Effects Of Grain Number And Intermetallic Compounds On Electromigration Of Sn-based Solder Joint

The rapid development of modern science and technology has brought the global information age,promoting vigorous development of the electronic industry,and putting forward higher requirements for the functions that integrated circuits can carry.The proposal of Hyper-Moore's Law points out that the direction of miniaturization and high density of microelectronic packaging has led to a continued decrease of solder joint dimension in the packaging systems,where high current density and joule heating in the solder joints increase exponentially,serving as key reliability issues for electromigration-induces failure of the electronic packages.At the same time,the increasing complexity of integrated circuit(IC)has brought about a sharp increase in the number of solder joints in the package.Therefore,the study on the electromigration reliability of solder joints has far-reaching significance in both acdemia and electroinc information industries.The extensive application of lead-free Sn-based solders and its processing temperature profiles usually induce finite number of Sn grains in solder joints.Such anisotropy of Sn will introduce serious polarization effect that causes premature failure of the solder joints.Refinement of Sn grains in a solder joint is believed to be one of the effective methods to improve electromigration resistence by alleviating its anisotropy.Therefore,it is very important to investigate the formation mechanism of polycrystalline solder joints and the analysis of electromigration behavior of solder joints containing different grain numbers in the current study,in an effort to propose an effective solution to improve the electromigration reliability of the solder joints.In this paper,the crystal growth model of Sn based polycrystal solder joints is analyzed,and its influence on improving the reliability of electromigration of solder joints is discussed.Another effective approach for such effort is to introduce appropriate types of intermetallic compound(IMC)in the solder matrix to reduce electromigration failure.In real industrial settings,Au-Ni-Cu is usually used as the bond pad metallization,where different IMCs are formed as compared to the Cu-Sn IMCs when Cu bond pad is used.These IMCs in the solder matrix are formed with different solder alloy composition,and Au-Sn IMC and Au-Sn-In IMCs behave differently during current stressing with Cu-Sn IMCs,the fundamental principles of which have not been paid much attention.Hence,the mechanisms of different types of IMC on the electromigration behavior of solder joints is systematically studied in this paper,suggesting the controlling mechanisms to retard electromigration from In element.The main research contents and conclusions are stated as follows.(1)The grain refinement mechanism of Sn based solder joints was studied by analyzing the formation of Sn crystal orientation with different cooling rate.The growth model of Sn crystal with interlace twins by In element is established.The addition of In was found to influence the growth mode of Sn crystals to form interlaced twin grains that have a grain orientation of 60°.In addition,the alloying elements of Bi and In could effectively reduce the melting points of Sn-Ag based solders,increase the degree of supercooling and the driving force of critical nucleation of the Sn grains.Under the cooling rate of 3.43?/s,Sn3.0Ag0.5Cu,Sn3.0Ag3.0Bi,Sn3.0Ag3.0In,and Sn3.0Ag3.0Bi3.0In solder were selected.The grain size and grain boundary type between Sn grains in the solder joint were statistically analyzed by observing the formation rule of Sn crystal orientations in the corresponding solder joint.The results showed the Bi and In provided more nucleation sites to promote the formation of polycrystalline solder joints.Moreover,through the observation of Sn crystal orientation formation in Sn3.0Ag0.5Cu and Sn3.0Ag3.0Bi3.0In solder joints at cooling rates of 3.43?/s and 81.67?/s,it was found that a higher cooling rate increased the undercooling,thus introduced a higher nucleation rate,so as to inhibit the growth of Sn grains resulting in reduced the average grain area.A stable Sn grain refining process through cooling rate control was proposed accordingly.(2)Electromigration behavior of solder joints with different Sn crysyal orientations was analyzed based on the Cu-Sn IMC in solder joints.The influence of the c-axis orientation of Sn grains and the angle(?)between the c-axis and the direction of electron flow on the electromigration behavior of solder joints were studied.General findings indicated that the c-axis of Sn grains determined the migration and diffusion direction of Cu atoms in the solder joint.When the c-axis was pointing to the observation surface of the solder joint,Cu atoms diffused and accumulated to the surface of the solder joint to form Cu-Sn IMC.On the contrary,the Cu atoms moved to the interior of the solder matrix and the IMC disappeared on the surface of the solder joint.The angle between the c-axis of Sn grain and the direction of electron flow affected the diffusion rate of Cu atoms.When the c-axis of Sn grain fell along the same direction with electron flow by forming acute angles,the smaller the?angle is,the larger the diffusion rate of Cu atoms is,resulting in a disastrous electromigration failure due to the fast growth of IMCs,as well as the growth rate of IMC layer thickness at the anode interface.The relationship between angle?and IMC growth rate was established,and the influence mechanisms of the c-axis and angle?on IMC layer at the anode interface and IMCs in the solder matrix were discussed.(3)The effects of Sn grain refinement on the electromigration reliability of Sn-based solder joints was studied.Electromigration features of solder joints containing single,double,and multiple interlaced twin grains were examined through the IMC growth behavior in the solder matrix.It was shown that the refinement of Sn grains in the solder joint could effectively slow down the electromigration-induced failure,weaken the anisotropy of Sn grain,and reduce the polarization effect.At the same time,the theory of grain boundary energy was applied and veridied that,twin boundaries with high density had obvious scattering and blocking effect on Cu atoms and electrons,avoiding the aggregation of IMCs in the solder matrix,and impeding the long-range directional migration of Cu atoms.(4)Based on the influence of Sn grain numbers and grain boundary types on the electromigration behavior of Cu-Sn phase in solder joints,the strengthening mechanism of In element to improve the reliability of solder joint electromigration by refining the IMC of Au-Sn phase was studied,The mechanism of In element to improve the reliability of electromigration of solder joints was specified.The electromigration behavior of Au-Sn IMC phase was affected by the anisotropy of Sn grain,but the formation of twin boundaries could delay the electromigration process.However,the Au-Sn-In IMC phase underwent a phase transformation process from needle shape to discontinuous granular shape under current stressing,and its electromigration behavior was not affected by the anisotropy of Sn.The microstructural evolution of IMC in Au-Sn and Au-Sn-In phases were analyzed by STEM observation.The results showed that In reduced the solubility of Au in Sn,and thus to induce a phase transition to form another Au-Sn-In phase with lower enthalpy of formation,and the lattice structure changed from Au Sn₄ to Au In₂.Such ternary Au-Sn-In IMCs was wrapped by Au with a loose morphological distribution in the solder matrix,serving as obstacles to reduce atomic mobility during electromigration process.