| The spontaneous growth of metal whiskers has been a troublesome industrial issue for many decades,which raises the risks of short circuit or metal-arcing accidents.The risk of Sn whisker growth was once successfully mitigated by alloying with Pb,but as the toxicity of Pb had been clearly identified,and the usage of lead element is gradually restricted or even banned in electronic assemblies.Thus,massive efforts have been made to find the long-waited lead-free mitigation strategy.During the seeking process for substitute alloying elements,the effect of some alloying elements on whisker growth has been reported,but a consensus on the mechanism still lacks up to now.In addition,the whiskering characteristics on Sn-based alloys also mount a great challenge to investigation,including the uncertain incubation period ranging from several days to years and the high randomness of results,which increases the difficulty of the research and leads to the slow progress of the related mechanisms and mitigation strategies.It has been reported that Ti2Sn C（a member of the MAX phase）can shorten the diffusion distance of Sn atoms by providing high-speed diffusion paths（along the basal plane）,which makes the incubation period of Sn whisker shorter and the growth rate faster.Therefore,Ti2Sn C phase was taken as the research platform for investigating the influence of alloying elements on the whiskering behaviors.The composition of whiskers from Ti2Sn C/Sn,Ti2Sn C/Sn-Bi and Ti2Sn C/Sn-Ga samples is free of alloying elments（Bi,Ga）,making the Sn element act as the sole metallic element.Sn whiskers are crystallineβ-Sn phase.However,there was no whisker growth behavior in Ti2Sn C/Sn-Pb system under various experimental parameters.Using High-Resolution Transmission Electron Microscope（HRTEM）to investigate the interface microstructure between Sn whisker and substrate in Ti2Sn C/Sn-Bi sample.The matter transfer during the whiskering process is realized as follows:Sn atom in the free Sn-Bi source escapes from its crystalline,and diffuses into the lattice of Ti2Sn C;the active Sn atom in Ti2Sn C lattice diffuses to the sample surface along the Ti2Sn C basal plane to form the whisker.While the free Sn phase with specific structure in Ti2Sn C/Sn sample can be used as the in-situ nucleation point of whiskers,and no incubation period.Therefore,compared with the growth process of whiskers in Ti2Sn C/Sn sample,an extra step of forming pure Sn nuclei is necessary for the following accumulation of Sn atoms in Ti2Sn C/Sn-X（X=Bi,Ga）systems.In consequence,the addition of alloying elements（Bi,Pb,Ga）into the Sn alloy helps to mitigate the whiskering process,by introducing an apparent incubation process prior to the growth of Sn whiskers.Moreover,the nucleation of whisker can be completed in Ti2Sn C/Sn-X（X=Bi,Ga）samples under proper culture conditions,while Pb element effectively mitigates the nucleation of whiskers.In the study of whisker growth behavior,to avoid the inconvenience of different nucleation time to the kinetics study,a double-layer structure of[Ti2Sn C/Sn]/[Ti2Sn C/Sn-X]were designed（taking[Sn-X]for abbreviation）,in which the thin[Ti2Sn C/Sn]layer（～40μm）provides nuclei,and the thick[Ti2Sn C/Sn-X]substrate（～400μm）provides enough source Sn atoms.Results indicated that whiskers with distinct densities,following a general sequence of:[Sn-Bi]>[Sn-Pb]>[Sn]>[Sn-Ga],indicating that Bi and Pb elements promote the diffusion of Sn atoms.To reveal the mechanism of the effect of alloying elements on the growth rate of Sn whiskers,the mobility of Sn atoms and the concentration of Sn vacancies were studied by first principle calculation based on density functional theory（DFT）and positron annihilation experiment.Firstly,the formation of Sn vacancy and transportation of Sn atoms in Sn-X（X=Bi,Pb,Ga）supercells were simulated using the Vienna Ab initio Simulation Package（VASP）in the framework of the Density Functional Theory（DFT）.Results show that the formation energy of a Sn vacancy in different supercells of Sn-X（X=Bi,Pb,Ga）represents a sequence of?#($%&’（）>?#$%>?#$%&*+>?#$%&,-.Thus,the sequence of the balanced concentration of thermal vacancies in these Sn-X alloys could be deduced:$%/$%&,->$%/$%&*+>$%/$%>$%/($%&’（）.Afterwards,the migration barrier of a Sn atom was investigated,and it implies that the diffusion energy barrier of a Sn atom inβ-Sn phase is as low as 0.134e V.With the presence of a Ga substitution at lattice site,a remarkably increased value is determined.While in the cases of Bi or Pb substitution,an opposite trend showing lower energy barriers can be observed.Consequently,the self-diffusion coefficient of Sn atoms in the Sn-X alloys follows the sequence of$%/$%&,-≈$%/$%&*+>$%/$%>$%/($%&’（）.And then,the vacancy concentration in Sn-X alloys was characterized by positron annihilation experiment（PALS）,demonstrating a sequence of:$%/$%&,->$%/$%&*+>$%/$%>$%/($%&’（）,which is consistent with the results from DFT simulation.Therefore,the effect of alloying elements on the growth rate of Sn whiskers is consistent with the results of DFT simulation and positron annihilation measurement. |
