Research On The Formation Mechanism Of Kirkendall Voids At The Sn-based Solders/Cu Interface

With the miniaturization and multifunction of electronic products, the size of chips andsolder joints becomes smaller and smaller, which represents an increasing proportion ofinterface area in the joints. Therefore, the microstructure at the interface plays a more andmore important role in the reliability of joints. During the solid-state aging treatment of solderjoints, a large number of Kirkendall voids often appear at the reaction interface, the effect ofwhich on the reliability of joints can not be neglected and has drawn many attentions fromscholars both at home and abroad.In this dissertation, Field emission scanning electron microscope （FESEM）, Transmissionelectron microscopy （TEM）, X-ray photoelectron spectroscopy （XPS） and X-RayDiffractormeter （XRD） are applied to characterize the microstructure, composition and phasestructure at the reaction interface of joints. A new method using the original reaction interfaceas the reference is introduced to study the diffusion of Cu and Sn in the Intermetalliccompound （IMC） layers. Magnetron sputtering and electroplating technique are used toprepare different kinds of substrates, studying the formation mechanism of Kirkendall voids.The diffusion of vacancies in the IMC crystals and the distribution of residual stress at thereaction interface are investigated by using first-principles method and finite-element method,respectively. Then, the relationship between Kirkendall voids and Cu₃Sn is analyzed.The rules for the migration of phase interfaces are discovered by using the originalreaction interface （the upper surface of unreacted Cu substrate） in the corner of joints as thereference. The results show that, with the extension of aging period, the solder/Cu₆Sn₅andCu₃Sn/Cu interfaces move toward the solder and Cu substrate, respectively. The migration ofCu₆Sn₅/Cu₃Sn interface is greatly affected by the aging temperature. At150℃, theCu₆Sn₅/Cu₃Sn interface moves toward the solder; At165℃, the interface shifts toward the Cu substrate; At180℃, the interface keeps still except for that in Sn0.7Cu/Cu joints whichmoves toward the Cu substrate. Based on the relative positions of each phase interfaces, amodel can be build to calculate the diffusion fluxes of Cu and Sn in IMC layers and theirevolution trend with aging time. The calculated results indicate that the interfacial diffusion ofCu and Sn is unbalanced, and Cu is the dominant diffusing species, which is the basis of theformation of Kirkendall voids; the diffusion fluxes of Cu and Sn in the IMC layer decreasewith extension of aging period. In addition, the diffusion flux of Cu in the IMC layer is largerthan that of Sn at the initial stage of aging treatment, and then the diffusion fluxes of twodiffusing species are gradually close to each other.The Sn/Cu system is built, in which various effect factors of the formation of Kirkendallvoids are studied by using different substrate in Sn/Cu joints. Kirkendall voids appear at theinterface using electroplated and sputter deposited Cu substrate rather than the interface usinghigh-purity Cu substrate. Thus, there is a direct relationship between Kirkendall voids andsubstrate. The grain size of electroplated and sputter deposited substrates is very small, and alarge amount of energy is stored in the grain boundary. Part of the stored energy can beintroduced into the interface during the reaction, accelerating the formation of Kirkendallvoids. Besides, the electroplated substrate contains a certain amount of impurities which canreduce the nucleation energy of voids. During the aging treatment, the formation ofKirkendall voids mainly consists of four stages, inoculation, nucleation, growth and closure.Kirkendall voids are formed at the Cu₃Sn/Cu interface and in the Cu₃Sn layer, while not in theCu₆Sn₅layer. In electroplated Sn/Cu couples, many impurity elements are detected in theelectroplated Sn. However, Kirkendall voids tend to form at the Cu₆Sn₅/Cu₃Sn and Cu₃Sn/Cuinterfaces rather than in the Cu₆Sn₅layer and at the electroplated Sn/Cu₆Sn₅interface, both ofwhich are close to the electroplated Sn. Therefore, there is a great relationship betweenKirkendall voids and Cu₃Sn layer.The relationship between Kirkendall voids and Cu₃Sn layer is studied from the macro andmicro perspective, respectively. The diffusion of vacancies in Cu₆Sn₅and Cu₃Sn crystals arestudied by using first-principles method. The calculated results indicate that the vacancyformation energy of Cu in both crystals is close to each other, but is lower than that of Sn intheir corresponding crystals. In both crystals, the vacancy diffusion barrier of Sn is higherthan that of Cu, and the vacancy diffusion barrier of Cu in Cu₆Sn₅crystal is higher than that inCu₃Sn crystal. These diffusion properties are beneficial for the nucleation of Kirkendall voids in Cu₃Sn crystal. At the reaction interface, the formation of IMC phase will cause an obviousvolume contraction, and then the residual stress will be produced. The distribution of residualstress is analyzed by using finite element method. The calculated results show that the wholeCu₃Sn layer is in the state of tensive stress. The stress gradient at the Cu₃Sn/Cu layer is thesteepest, that at the Cu₆Sn₅/Cu₃Sn interface takes the second place and that at the Sn/Cu₆Sn₅interfaces the third place. The area Kirkendall voids located is consistent with that the steepeststress gradient located. The interfacial stress gradient aggravates the unbalanced diffusion ofcomponent elements, accelerating the formation of voids. The transformation residual stress isconnected to Kirkendall voids through Cu₃Sn layer.The formation of Kirkendall voids can be inhibited by adding minor alloying elements （Cuor Zn or Ni） into the Sn/electroplated Cu system, but the inhibition mechanism of theseelements are different. The addition of Cu will not change the phases and microstructures ofIMC layers at the interface, but will decrease the concentration gradient, hampering thediffusion of Cu and reducing the unblanced diffusion of Cu and Sn. The addition of Zn canchange the phases and microstructures of IMC layers at the interface. When the content of Znin Sn solder is low （0.2and0.5wt.%）, the Cu₆（Sn,Zn）₅layer is formed; when the content ofZn increases to0.8wt.%, three layers can be found,（Cu,Zn）₆Sn₅, Cu₆（Sn,Zn）₅and Cu-Znsolid-solution alloy. The microstructure of （Cu,Zn）₆Sn₅and Cu₆（Sn,Zn）₅layers has minoreffect on the interfacial diffusion, but the Cu-Zn solid-solution alloy can hamper the diffusionof Cu effectively. Zn participates in the interfacial diffusion largely, which has the samediffusion direction as Sn. Zn can suppress the diffusion of Cu effectively and reduce theunbalanced diffusion of Cu and Sn. The addition of Ni can also change the phases andmicrostructures of IMC layers at the interface. The （Cu,Ni）6Sn5layer is formed, whichcomposes of several layers of small-sized grains. This kind of structure is helpful for thediffusion of Cu and Sn, the growth of layer and the reduction of the unbalanced diffusion ofCu and Sn. All three kinds of alloy elements can suppress the growth of Cu₃Sn layer, reducingthe introduction of impurities and transformation residual stress to the interface and inhibitingthe formation of Kirkendall voids.