Sn Whisker Research Articles

Mechanisms behind the spontaneous growth of Tin whiskers on the Ti2SnC ceramics

The spontaneous growth of Tin whiskers has been a reliability issue in electronic assemblies for around 70 years, but the underlying physics is still ambiguous. Herein, Sn whiskers formed on the Ti2SnC, a layered ternary carbide ceramic MAX phase, were studied to trace the atomic motion in whisker growth process. Free Sn source was found to be necessary for the formation of Sn whiskers, however, the interface microstructure suggests that Sn atoms feeding whiskers are diffusing through the Ti2SnC lattice. According to the simulation results on the formation and migration energies of vacancies in Ti2SnC, a low diffusion barrier is expected for Sn atoms to diffuse along the basal planes. When Sn atoms diffuse out of the substrate, Sn whiskers bounded by low-energy planes form to minimize the total energy. In contrast, the whiskers maintain the striated morphology inherited from the whisker root in the environment containing oxygen, which is mainly attributed to the confinement of the oxide film on whisker surface. The findings will also shed new light on understanding the general whiskering problems found in other materials.

Tin whisker growth on immiscible Al–Sn alloy

Spontaneous growth of Sn whiskers has seriously affected the reliability of electronic systems and caused many major accidents. Its growth mechanism and mitigation strategy have been the focus of research in the past decades. In this paper, a large number of Sn whiskers were found to grow quickly on Al–Sn alloys prepared by mechanical alloying when treated at 300 °C. Different from the large randomness of the whisker growth on metallic coatings and platings as in major research work of this topic, the Sn whisker growth from ball-milled Al–Sn substrate shows good repeatability. To demonstrate the compressive stress in the samples, X-ray diffraction residual stress analysis was detected. An average compressive stresses in Al phase after heat treatment was 81.2 MPa, which may have been responsible for the whiskers growing directly on the surface. Meanwhile, the interfacial energy caused by high-energy ball milling may have played a positive role in the growth of whiskers. Sn whiskers formed on Al–Sn alloy samples have shorter incubation period and faster growth rate, which makes it much more convenient to research the mechanism.

Tin (Sn) whisker growth from electroplated Sn finished

Electroplated tin (Sn)Limeited finishes are widely used in the electronics industry due to their excellent solderability, electrical conductivity and corrosion resistance. However, since prohibiting the use of lead (Pb) in electronics, the issue of tin (Sn) whisker has become one of the most imperative issues that need to be resolved. Moreover, with the increasing demand for electronics miniaturization, Sn whisker growth is a severe threat to the reliability of microelectronic device. This paper investigates the effect of electroplating current density (10, 30 and 50 mA/cm2) on whisker growth at room temperature. A simple micro- indentation test was used to accelerate continuously the growth of Sn whisker at room temperature storage for 24 h. Morphology analysis of Sn whisker growth was done using scanning electron microscope (SEM) and image-J Software. Axial length of the Sn whisker was calculated by using JEDEC Standards (JESD22-A121A).

Morphology Analysis of Spontaneous Tin (Sn) Whisker Growth on Lead-Free Solder

Due to the transition of the electronics industry to lead-free solders, the issue of tin (Sn) whisker has become one of the most imperative issues that need to be resolved. Moreover, with the increasing demand for electronics miniaturization, Sn whisker growth is a severe threat to the reliability of microelectronic devices. Sn whisker is a conductive tin structure that grows spontaneously from tin finished surfaces, which can lead to well-documented system failures in electronics industries. Assessment of the Sn whisker growth is difficult because of the slow and unpredictable nature of the Sn whisker formation. This paper presents the results of morphology analysis of Sn whisker growth in terms of Sn whiskers shape, average density and maximum length of Sn whisker on Sn-Cu solder finished using scanning electron microscope (SEM). Micro-indentation test was used to accelerate the growth of Sn whisker at room temperature storage.

Preliminary Study of Electrical Current Stressing on Tin Whisker Formation

The growth of Sn whiskers on Cu substrate Pb-free solder is considered as one of the emerging problems in electric and electronic devices as well as in aerospace interconnects. Electrical current stressing have been identified as a major driving force for Sn whisker formation. In this paper, we investigate the mechanism of Sn whisker growth through the effect of electromigration of Sn on copper substrate. Test samples were prepared by dipping method before proceed with current testing, and the characterization of whisker growths were evaluated using the Scanning Electron Microscopy (SEM) on the deposits near the anode and cathode areas. Results demonstrate that the accumulation took place at the anode side forming whiskers and depletion occured at the cathode side leading to the formation of void with the effect of current stressing associated with different exposure of time.

Study on the Length Distribution and the Growth Rate of Sn Whiskers Generated on Sn/Cu Plating System

Study on the Length Distribution and the Growth Rate of Sn Whiskers Generated on Sn/Cu Plating System

Microstructural Characteristics of Oxide Layer Growth on Tin Whisker and Finish Surface

The growth of oxide film on both Sn whisker and finish surfaces was observed using TEM and their crystal structures were analyzed using EELS. Only uniformly formed SnO2 structure of rutile phase was observed on the surface of Sn. In the samples stored at room temperature for 20 years, the thickness of oxide layer on finish surface (17 nm with polycrystalline structure) was thicker than that on whisker surface (6.5 nm with amorphous structure). The tin oxide growth tendency exhibited a parabolic curve with a gentle slope. However, at samples stored at 55 °C for 16 months, oxide layers consisted of 1 nm amorphous and 9 nm polycrystalline SnO2 were observed on whisker surfaces. The amorphous structure of SnO2 layer (5.5 nm) was also formed on the finish surfaces. No difference was found in the measured resistance of the Sn finish surface when the surface was etched by Ga+ ions.

Effect of Sn Film Grain Size and Thickness on Kinetics of Spontaneous Sn Whisker Growth

The effects of the grain size and thickness of Sn films on the kinetics of spontaneous Sn whisker growth have been quantitatively analyzed separately. The results reveal that the length and diameter of the whiskers are related to the parameters of the Sn film. A thicker film has a stronger influence on the whisker morphology than does the grain size. When the biaxial stress in the film reached a steady state, the growth rate was dependent on the grain size and thickness of the film. An equation is proposed to calculate the spontaneous growth rate of Sn whiskers in correlation with the dimension and microstructure of the Sn film. Spontaneous Sn whisker growth can be suppressed by increasing the thickness and grain size of the film.

Investigation into the effects of magnetic agitation and pulsed current on the development of Sn[sbnd]Cu alloy electrodeposits

SnCu alloy electrodeposits have found a wide range of applications in the electronics, automotive and aerospace industries. In the current research, the electrodeposition of SnCu alloy coatings produced from an electrolyte based on fluoroboric acids was thoroughly studied. The cathode current density was found to have a significant impact on the appearance and the surface morphology of SnCu alloy electrodeposits. Our experimental results suggested that deposition current density, magnetic agitation and pulsed current parameters (pulse frequency and duty cycle) were all found to be crucial factors significantly affecting the chemical composition of the SnCu alloy electrodeposits. In addition, the employment of pulsed current techniques significantly improved the edge effect and the surface roughness. Whisker growth on the SnCu alloy electrodeposits was assessed. All the samples subjected to different electroplating condition were prone to develop Sn whiskers after a few weeks of storage at room temperature.

Effect of Air-HAST Conditions on Sn Whisker Growth

Sn whiskers are one of the major causes of failure in fine-pitch electronics because of the short distance between the leads in an electronic package. In various whisker mechanisms, it takes a long time to grow a whisker due to Sn corrosion because the temperature is limited to below 100°C in high humidity. In the highly accelerated temperature and humidity stress test (HAST), the corrosion phenomenon rarely occurred because of the little air inside the chamber. Therefore, air is needed in the whisker accelerating test to meet the same whisker mechanism under high temperature and humidity conditions. In this study, an Air-HAST with air inputs in the chamber with highly temperature and humidity was introduced. Whisker growth was accelerated with temperatures at 130°C and various air pressures using the thin quad flat package (TQFP) sample with Sn plating lead. The whisker growth in the Air-HAST, HAST, and high temperature and high humidity test (85°C, 85%R.H.) was compared by plotting each growth length and number of whiskers. The accelerated relationships of whisker growth under the various test conditions will help the electronic industry meet the short development cycles. Key words: Sn whisker, Air-HAST, 85°C/85%RH, HAST, TQFP

Improved adhesion of polyurethane-based coatings to tin surface

Polyurethane (PU)—based conformal coatings which protect electronic components and boards from harsh environmental exposures were further developed to mitigate the tin (Sn) whisker growth in lead (Pb)—free electronics. As a means of achieving this goal, the adhesion mechanics of PU-based conformal coating on tin surfaces was investigated. We attempted to improve the interfacial adhesion between PU-based conformal coatings and tin (Sn) as it slows down the formation of tin whisker growth. A multi-layer sandwiched specimen made between two Si beams was employed to create an interface between PU coating and Sn for four-point bending testing (4PBT). The result shows that PU has strong adhesion to Sn while polyurethane acrylate (PUA) has much weaker adhesion to Sn. The PUA adhesion, however, increased by introducing a silane agent ((3-Aminopropyl)triethoxysilane) on the tin surface, which is covalently bonded to PUA. The adhesion improvement of a modified tin surface was also confirmed by a simple cross-cut tape peeling test. Post fracture analysis suggested that blocking the reaction between bare Sn surface and the acrylate component contribute to improved adhesion.

The influence of the crystallographic structure of the intermetallic grains on tin whisker growth

In this paper, the relationship between the crystallographic structure of Cu-Sn intermetallic grains and Sn whisker growth was investigated. In order to prevent the influence of the elements in the alloy composition and the effect of the soldering process on the formation of the intermetallic layer, 99.99% pure Sn was vacuum evaporated onto Cu substrates. The Sn layer thickness was sub-micron region (∼400 nm in average) to reach considerable and rapid compressive stress on the tin layer originated by the intermetallic formation. The samples were stored at room temperature for 1 month. Different types of whiskers (nodule and filament) and the layer structure underneath were studied with a scanning ion microscopy and transmission electron microscopy. It was found that not only the thickness of the intermetallic layer and shape of the intermetallic grains affects the whisker growth but the crystallographic structure of the intermetallic grains as well. The susceptibility of the Sn layer to whisker development is higher in those regions where the intermetallic layer is composed of monocrystalline grains instead of those regions, where it is composed of polycrystalline grains. This effect can be explained by the higher compressive stress generated by the monocrystalline intermetallics compared to the polycrystalline ones.

Studying the Effect of Grain Size on Whisker Nucleation and Growth Kinetics Using Thermal Strain

To understand how and why Sn whiskers nucleate, measurements were performed in which the thermal expansion mismatch was used to induce strain in the samples. The current work focuses on the effect of grain size by studying samples with a thickness of 8 μm to compare with previous studies on 2.5 μm samples. The stress evolution during cycles of heating is determined by monitoring the resulting curvature in the sample. At the same time, the number of whisker nuclei are measured with optical microscopy which enables the relationship between film stress and nucleation rate to be quantified, where the stress relaxation kinetics depend on the grain size. The rate of stress relaxation is greater in the thick samples than the thin samples for the same heating cycle and the final stress after relaxation is smaller for the thicker samples. The whisker volume evolution was determined by using short sequences of heating and then measuring the size of different features after each interval in the scanning electron microscope. This enabled quantification of the relationship between film stress and whisker growth rate. The growth rate depends on the stress above the final stress, similar to our previous results for the thinner samples. The whisker nucleation kinetics in the 8 μm samples can also be explained with the same model used for the 2.5 μm samples. However, the stress-dependent activation energy is shifted to lower values.

Spontaneous Tin (Sn) Whisker Growth from Electroplated Tin and Lead-Free Tin Alloys Coatings: A Short Review

Since the environmental regulations of Reduction of Hazardous Substances (RoHS) directive came into effect in Europe and Asia on July 1, 2006, requiring the removal of any lead (Pb) content from the electronics industry, the issue of tin (Sn) whisker growth from pure Sn and SnPb-free alloys has become one of the most imperative issues that need to be resolved. Moreover, with the increasing demand for electronics miniaturization, Sn whisker growth is a severe threat to the reliability of microelectronic devices. Sn whiskers grow spontaneously from an electrodeposited tin coating on a copper substrate at room temperature, which can lead to well-documented system failures in electronics industries. The Sn whisker phenomenon unavoidably gives rise to troubles. This paper briefly reviews to better understand the fundamental properties of Sn whisker growth and at the same time discover the effective mitigation practices for whisker growth in green electronic devices. It is generally accepted that compressive stress generated from the growth of Cu6Sn5 intermetallic compound (IMC) is the primary driving force for Sn whisker growth during room temperature storage. It is, therefore, important to determine that the relationship between IMC growth and Sn whisker growth. Reduction of stress in the IMC layer can therefore reduce the driving force for whisker formation and be used as a means for whisker mitigation. To date, there are no successful methods that can suppress the growth of Sn whisker as efficient as Pb addition. It is hoped that the Sn whisker growth mechanisms are understood better in the future, with better measuring and monitoring methodologies and systems being developed, the real solutions may be eventually developed to eliminate or mitigate the Sn whisker problems of green reliability lead-free electronic assemblies.

Sn Whiskers Nucleation and Growth - Short Review

Sn whisker growth on Cu substrate Pb-free solder is a serious problem in electric and electronic devices and as well as in aerospace applications. Due to the concern on the toxicity of lead by Restriction of Hazardous Substances Directive (RoHS), new lead free materials have been developed, and this resulted in the resurfacing of Sn whisker. The compressive stress, corrosionand surface oxide have been identified as the driving force for Sn whisker formation induced by mechanical alloying and oxidation. In this paper, we report the study to understand the mechanism of Sn whisker growth that control whisker formation on Sn finished.Based on the review, a preliminary conclusion has been made, where the analysis of the topography and microstructural characterization can be determined by evaluating under various environmental influences.Furthermore, the whisker growth happening on lead-free soldered can be considerably reduced by controlling the compressive stress in the solder which initiates the growth of intermetallic compounds (IMCs).

Control Al/Mg intermetallic compound formation during ultrasonic-assisted soldering Mg to Al

To prevent the formation of Al/Mg intermetallic compounds (IMCs) of Al3Mg2 and Al12Mg17, dissimilar Al/Mg were ultrasonic-assisted soldered using Sn-based filler metals. A new IMC of Mg2Sn formed in the soldered joints during this process and it was prone to crack at large thickness. The thickness of Mg2Sn was reduced to 22 μm at 285 °C when using Sn-3Cu as the filler metal. Cracks were still observed inside the blocky Mg2Sn. The thickness of Mg2Sn was significantly reduced when using Sn-9Zn as the filler metal. A 17 μm Mg2Sn layer without crack was obtained at a temperature of 200 °C, ultrasonic power of Mode I, and ultrasonic time of 2 s. The shear strengths of the joints using Sn-9Zn was much higher than those using Sn-3Cu because of the thinner Mg2Sn layer in the former joints. Sn whiskers were prevented by using Sn-9Zn. A cavitation model during ultrasonic assisted soldering was proposed.

The Effects of Oxidation Layer, Temperature, and Stress on Tin Whisker Growth: A Short Review

In order to reduce the Tin (Sn) whisker growth phenomenon in solder alloys, the researcher all the world has studied the factor of this behaviour. However, this phenomenon still hunted the electronic devices and industries. The whiskers growth were able to cause the electrical short, which would lead to the catastrophic such as plane crush, the failure of heart pacemaker, and the lost satellite connection. This article focuses on the three factors that influence the whiskers growth in solder alloys which is stress, oxidation layer and temperature. This findings were allowed the researchers to develop various method on how to reduce the growth of the Sn whiskers.

On Sn Whisker Suppressing Mechanism by Heat Treatment to the Double Layrer Plating System(Sn/Cu)

On Sn Whisker Suppressing Mechanism by Heat Treatment to the Double Layrer Plating System(Sn/Cu)

Comparative studies on microelectronic reliability issue of Sn whisker growth in Sn-0.3Ag-0.7Cu-1Pr solder under different environments

In this study, comparative studies on Sn whisker growth in Sn-0.3Ag-0.7Cu-1Pr solder under different environments were conducted to investigate factors like ambient temperature, oxygen level, and 3.5wt% NaCl solution on whisker growth. The experimental results revealed that ambient temperature and oxygen level are two important factors that could determine the oxidation rate of PrSn3 phase, thus indirectly affecting the growth rate of Sn whiskers. In addition, mechanisms of whisker growth under these three environments were established from the perspective of atom diffusion based on the “compressive stress-induced” theory. Although whiskers under different environments were all squeezed out from Pr oxides (hydroxides), the forms of their driving forces were different. For whiskers squeezed out in air whether at room temperature or 150°C, the driving force is the compressive stress produced by lattice expansion due to the oxidation of PrSn3 phase. The representative example was whiskers' growth at 150°C, which could be simplified as three stages: (1) squeezing out, (2) cracking and (3) bursting out. For whisker growth in 3.5wt% NaCl solution, the driving force for much fewer whiskers' growth was proposed to come from lateral stress provided by interfacial IMC layer growth. Moreover, Sn nanoparticles and their agglomerations were also found to form under the driving force of the potential difference between Sn atoms and Sn crystals. Their morphologies could also be affected by factors of ambient temperature, oxygen level and Cl− ions in corrosive liquid.

Sn whisker growth mitigation by using NiO sublayers

The potential of NiO sublayers for whisker growth mitigation has been examined. A thin NiO film was applied on a Cu-coated substrate before the deposition of a thicker Sn layer. The growth of Sn whiskers was then followed by optical and scanning electron microscopy and was compared with the whisker growth on a control sample without the NiO sublayer. No whiskers were observed on the sample with the NiO layer even after 12 months, whereas the control sample developed whiskers of size and density that is generally expected, based on the vast amount of published work on the topic. The mechanisms of whisker growth and whisker growth suppression are briefly discussed as well.