Investigation On The Microstructure And Service Behaviors Of Sn-Bi57-Ag0.7 Lead-free Solder

With the increasing development of micro-electronics industry,the development of various packaging materials has become the focus of attention.Among all the Sn-based lead-free solders,Sn-Ag-Cu ternary solder has always been the most promising alternative for Sn-Pb eutectic solder.However,due to the existence of other polymer materials,the packaging temperature of some consumer electronic products is required to be lower than 180℃,which makes SAC solder alloys with a melting point of about 220℃ no longer applicable.Sn-Bi solder has been gradually applied to contemporary electronic industry with its low cost and good performance in recent years.However,due to the brittleness of Bi element,the reliabilities of solder joint have been seriously affected.Therefore,the task of the development of new Sn-Bi solder is becoming more and more rigorous.According to the advantages that trace Ag element would reduce melting point and increase toughness of solder alloys,our present work aimed to investigate Sn-Bi57-Ag0.7 lead-free solder systematically in order to expand its applications.Not only the micro-morphology and mechanical properties of the solder alloy were analyzed,but also the reliability and microstructure evolution mechanism of solder joints in various service environments have been described.Our work provides deep theoretical guidance for the development of new Sn-Bi-X solders with good performances and high reliabilities.Firstly,we not only analyzed the evolution mechanism of β-Sn phase and Bi-rich phase in the matrix of Sn-Bi-Ag solder alloy,but also created an improved method to quantitatively characterize the average size of the microstructure of the alloy.Then the important law between the size of Bi-rich phase and the acceleration of cooling rate was determined.In addition,the results of mechanical tests of Sn-Bi-Ag alloy on the tensile strength,elongation,hardness and creep properties showed that the micro structure of the solder matrix became more uniform and refined with the increase of cooling rate during smelting,and both of the strength and hardness became higher,while the creep properties would be getting worse.The conclusion not only fills the gap in the studies on the micro structural evolution in Sn-Bi solder alloys under equilibrium and nonequilibrium cooling conditions,but also establishes the relationship among melting parameters,microstructure and properties.In order to evaluate the reliabilities of Sn-Bi-Ag solder joints,an isothermal aging test(85℃)and drop test were carried out.The reflowed solder joints were mainly composed of β-Sn phase,Bi-rich phase and intermetallic compounds(IMC).Few Ag3Sn second-phase particles were also distributed in the matrix.With the extension of aging time,the Bi-rich phase gradually coarsened,thicknesses of Cu6Sn5 and Cu3Sn layer also increased from 0.92 μm and invisible to 2.45 μm and 1.01 μm respectively,which were mainly related to the solid diffusion mechanism and Oswald ripening mechanism.According to the fractography of the failed solder joints after drop test,it could be concluded that the fracture mechanism changed from quasi-cleavage fracture in the matrix at the initial stage to brittle cleavage fracture at the Cu6Sn5/solder interface at the later stage,which indicated that the properties of Sn-Bi-Ag solder joints still need to further exploration and improvement.Although many studies have reported the behaviors of thermal cycling of Sn-based solder joints,the corresponding mechanism is difficult to describe universally due to the complexity of different cases.Therefore,the microstructure evolution and failure mechanism of Sn-Bi-Ag solder joints during thermal cycling of-40℃～85℃ from 0 to 1000 cycles were explored.The results indicated that with the extension of time during thermal cycling,the micro structure of Sn-Bi-Ag solder joint gradually coarsened and the IMC layer became thicker(from 0.82 μm to 2.38 μm).However,the solder joints failed after 3000 thermal cycles.Two different stages of failure were found and the mechanism,related to the increment of thermal mismatch stress,were detailedly illuminated.Furthermore,being different from the orientation change observed in SnBi eutectic solder joints in previous studies,the results demonstrated that both Sn and Bi grains did not present any preferred orientations after thermal cycling.And the reason of this phenomenon might be attributed to the Ag3Sn,which could be regarded as second-phase particles.This result would reduce the local anisotropy in the solder matrix compared with other solder joints.Moreover,the thermomigration(TM)behaviors of Sn-Bi-X solder joints and the orientations change of Bi grains under the temperature gradient are rarely reported.So Sn-Bi57-Ag0.7/Cu solder joints were used to conduct a TM test under a temperature gradient of 625℃/cm for 400 h.The results indicated that the Sn/Bi areal ratio after TM did not change significantly whether at the hot end(from 46.78%/52.12%to 50.90%/48.78%)or at the cold end(from 50.25%/49.64%to 48.71%/51.16%)due to the insufficient thermal energy compared with that of as-reflowed samples.And the thickness of intermetallic compound(IMC)after TM at hot end(2.49 μm)was very close to that at cold end(2.52μm),which was also close to that of the aged samples.In addition,both Sn and Bi grains had no preferred orientation whether after reflow or isothermal aging,while the orientation of Bi grains of the sample after TM changed from random direction to c-axis([0001]direction)parallel to the heat flow in the SnBi-Ag solder joints.Ag3Sn could hinder the change of orientation of Bi grains under the temperature gradient and the corresponding mechanisms have also been illuminated in this study.The present investigation firstly revealed the orientation change of Bi grains under the temperature gradient.We believe that Sn-Bi-Ag solder will have broad application prospects in the future.