Analysis of tin whisker growth on lead-free alloys with Ni presence under thermal shock stress

This paper presents the results of whiskers formation after thermal shocks on various tin-rich materials used in electronics: Sn100, Sn99Cu1, Sn97Cu3, Sn99.3Cu0.7Ni, Sn99.3Cu0.7AgNiGe and Sn99Ag0.3Cu0.7NiGe. Alloys plated over an Ni/Au sublayer were compared with those plated directly over a Cu layer. Scanning electron microscope (SEM) imaging was applied to determine whisker densities, lengths and types. Different degrees of susceptibility to whisker formation were found for all the investigated alloys subjected to 1500 shocks in a cyclic temperature range of −45 °C to +85 °C. Although the whisker growth rates for the alloys plated over an Ni/Au sublayer were lower than for the alloys plated directly over a Cu layer, all the materials tested were more or less prone to whisker formation.     

On the advantages of using a hypoeutectic Sn-Zn as lead-free solder material

Hypoeutectic Sn-Zn may be a better choice than the eutectic Sn-9Zn as a lead-free solder. We checked the non-equilibrium melting behaviors of a series of Sn-Zn alloys (2.5-9 wt.% Zn) by differential thermal analysis, and found that at a heating rate of 5 °C/min, Sn-6.5Zn behaves the same way as the eutectic Sn-9Zn in melting. Dipping and spreading tests were carried out to characterize the wettability of Sn-Zn alloys on Cu. Both tests indicated that Sn-6.5Zn has significantly better wettability to Cu than Sn-9Zn does. The reaction layers formed during the spreading tests were examined. For all samples with 2.5-9 wt.% Zn, two reaction layers are formed at the interface, a thick and flat Cu₅Zn₈ layer adjacent to Cu and a thin and irregular Cu-Zn-Sn layer adjacent to the alloy. The total thickness of the reaction layers between the alloy and Cu was found to remarkably decrease with decrease of the Zn concentration.     

Effects of humidity on tin whisker growth — Investigated on Ni and Ag underplated layer construction

The effect of humidity on nickel and silver underlayered tin platings has been observed in relation to whisker formation. Using nickel or silver underlayers between the copper substrate and the tin coating is known to be a useful technique for mitigating the occurrence of whiskers. The underlayer blocks the formation of Cu₆Sn₅ intermetallics which is one of the root causes of whisker growth. Samples with a bronze (94Cu/6Sn) substrate covered with a 1-2 μm underlayer (silver or nickel) and 5-7 μm of tin on the top were tested. The samples were stored in high humidity and at various temperature conditions; 40 °C/95% RH (Relative Humidity), 105 °C/100% RH and 50 °C/25% RH for over 4200 h to demonstrate the importance of humidity in whisker growth. Results have shown differences in whisker growth on the samples depending on the nature of the environmental conditions. The differences originate from the various stress-causing mechanisms. Growth differences have also been found on the two types of underlayer materials.     

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.     

Wetting behaviour of lead-free Sn-based alloys on Cu and Ni substrates

The present work was carried out in the framework of the study of new lead-free solder alloys for technical applications in electronic devices. In the focus of this characterisation the wetting behaviour of several Sn-rich alloys belonging to the In–Sn, Au–Sn and Cu–Sn systems has been studied by measuring the contact angle variations on Cu and Ni substrates as a function of time and temperature. The interface between the alloy and the substrate has been analysed by the use of optical microscopy and scanning electron microscopy combined with energy-dispersive X-ray spectrometry in order to study the reaction between the alloy and the solid substrate and the possible formation of different compounds at the interface. A remarkable effect of the two different substrates on the behaviour of the contact angle as a function of temperature and on the morphology of the interface between the liquid solder and the solid substrate was observed for the In–Sn and Cu–Sn, while the Au–Sn system shows a very similar wetting behaviour on Cu and Ni.     

The effect of wall-slip formation on the rheological behaviour of lead-free solder pastes

Wall-slip plays an important role in the flow behaviour of solder paste materials. The wall-slip arises due to the various attractive and repulsive forces acting between the solder particles and the walls of the measuring geometry. These interactions could lead to the presence of a thin liquid layer adjacent to the wall, which causes slippage. The aim of this study is to investigate the influence of the solder paste formulation on wall-slip formation and its effect on the printability of these pastes material. A wall-slip model is utilised to calculate the true viscosity and slip velocity for the lead-free solder pastes samples used in this study. The difference in the measured viscosity and the true viscosity could indicate wall-slip formation between the solder pastes and the parallel plate. Sample P1 showed a higher slip velocity compared to sample P2. The slip velocity calculated for the solder pastes could be used as a performance indicator to understand the paste release characteristics in the stencil printing process.     

Influence of alcohol on grain growth of tin oxide in chemical vapor deposition

Morphologies of tin oxide micro-grains in the early stage of film growth were analyzed for films deposited by chemical vapor deposition using tin chloride as a source material. Atomic force microscopy observations revealed increased micro-grain density and decreased size by adding methanol into the reaction system, but X-ray photoelectron spectroscopy analyses suggested that the total deposited volume was unchanged. The relative amount of chlorine contamination at the bottom of alcohol-added films increased in the order of isopropyl alcohol < ethanol < methanol. A model of chlorine desorption through reaction with alcohol, which occurred in the early stage of film growth, can explain the results.     

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.     

Effect of geometry on the fracture behavior of lead-free solder joints

Copper bars were soldered along their length with a thin layer of lead-free Sn3.0Ag05.Cu alloy under standard surface mount processing conditions to prepare double cantilever beam (DCB) specimens. The geometry of the DCBs was varied by changing the thickness of the solder layer and the copper bars. These specimens were then fractured under mode-I and two mixed-mode loading conditions. The initiation strain energy release rate, G_ci, increased with the relative fraction of mode-II, but was unaffected by the changes in either the substrate stiffness or the solder layer thickness. However, the steady-state strain energy release rate, realized after several millimeters of crack growth, was found to increase with the solder layer thickness at the various mode ratios. The crack path was found to be influenced by mode ratio of loading and followed a path that maximizes the von Mises strain rather than maximum principal stress. Finally, some preliminary results indicated that the loading rate significantly affects the G_ci.     

激光照射原位生成SiC晶须时粘结剂对晶须的影响

进行了含粘结剂和不含粘结剂的SiC纳米颗粒激光照射原位生成SiC晶须的比对试验,用X射线衍射仪(XRD)和扫描电子显微镜(SEM)分析了晶须的相组成和微观形貌,研究了粘结剂对生成晶须的影响。结果表明,在适当的激光参数下,不含粘结剂和含粘结剂的样品中均有晶须生成,不含粘结剂样品中晶须数量较少,而含粘结剂样品中晶须数量众多,且尺寸、形态各异。SiC纳米颗粒中掺入粘结剂可大幅度提高SiC晶须的生成数量,但并不影响晶须的纯度。     

Influence of tin and bismuth on machinability of lead free 6000 series aluminium alloys

Abstract

The present paper presents the microstructural characteristics of an extruded AA6012A-T6 (AlMgSiBiSn) alloy and the microstructural changes occurring during turning operations, analysing the mechanism involved in chip breaking. An experimental investigation has been conducted to determine the effects of different cutting speed and feedrate on the machinability of the alloy. The machinability of the AA6012A-T6 alloy, where Pb is substituted by Bi and Sn, has then been compared to the standard AA6012-T6 (AlMgSiPb) and AA6082-T6 (AlSiMg) alloys. The results indicate that the extensive plastic deformation induces a preferred orientation of the grain structure and secondary phases along the shear plane, and a local increase in the alloy temperature. Low melting point compounds, such as the Sn and Bi bearing particles, transform into a soft or liquid state, changing their initial compact shape to assume a needle-like morphology. The β-Mg₂Si and α-Al(FeMn)Si particles are not influenced by the working temperature and keep their initial shape. The AA6082-T6 alloy shows a very poor machinability, with long and continuous strips, while the AA6012A-T6 alloy reveals a good chip formation with small and discontinuous C shaped chips, similar to the AA6012-T6 (AlMgSiPb) alloy. In particular, a feedrate higher than 0·2 mm rev^?1 provides short and suitable chips, independently of cutting speed.     

Characterization of Co-Sn intermetallic compounds in Sn-3.0Ag-0.5Cu-0.5Co lead-free solder alloy

The minor addition of Co into Sn-3.0Ag-0.5Cu lead-free solder alloy triggered the formation of Co-Sn intermetallic compounds. The Sn-3.0Ag-0.5Cu-0.5Co solder alloy was heated up to 300 °C or 400 °C and then cooled down to the room temperature at different rates. A new Co-Sn intermetallic phase, say, CoSn₃ containing small amount of Cu, were detected. Only CoSn₃ phase was formed in the solder alloy from 300 °C regardless of the cooling rate. However, during the solidification from 400 °C, the CoSn₂ + CoSn₃ cascade structures were illustrated after slow furnace cooling due to the peritectical reaction, i.e., CoSn₂ + L(Sn) → CoSn₃, while only CoSn₂ was observed after rapid quench. A novel DSC technique was employed herein to demonstrate the presence of this peritectical reaction. The mechanical properties of the individual phases of Co-Sn intermetallics were measured and compared with other sole phases in the solder alloy.     

Whisker growth on annealed and recrystallized tin platings

The aim of this research is to examine the effects of annealing and recrystallization of electroplated tin layers on mitigating the growth of tin whiskers. Tin plating electroplated on copper substrate has been tested. Three different groups of samples have been investigated: reference, annealed and recrystallized. Three types of ageing were being used to accelerate the whisker appearance: two life-like circumstances with a constant elevated temperature in low humidity and a temperature cycling test. The whisker growth was observed by using scanning electron microscopy and was evaluated by the average and maximum whisker length and the average whisker density. It is shown that at different ageing temperatures whisker appearance and growth are decreasing in aspect of the applied pre-treatments. The effects of the pre-treatments were explained by changes of the grain structure in the tin layer which were observed with focused ion beam.     

Effective diffusivity of lead free solder alloys

A Monte Carlo simulation based algorithm is developed to compute the effective diffusivity of lead free solder alloys. Simulations are performed to determine the vacancy diffusivity for 95.5Sn–4.0Ag–0.5Cu (SAC405) solder alloy for different paths. Temperature and micro-structural influence on diffusivity are studied. A map of diffusivity versus temperature and average grain size is developed. Significant differences between diffusivity values reported in the literature for the same solder alloy is also discussed.     

The effects of suppressed beta tin nucleation on the microstructural evolution of lead-free solder joints

Most lead-free solders comprise tin (Sn) as the majority component, and nominally pure β-Sn is the majority phase in the microstructure of these solders. It is well established that nucleation of β-Sn from Sn-base liquid alloys is generally difficult. Delays in the onset of β-Sn formation have a profound effect upon the microstructural development of solidified Sn-base alloys. Utilizing stable and metastable phase diagrams, along with solidification principles, the effects of inhibited β-Sn nucleation on microstructural development are discussed, employing the widely studied Sn–Ag–Cu (SAC) alloy as a model system. This analysis shows that the main effect of suppressed β-Sn nucleation on near-eutectic SAC solders is to increase the number and/or volume fraction of primary or primary-like microconstituents, while simultaneously decreasing the volume fraction of eutectic microconstituent. General strategies are outlined for avoiding unwanted microconstituent development in these materials, including the use of metastable phase diagrams for selecting alloy compositions, employment of inoculants to promote β-Sn nucleation, and utilization of high cooling rates to limit solid phase growth. Finally, areas for future research on the development of inoculated Sn-base solder alloys are outlined.     

Epitaxial growth of Cu6Sn5 formed at Sn-based lead-free solder/non-textured polycrystalline Cu plate interface

This study examines the epitaxial growth of the intermetallic compound (IMC) of Cu₆Sn₅ (or (Cu,Ni)₆Sn₅) that forms at the interface between molten Sn-based lead-free solders and non-textured polycrystalline Cu substrates. Sn, Sn–Cu, Sn–Cu–Ni and Sn–Ag–Cu solders were investigated. The dominant growing planes in a hexagonal structure of this IMC on Cu substrates are (101) and (102). Addition of trace Ni into Sn–Cu solders leads to an increase in (101) growth and a decrease in (102) growth. The presence of Ag in Sn–Ag–Cu solders facilitates (102) growth and suppresses (101) growth. Such an epitaxial growth should have a large influence on the mechanical and electrical characteristics of the Sn-based solder/Cu joints.     

Influence of SiC whisker on planar slip in Al-Li based alloys

The tensile deformation microstructure characteristics of silicon carbide whisker (SiC_w) reinforced Al-Li and Al-Li-Cu-Mg-Zr composites prepared by squeeze casting technique were studied by means of transmission electron microscopy (TEM), in order to evaluate the influence of SiC whisker on planar slip in Al-Li based alloys. For the purpose of comparison, the microstructural features of the unreinforced matrix alloys with the identical fabrication, thermal processing and tensile deformation history were also investigated. Dislocation pairs and intense slip bands originated from cutting of (Al₃Li) phases by moving dislocations, could be found in the specimens of Al-Li and Al-Li-Cu-Mg-Zr alloys, whereas absent in either SiC_w/Al-Li or SiC_w/Al-Li-Cu-Mg-Zr composites. The results demonstrate that the addition of SiC whisker to Al-Li based alloys, has a considerable effect on suppressing planar slip which is a general phenomenon in Al-Li based alloys resulting from the interaction between phase and dislocations.     

Fatigue crack initiation and growth in solder alloys

In modern electronic packaging, especially surface mount technology (SMT), thermal strain is usually induced between components during processing, and in service, by a mismatch in the thermal expansion coefficients. Since solder has a low melting temperature and is softer than other components in electronic packaging, most of the cyclic stresses and strains take place in the solder. Fatigue crack initiation and fatigue crack propagation are likely to occur in the solder even when the cyclic stress is below the yield stress. It is an objective of this research to study the behaviour of fatigue crack initiation and propagation in both lead‐containing solder (63Sn‐37Pb), and lead‐free solders (Sn‐3.5Ag). The effect of alloying (Cu and Bi addition), frequency, tensile hold time and temperature on low cycle fatigue (LCF) behaviour of the solders is discussed. Mechanisms of LCF crack initiation and propagation are proposed and LCF life prediction, based on the various models, is carried out.     

Influence of antimony doping on structure and conductivity of tin oxide whiskers

Tin dioxide whiskers doped with different concentrations of antimony (0-0.25 at.%) have been grown from SnO and Sb₂O₃ mixture in a tube furnace in a flowing mixture of argon and oxygen at a constant source temperature. The whiskers possess high structural perfection. Influence of Sb on crystal structure, morphology and conductivity of SnO₂ whiskers is investigated. Antimony doping allows a decrease in the resistance of SnO₂ whiskers up to 10⁶ times.