Early Stage Interfacial Reaction And Undercooling Solidification Behavior Of Microscale Lead-free Solder Interconnects During Reflowing Process

In recent years, the dimension of lead-free solder micro-interconnects has been scalingdown to meet the trend of miniaturization, lightweight and multifunction of electronicproducts and devices. Accordingly, there is an increasing demand for high reliability lead-freesolder interconnects whose size has been scaled down gradually. Currently, one of the majorreliability concerns for the microscale solder interconnects is the existence of the brittleintermetallic compounds (IMC) which spread over the joints' solder matrix and interface.Generally, the microstructure and defects in solder joints are directly related to the interfacialreaction between the solder and under bump metallization (UBM) and the solidificationbehavior of the undercooled solder during reflow process. However, thus far, relatively littleis known about the undercooling behavior of the micro-sized solder melt on metallicsubstrates and the interfacial mass transition as well as its influence on the solidificationmicrostructural evolution of microscale solder interconnects during the soldering process. Inthis study, the DSC-reflow experimental approach was employed to simulate the reflowsoldering process with extensive microstructure characterization, the interfacial reactionbetween lead-free solders (i.e., pure Sn and a few other Sn-based solders) and different UBMlayers during reflow soldering process and the melting/solidification characteristics of eachreaction system were investigated using a differential scanning calorimeter (DSC). Thefocuses were placed on revealing the mechanism of formation and morphology transition ofthe interfacial IMC phases at the early stage interfacial reaction of the typical solderingsystems during reflow process, and clarifying the influence of interfacial UBM, dimensionand composition of the solders on the undercooling solidification behavior, theundercooling-dependent solidification behavior and subsequent microstructural evolution ofthe solders and their joints. In the end, the growth kinetics of interfacial IMC and the CuUBM dissolution behavior of the ball grid array (BGA) structure Sn-3.0Ag-0.5Cu/Cu jointsduring the liquid isothermal aging at the temperatures near or above the solder's meltingtemperature were studied systematically.The results show the minute amount addition of mixed rare-earth La-Ce has broughtabout an obvious decrease of the undercooling for Sn-Cu-Ni series of solders, consequentlythe growth of primary solidification phase of (Cu,Ni)₆Sn₅IMC has been suppressed. For theSn/Cu(Ag, Ni, Co) binary soldering system, due to the big difference of interfacial energyamong different soldering systems, the undercooling values of Sn/Ni and Sn/Co solderingsystems are much smaller than that of Sn/Cu and Sn/Ag ones. Consequently both of Sn/Cu and Sn/Ag soldering systems, which possessed higher undercooling values, exhibitedscallop-like morphology of the interfacial IMC layer, and the interfacial morphologies ofSn/Ni and Sn/Co systems, which possessed lower undercooling values, showed denselyneedle-like interfacial IMC layer. The Cu UBM plays the role of heterogeneous nucleationsites, and the undercooling value of Sn-3.5Ag/Cu soldering system was significantly lowerthan that of Sn-3.5Ag solder. The alloying effect of Cu in Sn-3.5Ag solder also leads to anobvious decrease of the undercooling value of the solder, but the Cu UBM shows a strongerrole in decreasing the undercooling value of Sn-3.5Ag solder than that of the alloying effectof Cu in the solder. Further, the undercooling behavior and solidification microstructuralevolution of Sn-3.0Ag-0.5Cu solder balls and BGA structure Sn-3.0Ag-0.5Cu/Cu joints withdifferent dimensions were characterized using DSC-reflow approach. It has been shown thatthe decrease in diameter of solder balls leads to an obvious increase of the undercooling of thesolder; and the undercooling of solder joints show a dependence on both the diameter ofsolder balls and the interfacial reaction of Cu open pads, but the dimension of Cu open padshas a slight influence on the undercooling of the joints. Coarse primary IMC solidificationphase formed in smaller solder balls and joints, in which the bulk Ag₃Sn IMC was the primarysolidification phase in the solder balls with a large undercooling value and the large primaryCu₆Sn₅phase formed in all of reflowed joints. The interfacial reaction and dissolution of Cuatoms into the solder matrix resulted in a decrease in the degree of the undercooling of thesolder joints during solidification process, and the primary Ag₃Sn phase can be suppresseddue to the increase of Cu content in the solder matrix of solder joints.Moreover, the results of the early stage interfacial reaction and microstructural evolutionof the Sn/Cu and Sn/Ag soldering systems show that the initial interfacial reaction arisingfrom atomic interdiffusion in solid-state Sn/Cu and Sn/Ag systems led to the formation of theplanar-like IMCs and a mushy layer of a binary alloy at the interface in each of the twosystems; subsequently the interfacial mushy layer of the binary alloy in the two systemsmelted at a temperature4.9°C and10.6°C respectively lower than the actual melting point ofpure tin, and this phenomenon can be called premelting of the interfacial mushy layer due toeutectic reaction. After premelting occurred at the interface, there was a big change ininterfacial energy at the interface of Sn/Cu₆Sn₅as the interfacial solder experienced atransition from solid-state to liquid-state in a very small temperature range; and consequentlythe Sn/Cu soldering systems exhibited morphology change of the interfacial IMC fromplanar-like in solid-status to scallop-like in liquid status. However, the solder located at theinterfacial areas experienced the transition from solid-state to liquid-state did not lead to the morphology change of the interfacial IMC layer in the Sn/Ag soldering system. It isinteresting that the interfacial Ag₃Sn grains show spherical shape at the initial growing stage,and would begin to transform to polyhedron-shaped when the spherical particles grew to acritical size. The results also show that during heating process, the atomic interdiffusionbetween Cu UBM and Sn-3.5Ag solder in solid-state Sn-3.5Ag/Cu system resulted in theformation of planar-like IMC and a mushy layer of Sn-Ag-Cu ternary alloy at the interface,and the mushy layer melted at a temperature nearly4oC lower than Sn-3.5Ag solder's meltingpoint. The premelting may tend to first appear in the tripe junction of the Sn/Ag₃Sn/Cu₆Sn₅grain boundaries due to more severe lattice mismatch in these places. On the one hand, theinterfacial premelting resulted in the morphology change of the interfacial IMC fromplanar-like in solid-status to scallop-like in liquid status; on the other hand, Ag and Sn atomsin the liquid phase continued to accumulate in the original Ag₃Sn grains located in the tripejunction area and then Ag₃Sn grains may grow up into large needle-like Ag₃Sn in the top ofCu₆Sn₅layer.The results of study on the interfacial IMC growth and the Cu UBM dissolutionbehaviors in BGA structure Sn-3.0Ag-0.5Cu/Cu joints during liquid isothermal aging at thetemperatures near or above the solder's melting temperature show that a slight increase of theaging temperature from217oC up to218oC had a significant influence on the degree of CuUBM consumption during isothermal aging process, the proportion of the Cu UBM dissolvedinto the Sn matrix in the total consumption of Cu UBM also increased greatly from15.5%to71.0%. However, the thickness of the interfacial Cu₆Sn₅layer and total Cu-Sn IMC layer hada slight decrease due to the fact that Cu UBM rapidly dissolved into the solder matrix andresulted in less supply in the growth of interfacial IMC layers. When the aging temperaturewas increased to230oC, the proportion of Cu UBM dissolution with total consumption justincreased to72.6%, and the thickness of interfacial Cu-Sn IMC layers reached the highestvalue. The results of interfacial IMCs growth kinetics show the growth of the interfacialCu₆Sn₅and Cu₃Sn was mainly controlled by grain boundary diffusion and bulk diffusion,respectively; and the grain boundary grooving and grain coarsening can also influence thegrowth kinetics of the interfacial Cu-Sn IMCs.