Growth Kinetics Of Interfacial Intermetallic Compounds Research Articles

Effects of bismuth and nickel on the microstructure evolution of Sn-Ag-Cu (SAC)-based solders

The addition of alloying elements to the SAC alloy composition changes the growth behavior of thick Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) and their influences on mechanical responses. The purpose of this study is to investigate the effects of Bi and Ni on the growth kinetics of interfacial IMCs (Cu6Sn5 and Cu3Sn) and the mechanical strength of the bulk solders during high temperature aging. Reflowed samples of the SAC405 solder balls and the SAC + Bi,Ni solder balls were stored in high-temperature storage (HTS) chamber at 150 °C and 175 °C each for 0 h, 500 hs, 1000 hs, and 2000 hs. The microstructure evolutions of bulk solder and its interfacial areas were evaluated and the associated microhardness values were measured on the solder balls. During aging, the presence of Bi was seen only in the bulk solder region, resulting in increased hardness by a factor of ∼ 2 and resistance to the age softening due to solid solution strengthening. On the other hand, Ni was observed in both the solder ball and IMC regions. After aging at 150 °C, SAC + Bi,Ni solder balls had a thinner Cu3Sn IMC layer at its interface when compared to that in SAC solders with no Ni. At 175 °C, however, Ni addition did not decrease the growth of Cu3Sn. Long aging (2000 h) of the SAC405 sample at 175 °C resulted in the complete consumption of the Cu layer, which was slowed in SAC + Bi,Ni. The presence of Bi and Ni, therefore, show the potential to improve the reliability of the SAC solder ball joint during aging by restricting the solder deformation and the growth of the Cu3Sn layer.

Effect of thermomigration on the growth kinetics of Cu6Sn5 at liquid-solid interfaces in Cu/Sn/Cu solder joints

With the continuous miniaturization of electronic packaging, micro bumps for chip interconnects are smaller in size, and thus the reliability of interconnects becomes more and more sensitive to the formation and growth of intermetallic compounds (IMCs) at liquid-solid interface during soldering. Thermomigration (TM) is one of the simultaneous heat and mass transfer phenomena, and occurs in a mixture under certain external temperature gradient. In the process of interconnection, micro bumps usually undergo multiple reflows during which nonuniform temperature distribution may occur, resulting in TM of metal atoms. Since the interdiffusion of atoms between solders and under bump metallization (UBM) dominates the formation of interfacial IMCs, TM which enhances the directional diffusion of metal atoms and induces the redistribution of elements, will markedly influence the growth behaviors of interfacial IMCs and consequently the reliability of solder joints. The diffusivity of atoms in liquid solder is significantly larger than that in solid solder and in consequence a small temperature gradient may induce the mass migration of atoms. As a result, the growth of interfacial IMCs becomes more sensitive to temperature difference between solder joints in soldering process. So far, however, few studies have focused on liquid state TM in solder joints, and the growth kinetics of interfacial IMCs under TM during soldering is still unknown to us. In this study, Cu/Sn/Cu solder joints are used to investigate the migration behavior of Cu atoms and its effect on the growth kinetics of interfacial Cu6Sn5 under temperature gradients of 35.33℃/cm at 250℃ and 40.0℃/cm at 280℃, respectively. TM experiments are carried out by reflowing the Cu/Sn/Cu interconnects on a hot plate at 250℃ and 280℃ for different durations. For comparison, isothermal aging experiments are conducted in a high temperature chamber under the same temperatures and reaction durations. During isothermal aging, the growth of interfacial Cu6Sn5 follows a parabolic law and is controlled by bulk diffusion. Under the temperature gradient, asymmetrical growth of interfacial Cu6Sn5 is observed between cold and hot ends. At the cold end, the growth of the interfacial Cu6Sn5 is significantly enhanced and follows a linear law, indicating a reaction-controlled growth mechanism; while at the hot end, the growth of the interfacial Cu6Sn5 is inhibited and follows a parabolic law, indicating a diffusion-controlled growth mechanism. The dissolved Cu atoms from the Cu substrate at the hot end are driven to migration toward the cold end by temperature gradient, providing the Cu atomic flux for the fast growth of the interfacial Cu6Sn5 at the cold end. With the variation of the measured thickness of Cu6Sn5 IMC at the cold end and the simulated temperature gradients, the molar heat of transport Q^* of Cu atoms in molten Sn is calculated to +14.11 kJ/mol at 250℃ and +14.44 kJ/mol at 280℃. Accordingly, the driving forces of thermomigration in molten solder FL are estimated to be 1.62×10-19 N and 1.70×10-19 N, respectively.

Modeling of interfacial intermetallic compounds in the application of very fine lead-free solder interconnections

Intermetallic compounds (IMCs) are formed between lead-free solders and base metals during soldering processes. The morphology as well as the amount of the IMCs, in particular the interfacial IMCs, are important to mechanical performance of the solder joints in their service environment. This is especially the case in state-of-art ultrafine-pitch wafer-level packaging, in which solder joints could become as small as 50 μm in diameter or even less. However, research addressing growth kinetics of the IMCs in the existing literature has concentrated on fitting experimental data onto a simple parabolic growth equation, which cannot account for all the complicated physics involved in the process of IMC formation. There are a few models based on Fick’s diffusion equations in the literature, which assume the growing IMC interface is planar. A combined thermodynamic–kinetic model has been derived from the relationships amongst some thermodynamic parameters; however, this model is only one dimension (1-D) in nature. 2-D models have also been proposed, e.g. an implicit boundary tracking method, and a phase field model. This paper reviews the modeling techniques in the literature for both the growth kinetics and morphology of the interfacial IMCs, under the context of lead-free soldering in very fine interconnections. The advantages and disadvantages of each method are discussed. Possible solutions to include the geometrical features of micro solder joints in the modeling of growth kinetics of interfacial IMC are presented.

Morphology and growth kinetics of intermetallic compounds in solid-state interfacial reaction of electroless Ni-P with Sn-based lead-free solders

A comparative study of solid/solid interfacial reactions of electroless Ni-P (15 at.% P) with lead-free solders, Sn-0.7Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn, was carried out by performing thermal aging at 150°C up to 1000 h. For pure Sn and Sn-3.5Ag solder, three distinctive layers, Ni3Sn4, SnNiP, and Ni3P, were observed in between the solder and electroless Ni-P; while for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders, two distinctive layers, (CuNi)6Sn5 and Ni3P, were observed. The differences in morphology and growth kinetics of the intermetallic compounds (IMCs) at the interfaces between electroless Ni-P and lead-free solders were investigated, as well as the growth kinetics of the P-enriched layers underneath the interfacial IMC layers. With increasing aging time, the coarsening of interfacial Ni3Sn4 IMC grains for pure Sn and Sn-3.5Ag solder was significantly greater than that of the interfacial (CuNi)6Sn5 IMC grains for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders. Furthermore, the Ni content in interfacial (CuNi)6Sn5 phase slightly increased during aging. A small addition of Cu (0.7 wt.%) resulted in differences in the type, morphology, and growth kinetics of interfacial IMCs. By comparing the metallurgical aspects and growth kinetics of the interfacial IMCs and the underneath P-enriched layers, the role of initial Cu and Ag in lead-free solders is better understood.