Spontaneous Growth Of Tin Whiskers Research Articles

The study on modulable tin whisker growth behaviors and mechanisms on Ti2SnC

The spontaneous growth of tin whiskers has resisted interpretation for more than 70 years. Herein, tin whisker growth behaviours on Ti2SnC bulks cold-pressed under different pressures were studied. It was found that higher whisker growth propensity is reached when Ti2SnC cold-pressed under higher pressure. Several thicker whiskers with diameter more than 3 μm were observed on the samples formed under higher pressure, while the diameter of more than 50% whiskers ranges from 0.5 μm to 1.5 μm for all of the samples. An atomic motion mechanism, in which the element source supplying Sn whisker growth diffuse through the Sn atom layer in Ti2SnC lattice, and the interaction between Ti2SnC and Sn is considered to be the fundamental factor initiating the Sn whisker growth, was employed to understand the phenomena. The findings herein provide new clues to understand the tin whisker growth behaviors and modulable metal whisker fabrication.

Long-term thermal aging of parylene conformal coating under high humidity and its effects on tin whisker mitigation

Usage of pure tin (Sn) or Sn-rich alloy in electronics industry is growing, which makes them more susceptible to the spontaneous growth of tin whiskers. The risk of having these whiskers can be dramatically mitigated by applying a conformal coating such as parylene C. The purpose of this paper is to investigate the degradation of parylene C coating (with two thicknesses, 12.5 μm and 25 μm) during thermal aging in high humidity environment (85 °C/85% relative humidity for up to 5000 h) and its effect on the tin whisker mitigation property. It was observed that under the high temperature and high humidity (HTHH) environment, the oxidation occurs at early times, followed by bond scission, resulting in the loss in strength and modulus of the parylene coating. After 2500 h of aging, due to the degradation of the silane agent at the tin/parylene interface, the adhesion of the parylene coating on the tin surface was significantly weakened, which resulted in extensive corrosion of the tin solderpads. The loss of adhesion of the coating, along with the corrosion of the tin surface facilitated tin whisker nucleation at the interface while the loss of mechanical strength and the brittleness made the growing whiskers penetrate easily through the coating. Tin whiskers with a smaller diameter were buckled and grew along the delaminated interfacial region while those with a larger diameter penetrated and continued to grow through the coating. Although the thicker parylene coating helped delay the complete penetration and increase the critical diameter of whiskers for complete penetration, it showed more tin whisker activities and easy penetration by large diameter whiskers due to poorer adhesion. Hence, the effectiveness of the parylene coating on tin whisker mitigation will depend on its degradation kinetics of mechanical properties and adhesion under such harsh conditions.

The effect of Bi addition on the formation of metal whiskers in Ti2SnC/Sn-xBi system

The spontaneous growth of tin whiskers on tin-rich solders and platings has haunted the electronics industry for more than 70 years. Reliable mitigation strategies are long-waited with the proceeding of lead-free movement. However, the development of mitigation strategies has been retarded for lack of a clear whisker growth mechanism. Herein, a new research platform of Ti2SnC is developed, in which fast tin whisker growth with excellent repeatability is realized. The sample of Ti2SnC/Sn-xBi is prepared using ball-milling method, to reveal the effect of Bi addition on the growth behavior of Sn whiskers. Compared with the Ti2SnC/Sn system, an extended nucleation period is identified in Ti2SnC–Sn5Bi, Ti2SnC–Sn15Bi, Ti2SnC–Sn20Bi, which mitigates the growth of Sn whiskers in the initial stage; however, when cultivated at proper parameters, the density of Sn whiskers is found to increase with the rising of Bi content. Finally, the growth kinetics was studied using ESEM. A linear relationship between whisker length and square root of the holding time is obtained, suggesting a diffusion-controlled mechanism for the growth of Sn whiskers in this work.

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.

Confining effect of oxide film on tin whisker growth

Spontaneous tin whisker growth has been mysterious and catastrophic for more than half century. The difficulty in the research on this topic consists of the randomness of the whisker growth, the slow growth rate and many other tricky factors. Herein, with Ti2SnC-Sn as a new platform, fast tin whisker growth is realized to facilitate the research. The whisker morphology is found to be modulated by oxide film. A striated whisker morphology forms as growing in air, whereas a faceted morphology forms in vacuum. Furthermore, the evolution to the faceted morphology is attributed to the reconstruction of the whisker surface driven by surface energy reduction. The findings might open a new avenue to uncover the myths of this long-standing issue, and thus develop a long-awaited lead-free tin whisker mitigation strategy.

The effect of electroplating parameters and substrate material on tin whisker formation

Electroplated tin finishes are widely used in the electronics industry due to their excellent solderability, electrical conductivity and corrosion resistance. However, the spontaneous growth of tin whiskers during service can result in localised electrical shorting or other harmful effects. Until recently, the growth of tin whiskers was successfully mitigated by alloying the tin with lead. However, restriction in the use of lead in electronics as a result of EU legislation (RoHS) has led to renewed interest in finding a successful alternative mitigation strategy.Whisker formation has been investigated for a bright tin electrodeposit to determine whether whisker growth can, at least partially, be mitigated by control of electroplating parameters such as deposition current density and deposit thickness. The influence of substrate material and storage at 55°C/85% humidity on whisker growth have also been investigated.Whisker growth studies indicate that deposition parameters have a significant effect on both whisker density and whisker morphology. As deposition current density is increased there is a reduction in whisker density and a transition towards the formation of large eruptions rather than potentially more harmful filament whiskers. Increasing the tin coating thickness also results in a reduction in whisker density. Results demonstrate that whisker growth is most prolific from tin deposits on brass, whilst that from tin deposits on rolled silver is greater than that observed for tin deposits on copper.

Effect of direct current and pulse plating parameters on tin whisker growth from tin electrodeposits on copper and brass substrates

Electroplated tin finishes are widely utilised in the electronics industry due to their advantageous properties such as excellent solderability, electrical conductivity and corrosion resistance. However, the spontaneous growth of tin whiskers during service can be highly deleterious, resulting in localised electrical shorting or other harmful effects. The formation of tin whiskers, widely accepted as resulting from the formation of compressive stresses within the electrodeposit, has been responsible for a wide range of equipment failures in consumer products, safety critical industrial and aerospace based applications. The numbers of failures associated with tin whiskers is likely to increase in the future following legislation banning the use of lead in electronics, the latter when alloyed with tin, being an acknowledged tin whisker mitigator. Using a bright tin electroplating bath, the effect of process parameters on the characteristic structure of the deposit has been evaluated for deposition onto both brass and copper substrates. The effect on whisker growth rate of process variables, such as current density and deposit thickness, has been evaluated. In addition, the effect of pulse plating on subsequent whisker growth rates has also been investigated, particularly by varying duty cycle and pulse frequency. Whisker growth has been investigated under both ambient conditions and also using elevated temperature and humidity to accelerate the growth of whiskers. Studies have shown that whisker formation is strongly influenced by pulse plating parameters. Furthermore, increasing both current density and thickness of the deposit reduce whisker growth rates. It is also observed that whisker formation is greatly accelerated on brass substrates compared with copper. The basis for this observation is explained.

A Model for Rapid Tin Whisker Growth on the Surface of ErSn3 Phase

Spontaneous growth of tin whiskers on the finish of leadframes is an extremely slow process under moderate temperature conditions. It therefore becomes difficult to track the continuous growth of tin whiskers and to vary the experimental conditions to determine their root causes. Accordingly, the fundamental growth behaviors of tin whiskers are still not fully understood. In this study, rapid tin whisker growth was achieved by adding 1 wt.% Er to Sn-3.8Ag-0.7Cu solder alloy. The results showed unique tin whisker morphology with nonconstant cross-section. An explanation is proposed by adding kinetic energy to the conventional energy balance equation. In addition, a double compressive stress zone is proposed to demonstrate the driving force for tin whisker growth in rare-earth-bearing phases.

Controlled positions and kinetic analysis of spontaneous tin whisker growth

This study achieved controlling the positions of spontaneous growth of tin whiskers. We surmounted the unpredictable growing nature of such whiskers and performed accurately quantitative analyses of the growth kinetics and yielded precise measurement of the growth rate. Furthermore, using synchrotron radiation x-ray, this study determined the stress variations in conjunction with whisker growth that fitted appropriately to the model. Accordingly, the results could address the debate held for decades and prove that forming a surface oxide layer is one of the required and necessary conditions for controlling the positions of spontaneous growth of tin whiskers.

Changes of internal stress and microstructure in spontaneous tin whisker growth

Tin thin films with various thicknesses were electrodeposited on Cu substrate with and without Ni undercoat. The changes of the internal stresses in tin films were measured by the X-ray diffractions. For the case of 2.2 μm thick film without Ni undercoat, the initial internal stress was tension and then gradually changed into compression. After 20 h, the internal compressive stress became around −30 MPa and was remained constant afterward. Filament-type whiskers were formed on the surface after the stress changed into compression. For the case of 2.2 μm thick film with Ni undercoat, the stress maintained in tension and no whisker was observed. For the cases of thicker films with and without Ni undercoat, the stress was compression around −30 MPa just after deposition, and remained constant. Nodule-type and Mount-type whiskers were formed on the surface, but no Filament-type whiskers were observed. The compressive stress applied by bending on 2.2 μm thick film with Ni undercoat resulted in the formation of Mount-type whiskers. The postbake process induced the tensile stresses in 2.2 and 16.5 μm thick films. Whisker formation was prevented in thin film and only Mount-type whiskers were formed in thicker films.

Spontaneous growth of tin whiskers on tin−rare-earth alloys

Tin whisker growth has been observed on the surface of bulk Sn–Nd alloys with Nd contents of 30, 5 and 0.5 wt%, under ambient conditions of temperature and atmosphere. It is shown that the whiskers originate from the area of Sn–Nd intermetallic compounds after a very short incubation period and that they exhibit a relatively high growth rate on the surface of the bulk alloys. The results show that these Sn-based, rare-earth alloys have a very strong propensity to grow tin whiskers, i.e. this rare-earth element in tin is a catalytic agent for whisker growth. Direct evidence is also shown that whiskers can grow from the surface of a bulk Sn-based alloy.

Tin-Manganese Alloy Electrodeposits: I. Electrodeposition and Microstructural Characterization

The electrodeposition of tin-manganese alloys from aqueous solutions has been proven to be extremely difficult and only limited success has been achieved in the past. In this study, silvery matte and adherent tin-manganese alloys were electrodeposited from a modified sulfate/gluconate electrolyte within a current density range of at room temperature. The cathode current efficiency was generally greater than 50%, which is relatively high for the deposition of manganese alloys. The manganese content in the deposits varied with the applied current density from zero to a maximum of . No oxygen was detected in the deposits. The result of X-ray diffraction analysis indicated that tin-manganese alloys deposited at mainly consisted of fine-grained , while alloys deposited at higher current densities were close to amorphous. The successful electrodeposition was attributed to the five-fold (minimum) increase in the concentration of tin(II) ions in the electrolyte compared to that reported in the literature, and the addition of a non-ionic surfactant, Tween 20, which suppressed the preferential deposition of tin, thus enabling the co-deposition of manganese. While the electrodeposition process was successful, the deposits were prone to rapid and spontaneous growth of tin whiskers during storage.

Effects of Lead on Tin Whisker Elimination: Efforts toward Lead-Free and Whisker-Free Electrodeposition of Tin

Spontaneous tin whisker growth is one of the direct consequences of complete removal of Pb from tin electrodeposits. To pursue a greater understanding of the whisker growth mechanism, the effects of Pb on the properties of matte tin electrodeposition at different alloy compositions were investigated with respect to cathodic polarization, crystallographic and microstructural characteristics of the deposits, and the formation of intermetallic compounds (IMC) under ambient conditions at the interface between the Sn or films and the Cu substrate. Furthermore, internal stress of pure Sn and deposits was measured as a function of storage time using the bent strip method. The observations strongly suggest that Pb prevents tin whisker growth due to the formation of an even IMC. In turn, upon the IMC growth the deposit exhibits lessening of compressive stress. It is evident that the presence of Pb enhances the lattice diffusion of Cu and increases the ratio of IMC formed through lattice diffusion and grain boundary diffusion. Other contributing factors such as predominant tin [220] textures and the equiaxed grain structure may also play supporting roles in the whisker retarding effect of Pb.

High spatial resolution stress measurements using synchrotron based scanning X-ray microdiffraction with white or monochromatic beam

Scanning X-ray microdiffraction (μSXRD) combines the use of high brilliance synchrotron sources with state-of-the-art achromatic X-ray focusing optics and large area detector technology. Using either white or monochromatic beams, it allows for orientation and strain/stress mapping of polycrystalline thin films with submicron spatial resolution. The present paper will focus on three applications performed at the Advanced Light Source: the study of local plasticity in aluminum thin films, the study of the spontaneous growth of tin whiskers in lead-free solder finish and the measurement of strain field around thin film bucklings.

Observations of the Spontaneous Growth of Tin Whiskers on Tin-Manganese Alloy Electrodeposits

The spontaneous growth of tin whiskers on electrodeposited tin-manganese alloy coatings has been observed. This growth is distinct from any previously reported whisker growth on either pure tin or other tin-based alloy electrodeposits. It has an extremely short incubation period of a few hours only, followed by a spectacularly rapid and profuse growth. During the whole period of whisker growth, the tin-manganese electrodeposits were found to be in a tensile residual stress state. This rules out the commonly accepted explanation of a compressive stress as the driving force for tin whisker growth.

Mechanism and Prevention of Spontaneous Tin Whisker Growth

Spontaneous Sn whisker growth on Cu leadframe finished withPb-free solder is a serious reliability problem in electrical andelectronic devices. Recently, Fortune magazine had an article to describethe urgency of the problem. The spontaneous growth is an irreversibleprocess, in which there are two atomic fluxes driven by two kinds ofdriving force. There are a flux of Cu atoms and a flux of Sn atoms. TheCu atoms diffuse from the leadframe into the solder finish driven bychemical potential gradient to form intermetallic compound of Cu6Sn5 inthe grain boundaries of the solder, and the growth of the compound atroom temperature generates a compressive stress in the solder. To relievethe stress, a flux of Snatoms driven by the stress gradient diffuses awayto grow a spontaneous Sn whisker which is stress-free. The typicalindustry solution is to inserta diffusion barrier of Ni between the Cuand solder to prevent the diffusion of Cu into the solder. It isinsufficient, because we have to uncouplethe irreversible processes andstop both the fluxes of Cu and Sn. A solution is presentedhere.

Spontaneous growth mechanism of tin whiskers

A study has been made of the spontaneous growth of tin whiskers from tin electrodeposits on phosphor bronze sheet. The driving force for the evolution of tin whiskers is a biaxial compressive stress of about 8 MPa developed in tin deposits by the formation of an intermetallic compound of Cu 6Sn 5, especially in grain boundaries of tin films. The biaxial compressive stress gives rise to strains normal to the film plane, which are dependent on the tin grain orientations. The shear stresses due to differences between strains of different grains along the thickness direction make the tin surface oxide film sheared approximately along boundaries of grains with particular orientations which are different from the major texture of the film. Tin extrudes from the grains, whose surface oxide films are sheared. The extrusion takes place continuously by expansion of the dislocation loops by climb and their subsequent glide toward the surface in the slip direction to form whiskers.

Zinc whiskers again

A study has been made of the spontaneous growth of tin whiskers from tin electrodeposits on phosphor bronze sheet. The driving force for the evolution of tin whiskers is a biaxial compressive stress of about 8 MPa developed in tin deposits by the formation of an intermetallic compound of Cu6Sn5, especially in grain boundaries of tin films. The biaxial compressive stress gives rise to strains normal to the film plane, which are dependent on the tin grain orientations. The shear stresses due to differences between strains of different grains along the thickness direction make the tin surface oxide film sheared approximately along boundaries of grains with particular orientations which are different from the major texture of the film. Tin extrudes from the grains, whose surface oxide films are sheared. The extrusion takes place continuously by expansion of the dislocation loops by climb and their subsequent glide toward the surface in the slip direction to form whiskers.