn–0.5 Ga–1Nd solders. XRD analysis shows that the formation of GaNd and GaNd3 compounds instead of SnNd compound is the key factor. Because the Gibbs formation free energies of GaNd and GaNd3 are more negative than that of SnNd, the GaNd and GaNd3 compounds are relatively more stable, no free Sn atoms released during exposure and oxidizing reaction to feed the Sn whisker growth. EDX analysis was also identified that the formation of Ga–Nd compounds substituted for the Sn–Nd IMC is the main factor that inhibited the spontaneous growth of Sn whisker.

An Investigation Into the Role of Lead as a Suppressant for Tin Whisker Growth in Electronics

The effect on Sn electrodeposits of up to 10 wt.% codeposited Pb was investigated. The influence of Pb on cathodic polarization of the electrodeposition process, the microstructural and crystallographic characteristics of as-deposited films, the Sn-Cu intermetallic compound (IMC) formation between the deposits and Cu substrate and surface oxidation after ambient storage were examined. During electrodeposition, the plumbous (Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ) ions were observed to inhibit stannous (Sn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ) ion reduction. The use of a metallographic cross sectioning approach enabled the observation of a grain structure evolution toward a more equiaxed morphology with increasing Pb content. This was accompanied by a duplex-phase structure consisting of isolated Pb-rich particles at grain boundary intersections together with a continuous Sn-rich phase. X-ray diffraction measurements corroborate the existence of the duplex-phase structure and also indicate an as-deposited texture transition from (112) through (200) and further toward (220) with an increase in Pb content from 0 to 8 wt.%. A marked topological transition of the interfacial Cu-Sn IMCs from wedge-shaped and discretely distributed to relatively even and uniformly distributed was found to occur concurrently. After tin whisker growth had occurred from low-Pb content (up to 5 wt.%) electrodeposits on brass, Pb was generally observed to be strongly associated with actual Sn whiskers themselves, thus the spontaneous redistribution of Pb after electrodeposition has been evidenced. X-ray photoelectron spectroscopy sputter profiling indicates the absence of Pb in the Sn oxide layer and the enrichment of metallic Pb beneath the Sn oxide from a 90Sn-10Pb deposit ambient stored for 270 days. The whisker-inhibiting effect of Pb codeposition is postulated to emanate from more than one source: not only those reflected in grain structure modification, grain texture transition, and changes in the growth mode of the interfacial Cu-Sn IMCs as prevailingly acknowledged, but also significantly, the universal distribution of the soft Pb-rich phase, providing simultaneous diffusion toward potential whisker sites as well as effective strain dissipation.

Tensile Behavior of Single-Crystal Tin Whiskers

The growth of metallic (predominantly Sn) whiskers from pure metallic platings has been studied for over 50 years. While the phenomenon of Sn whiskering has been studied for decades, very little is known about the mechanical properties of these materials. This can be attributed to the difficulty in handling, gripping, and testing such fine-diameter and high-aspect-ratio whiskers. We report on the stress–strain behavior of Sn whiskers inside a dual-beam focused ion beam (FIB) with a scanning electron microscope (SEM). Lift-out of the whiskers was conducted in situ in the FIB, and the whiskers were tested using a microelectromechanical system tensile testing stage. Using this technique, the whiskers had minimum exposure to ambient air and were not handled by hand. SEM images after fracture enabled reliable calculation of the whisker cross-sectional area. Tests on two different whiskers revealed relatively high tensile strengths of 720 MPa and 880 MPa, respectively, and a limited strain to failure of 2% to 3%. For both whiskers, the Young’s modulus was between 42 GPa and 45 GPa. It is interesting to note that the whiskers were quite strong and had limited ductility. These findings are intriguing and provide a basis for further work to understand the effect of Sn whisker mechanical properties on short circuits in electronics.

Sn whisker mitigation by a thin metallic-glass underlayer in Cu-Sn

This study investigates the effects of thin film metallic glass (TFMG) as an underlayer on the Sn whisker mitigation in the Cu-Sn couples. Samples with and without TFMG underlayers were subjected to various heat treatments at elevated temperatures up to 85 °C in monotonic and cyclic modes. TFMG underlayer effectively blocks the Cu/Sn interaction even with the thickness as thin as 25 nm. In addition, with very thin thickness, the introduction of TFMG underlayer is expected to yield insignificant degrees of compressive stress, which is anticipated to occur when the samples are exposed to thermal cycling. Their thin thickness and amorphous nature are considered beneficial to make TFMGs as a promising diffusion barrier for Sn whisker mitigation.

A study of electrodepositing Sn–SiC composite coatings with good welding performances

Sn–SiC composite coatings with a good welding performance were electrodeposited in a bath containing 55 g L−1 Sn(CH3SO3)2, 160 g L−1 methyl sulfonic acid (MSA), 10 mg L−1 OP-21, 30 g L−1 SiC nano-particles and 50 mg L−1 sodium dodecyl sulfate (SDS), at an inclination angle of the plating tank θ of 45°, a temperature of 20 °C and a cathode–current density of 3 A dm−2 for 15 minutes. The incorporation of SiC nano-particles in the Sn deposits refines the crystalline grains. The solder spreading performance and Sn whiskers of the Sn–SiC coatings composited with 1.0–2.3 wt% SiC are better and smaller than that of the pure Sn coating, respectively. Only when the intercalation of SiC nano-particles into the Sn grain boundary is firm and homogeneous could coatings with a good welding performance be obtained.

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.

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.

Sn Whisker Growth in Cu(Top)-Sn(Bottom) Bilayer System upon Room Temperature Aging

The Sn whisker growth in Cu(top)-Sn(bottom) bilayer system upon room temperature aging was investigated by scanning electron microscope and X-ray diffraction techniques. The experimental observations indicate that the Sn whisker growth on the Cu surface in Cu-Sn bilayer system is different from that on the Sn surface in Sn-Cu bilayer system. When the Sn sublayer thickness is less than 0.5μm, the Sn whisker growth can take place in Cu-Sn system but not in Sn-Cu system. An explanation for Sn whisker growth in Cu-Sn bilayer system is given.

Thin film metallic glass as an underlayer for tin whisker mitigation: A room-temperature evaluation

Introduction of underlayer is one of the mitigation methods commonly used for the suppression of the Sn whiskering phenomenon in electronic packaging. The presence of a proper underlayer prevents the intermetallic compound formation resulting from a Cu/Sn interaction believed to be the major driving force of Sn whisker growth. Ni as an underlayer has been widely studied and industrially accepted. However, Ni underlayer suffers from its polycrystalline grain structure where grain boundaries can potentially act as a diffusion path for the Cu/Sn interaction to take place. In this study, a 100-nm-thick Zr46Ti26Ni28 thin film metallic glass (TFMG) is introduced to block the Cu/Sn interaction. Samples with and without TFMG underlayer were aged at room temperature for up to 33days in ambient atmosphere. No Sn whisker is observed in the sample with TFMG underlayer after aging. In contrast, Sn whiskers are found in the absence of the underlayer and the whisker density increases with increasing aging time. It is found that TFMG underlayer plays an important role in effectively suppressing Sn whisker growth.

Growth behaviors of tin whisker in RE‐doped Sn‐Zn‐Ga solder

PurposeThe purpose of this paper is to investigate the growth behavior and mechanism of Sn whisker induced by RE addition in Sn‐Zn‐Ga‐Pr solder at ambient condition.Design/methodology/approachBy means of aging treatment, FIB and SEM microstructure analysis, the whisker growth behavior was investigated.FindingsIt was found that the morphologies of tin whisker are changed during air exposure. After 60 days aging, the average length of the longest whiskers could reach up to 70 μm, some whiskers even can grow to a length of 100 μm. It was discussed that the oxidation of Pr‐Sn intermetallics provides driving force for whisker growth.Originality/valueTin whisker growth is a complex reliability issue for lead‐free solder. The current research can be helpful in re‐understanding the issue of tin whisker growth as well as an enriched understanding on the effects of REs on lead‐free solders.

Thermal stress driven Sn whisker growth: in air and in vacuum

Whisker growths from matte tin electroplating have been observed during thermal cycling up to 1,000 cycles either in air or in vacuum. The density, length, and width of thermal stress whiskers depend on the plating thickness of 2 and 5 μm in the present study. Whiskers grown on the 2 μm plating are longer and thinner than those on 5 μm plating. In both cases, whiskers grow thinner and faster in vacuum than in air. These apparent variations come from the grain sizes and the thermal stress distributions in the electroplating, intrinsically different in 2 and 5 μm thick films. The grain structure of whisker root, particularly grain boundary cracks oxidized in air, determines the stress concentration to drive the whisker growth. Cracking caused by oxidation was rarely observed in vacuum hence causes thin and straight whiskers even from thick plating. Our results indicate that the stress concentration at whisker root grain is essential for controlling whisker growth morphology, and has a critical impact on various electronic applications.

リード線溶接における Sn ウィスカ発生メカニズムに関する研究

This paper describes the mechanism of Sn whisker growth in the welding leads of aluminum electrolytic condenser. Such whiskers are crystalline, have lengths of several microns to several millimeters, and can occasionally result in device failure. The leads used for aluminum electrolytic condensers are composed of Sn- and Cu-plated Fe and Al wire by arc-stud welding. Observations have shown that the Sn whiskers grow in Al-Sn alloy regions at the welding point, where a three-dimension network of Sn exists as a result of rapid solidification from the melt in Al-Sn alloy. This network is microstructure of rapidly solidified from the melt. This one for the most part is occluded by Al, and has slightly opening to surface. It is considered that the driving force for whisker formation is the compressive stress that is present in the Sn network as a result of the different shrinkage rates of Al and Sn during solidification.

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 (Ecorr) 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.

Sn whisker growth on Sn plating with or without surface treatment during the room temperature exposure

The establishment of lead-free plating technology and countermeasures for whisker formation are some of the critical issues remaining to be solved for lead-free electronics packaging. This study examined a method for mitigating Sn whisker formation by depositing a thin metal layer, such as Au, Pd and Ni, on pure Sn plating. Au, Pd and Ni layers with thicknesses ranging from 50nm to 200nm were deposited on matte Sn plating using a flash-coating process. The Sn whisker growth behavior of pure Sn plating and metal layer/Sn plating samples at room ambient over a period of 10,000h was observed. The metal layer/Sn plating was considerably more stable against Sn whisker formation in room ambient environment than the pure Sn plating.

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.

Effects of thermal cycling on rare earth (Pr)-induced Sn whisker/hillock growth

The addition of rare earth (RE) into Pb-free solders has been shown to induce spontaneous Sn whisker growth on RE-rich intermetallics (IMCs). In this research, whisker growth behaviors under ambient and thermal cycling conditions were investigated to address the RE-induced Sn whisker growth mechanism. Unlike the needle-like whiskers grown mainly in ambience, it was found that coarse Sn hillocks were the dominant growth morphology under thermal cycling condition. It was discussed that the CTE mismatch between RE-rich PrSn3 IMCs and surrounding solder matrix induces compressive stress within the IMCs during thermal cycling, which contributes to the driving force for hillock growth.

Sn 휘스커 연구동향

Sn whiskers are a serious cause of failure in electronic devices as they create short circuits. Sn whisker growth and mitigation have been investigated by many Japanese consortia including JEITA and JAXA. This paper gives an overview about recent researches of JEITA and JAXA.