The Study On Growth Mechanism Of Kirkendall Void And Its Inhibition At Interface Of Micron Solder Joints

With the development of micro-electronics industry, consumer electronics products become to have small size, high performance. This has put forward higher requirements about the reliability of miniaturized solder joints. The reliability of solder joints depends primarily on the microstructure of solder joints, which is greatly affected by the interfacial interaction during the process of soldering and service. Kirkendall voids formed in electrical components served in high temperature may be a concern for the solder joint reliability. It is known that Kirkendall voids are the result of unbalanced atoms(Sn-Cu)diffusion in solid state reaction between Sn-rich solders and Cu metallization, The voids will lead to rapid decline in strength of micro solder joints and reduce the actual electrical contact area of solder joints if a mass of Kirkendall voids formed at the interface of Cu₃Sn/Cu.In present paper, four kinds of solder, Sn3.0Ag0.5Cu,Sn,Sn3.5Ag and Sn37Pb were used to join Cu and Ni for obtaining the solder joints with different structures. These researches revealed the influence of different factors on the formation of Kirkendall voids and the influence of Kirkendall voids on the mechanical property of solder joints. The research results are listed as follows:The study of the effect of thermal aging on formation of Kirkendall voids revealed that Kirkendall voids normally formed inside the Cu₃Sn layer and the density of the voided area further increases as the aging temperature and time increases. Base on the fitted curve according equation of the experimental date, we obtained an empirical relation of Kirkendall voids area ratio. With this empirical relation, we may estimate the time required at a particular temperature to reach certain percentage of the voided area and predict life of the components.We also investigate effect of substrate and solder alloy on the formation of Kirkendall voids in the solder joints during thermal aging. The substrates include Cu bonding pads (electroplated Cu, polycrystalline Cu and single-crystal Cu) and Ni films (pure Ni, electroplated Ni and electrolessplated Ni ).In the present study, lots of Kirkendall voids are found inside the Cu₃Sn layers of the solder joints on the electroplated Cu and polycrystalline Cu after thermal aging 175℃for 300h, but no void is observed inside the Cu₃Sn layer of the solder joints on the single-crystal Cu substrate even aged at 175℃for 660 h. For both solders, the voids in Cu₃Sn on electroplated Cu substrate is much higher in density and bigger in size than that on polycrystalline Cu. The microstructure of the Cu substrates may influence the grain structure of Cu₃Sn layer during reflow and thermal aging, and the unbalance of Cu-Sn interdiffusion speed is determinate by the grain structure of Cu₃Sn layer . It is known that Kirkendall voids are the result of unbalanced element(Sn-Cu)diffusion in solid state reaction between Sn-rich solders and Cu metallization. We find the propensity for voiding could be correlated with some properties like Cu grain size, crystal defect and purity of Cu material, presumably affected by manufacturing process.For the Sn3.0AgCu lead-free solder, a small quantity of Kirkendall voids formed at the interface of solder joint with electroless plated Ni as the result of presence of Ni-Sn-P layer between (Ni,Cu)3Sn4 and Ni film. However no voids are observed at the solder joints with pure Ni and electroplated Ni film. We attribute it to the unbalance of influx and outflux Sn atoms through Ni-Sn-P pahse.For he Cu/SAC305/Cu solder joints, the thickness of the IMC (Cu6Sn5 and Cu₃Sn) layer and density of Kirkendall voids increase with the aging time. On the contrary, the tensile strength of the solder joints decrease as the aging time increased. The result indicates that the existence of Kirkendall voids inside the solder joints could lead to rapid decline in strength of micro solder joints. The tensile failure remaine though the bulk of solder for the solder joints after reflow or short aging. On the other hand, the failure mode quickly switched into mixed fracture mode and then interface failure as the aging temperature increased as the result of degradation of IMC toughness.