Sihan Liu

The failure models of Sn-based solder joints under coupling effects of electromigration and thermal cycling

Currently, the main concerns of Pb-free solder joints are focusing on electromigration (EM) and thermomechanical fatigue (TMF) problems. Many models have been established to understand the failure mechanisms of the joint under such single test conditions. Based on the fact that almost all microelectronic devices serve in combination conditions of fluctuated temperature and electric current stressing, the coupling effects of EM and TMF on evolution of microstructure and resistance of solder joint had been investigated. The failure models of binary SnBi alloy and ternary SnAgCu (SAC) solder under the coupling stressing were divided into four and three different stages, respectively. The failure mechanisms were dominant by the relationship of phase segregation, polarity effect, phase coarsening, and the coefficient of thermal expansion mismatch. Cracks tend to form and propagate along the interface between intermetallic compound layers and solder matrix in SAC solder. However, grain boundary was considered a...

The coupling effects of thermal cycling and high current density on Sn58Bi solder joints

Currently, one of the serious challenges in microelectronic devices is the miniaturization trend of packaging. As the decrease of joint dimension, electromigration (EM) and thermomechanical fatigue become critical issues for fine pitch packaging. The independent mechanisms of EM and thermomechanical fatigue are widely investigated and understood. However, the coupling effect of both conditions needs further exploration. The current study established the correlation between resistance and microstructure evolution of solder joint under the combination effect of thermal cycling and high current density and illustrated the different contributions of these two factors to the reliability of the joint through the comparison monopoly tests. The results revealed that cracks had more impact on resistance increase than phase segregation. The resistance evolution could be divided into three stages. First, the resistance mitigated due to the phase coarsening. Second, Joule heating effect made the resistance increase slowly. Third, EM led to the resistance increase rapidly. The high current density can help to improve the reliability of the solder joint under the coupling effect of thermal cycling and EM at the initial stage, but harmful to the consequence process.

First-principles study of the stability of free-standing germanene in oxygen atmosphere

The O2 dissociation and O atoms adsorption on free-standing germanene are studied by using first-principles calculations in this paper. Compared with the extremely active silicene in oxygen atmosphere, germanene is found to be less active due to an energy barrier for dissociation of about 0.57 eV. Moreover, the dissociated oxygen atom follows two opposite migration pathways on the germanene surface, which is quite different from the case of silicene. Furthermore, the migration and desorption of O atoms at room temperature are relatively difficult due to the strong Ge-O bonding, resulting in the formation of germanium oxides. Our results reveal the interplay between germanene and O2 and suggest the enhanced stability of germanene in oxygen atmosphere compared with silicene.

The effects of thermal cycling on electromigration behaviors in lead-free solder joints

With packaging density constantly increasing and the size of solder joints scaling down in microelectronic products, electromigration (EM) becomes serious reliability problem in interconnections which induced by the high current density. Currently, investigations on EM phenomenon mainly focus on microstructure evolution, the change of grain orientation and the dominant diffusion species, etc. Almost all these studies and EM damages are carried out under constant ambient temperature. However, electronic devices usually service in a non-equilibrium state environment in practical. The purpose of this research is investigating the EM behavior under the thermal cycling conditions and trying to understand the relationship between microstructure evolution and resistance change of the solder joints. The one-dimensional Cu/Sn3.0Ag0.5Cu/Cu linear solder joint was employed to study the EM behavior of Sn-3.0Ag-0.5Cu (SAC305) under combined conditions of cycling temperature ranges from -20 to 100 °C and a high current density of 104A/cm2. Resistance change and microstructure evolution were analyzed in this paper. Resistance and microstructure data was collected and observed by paperless recorder and scanning electron microscope (SEM), respectively. The results revealed that high current density can cause voids formation. The propagation of cracks was accelerated by cycling temperature due to the mismatch of coefficient of thermal (CTE) between solder matrix and Cu substrate. Moreover, the coupling effects of high density and cycling temperature can lead to a rapid failure when compare to the monolithic loading condition.

Whisker Growth Behavior of Sn58Bi Solder Coatings Under Isothermal Aging

Whisker formation is a frequently occurring problem in the electronica industry, causing damage to fine-pitch electrical components. Several theories and models have been developed to describe the whisker growth, and many attempts have been made to find solutions for this issue. Most of the previous literature addressed the formation of Sn whiskers, while some attention was focused on Bi-rich whiskers. Moreover, investigation of different types of whiskers would be beneficial for understanding of the fundamental processes behind of the whisker growth. In our work, we analyze and discuss the growth of Bi-rich whiskers in eutectic Sn58Bi solder coatings under isothermal aging conditions. A possible growth mechanism for Bi-whiskers in the coating is proposed. Two processes contributed to the whisker growth. One process was the chemical reaction between Cu and Sn atoms to form Cu6Sn5. Cu atoms were inexhaustibly introduced into the coating from the substrate and caused the formation of Cu6Sn5. This process provided the driving force and was a sufficient condition for the growth of Bi-rich whiskers. The second process was the segregation of Bi atoms driven by the gradient of the Bi atoms’ concentration. This process was the necessary condition for Bi-whiskers growth.

Effects of POSS-silanol addition on the whisker formation in Sn3.0Ag0.5Cu Pb-free solder

The whisker formation behaviors of Sn3.0Ag0.5Cu composite solder which reinforced by an organic-inorganic hybrid materials, silanol of polyhedral oligomeric silsesquioxane (POSS-silanol) was investigated. The thin solder films were obtained by reflow soldering and a thermal cycle from -40°C to +85°C was applied to accelerate the test process. Both surface and interface microstructures of the specimens were evaluated using scanning electron microscopy (SEM). The results revealed that the growth of whiskers was inhibited significantly in solders with the addition of POSS-silanol. Fewer whiskers with smaller size were observed on the Sn3.0Ag0.5Cu+POSS solder when compared with Sn3.0Ag0.5Cu solder. The main driving force for the whisker formation under such condition was considered to be the squeeze due to the deformation of the solder matrix which induced by the coefficient of thermal expansion (CTE) mismatch. The POSS addition alleviated the deformation which dominated by CET mismatch. Thus, the driving force of whisker formation was weaken through releasing of the stress concentration during the iterative of heating and cooling process.

The effects of creep on electromigration behaviors in Sn-based Pb-free solder joints

Nowadays, the combination effects of thermal, mechanical and electrical on the reliability issues of solder joint gain more and more attentions. The independent failure mechanisms of solder joints are widely investigated and comprehended under single conditions as listed above. However, the coupling effect of thermal, mechanical and electrical needs further exploration. Consequently, the failure behaviors of solder joint under coupling of high current density and creep become a critical problem. The sandwich Cu/Sn3.0Ag0.5Cu/Cu solder joints which subject to constant ambient temperature and creep stress but different current density are systematically investigated in the study. The failure behaviors of solder is analyzed and discussed by observation of the resistance change, microstructure evolution and orientation transition of solder joints. The effects of mechanical, thermal, and electrical on the damage calculation are also illustrated individually. The results indicate that, failure mechanism of solder joint under a constant creep stress of 5.5MPa and low current density was controlled by grain boundary sliding. The damage easily accumulated near both interfaces of solder/Cu. The electromigration process could alleviate creep damage by released grain boundary sliding and changed stress distribution in the solder joint at initial stage.