Yong Zuo

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.

Effects of Electromigration on the Creep and Thermal Fatigue Behavior of Sn58Bi Solder Joints

Electromigration (EM), creep, and thermal fatigue (TF) are the most important aspects of the reliability of electronic solder joints, the failure mechanisms of which used to be investigated separately. However, current, mechanical loading, and temperature fluctuation usually co-exist under real service conditions, especially as the magnitude of current density is increasing with joint miniaturization. The importance of EM can no longer be simply ignored when analyzing the creep and TF behavior of a solder joint. The published literature reports that current density substantially changes creep rate, but the intrinsic mechanism is still unclear. Hence, the purpose of this study was to investigate the effects of EM on the creep and TF behavior of Sn58Bi solder joints by analyzing the evolution of electrical resistance and microstructure. The results indicated that EM shortens the lifetime of creep or TF of Sn58Bi solder joints. During creep, EM delays or suppresses the cracking and deforming process, so fracture occurs at the cathode interface. During TF, EM suppresses the cracking process and changes the interfacial structure.

Study of EM and TM of Sn0.3Ag0.7Cu solder joints under high current stress and thermal gradient

Electromigration (EM) of solder joints in microelectronic devices have become a critical reliability issue due to the continuous miniaturization and requirements for enhanced performance of the devices. At present, besides EM, thermomigration(TM) has been regarded as another reliability concern as it has been found to accompany EM in flip chip solder joints. Both EM and TM effect are the main factors leading to the failure of the solder joints. In this study, TM and EM were studied in Cu/Sn0.3Ag0.7Cu/Cu solder joints by applying a 1×10(4) A/cm(2) DC current and a thermal gradient of 1×10(3)°C/cm to the joints respectively. The microstructure and composition of the solder joints were observed by means of Scanning Electron Microscope. This research include two possible aspects. Firstly, the solder joint was subjected to current stress and thermal gradient. Secondly, analyzing the microstructure and composition of the joints. According to the microstructure and composition, the behavior of the joints under current stress and thermal gradient were found. The results indicated that, with a thermal gradient of 1×10(3)°C/cm, the Cu atoms were found to migrate towards the lower temperature side while the Sn atoms were found to migrate towards the higher temperature side firstly, with the time prolonging, the Sn atoms were found to migrate towards the slower temperature side. And with a 1×10(4) A/cm(2) DC current stressing, The IMC in cathode decrease much, and some voids and cracks occur. In anode, the IMC increase a lot. And there are some bump. The couple of thermal and electrical field of solder joint will be studied in the future.

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 magnetic field on microstructure and mechanical properties of lead-free solder joint

Magnetic field is believed to have a significant impact on the evolution of microstructure during solidification or service condition. Many researches apply magnetic field to obtain a material with textures or special microstructure, such as turbine blade. The magnetic field is able to influence the crystal growth process, constrain crystal arrangement direction; and effectively inhibit the conductive fluid in thermo solute convection. However, the effect of magnetic field on the reliability of solder joint or related research is rarely reported. The purpose of this research is to investigate how magnetic field impact on the microstructure and mechanical performance of Sn-based lead-free solder joint. This research contains two possible aspects. Firstly, magnetic field with different intensities was posed on Sn0.3Ag0.7Cu to analyze the solidification microstructure. The roles of the second phase particles, such as Ni, were also considered. Intensity of magnetic field ranged from 0T-7T. Secondly, the solder joint was subjected to current density with magnetic field. The coupling effect of magnetic and electric fields was studied. The result indicated that, magnetic field influenced migration rate and migration direction of atoms. Consequently, the thicknesses of the intermetallic compounds (IMCs) at two interfaces were different. The appearance of interfacial IMCs changed. The couple of magnetic and electric made the solder joint failure more quickly. This study contributes to understand the reliability of solder joint under multi-field condition.

Creep behaviors of Pb-free solder joints during current stressing

Electromigration and creep as two of the most important reliability issues gain much attention, and flurry of academic activities dedicate to understand fundamental physics of them. However, since real solder joints are usually exposure to complicated conditions with high current density, temperature excursion, and mechanical loading, when analyzing creep of a real solder joint, Electromigration must be taken into account because of the interaction between them. In this study, simple shear to failure was performed at different temperature and stress level to evaluate failure behavior of solder joints. Activation energy and stress exponent under high current density were also calculated and discussed. This research indicated that, strain rate was accelerated by high current density at different levels of stress and temperature. The effect of high current density played more important roles at low stress and low temperature condition. At high stress and high temperature, the accelerating effect was not that significance. When high current density was introduced, both activation energy and stress exponent tend to decrease.

Failure behaviors of Sn-0.3Ag-0.7Cu solder joint under creep and current stressing

The reliability issues such as creep, electromigration (EM), and thermal fatigue of solder joints in real service condition have gained much attention due to the rapid development of electronic devices towards multi-function, miniaturizing and portable tendency. There are many studies focusing on microstructure changes of solder joints under the single condition of creep, EM, or thermal fatigue. However, few studies focus on the coupling effect(s) of the above mentioned impact factors during the failure process of a solder joint. In this study, Sn-0.3Ag-0.7Cu which has much lower cost than Sn-3.0Ag-0.5Cu under the coupling stressing of EM and creep were investigated. The in-situ microstructure and resistance of the solder joints were observed and measured during the failure process. The results indicated that the lifetime, the fracture location, and the fracture mode of solder joint were significantly changed by the coupling effect(s) of creep and EM.

Effects of electromigration on the creep of Sn0.3Ag0.7Cu solder joint

For real solder joints used in electronic devices, EM and creep may be two relevant reliability problems because solder joints usually serves under a combination of current stressing, elevated temperatures, and mechanical loading conditions. Especially when the size of solder joints scales down and current density escalates, EM-induced microstructure change should have certain impact on creep behavior. In this study, we analyzed and discussed how EM influenced the creep behavior of lead-free solder joint from the perspective of EM-induced IMCs change. Behaviors of both interfacial and matrix IMCs were observed and attempt to reveal the roles they played in the failure of real lead-free solder joint. From this study we can conclude that: during EM, grain size of interfacial IMCs at both sides was increased, and the size at anode side was much larger. The shape of interfacial IMCs was changed from scallop shape to hexagonal prism. Besides, there were many Cu6Sn5 migrated into solder matrix. During creep, pre-EM process alleviated deformation of solder matrix and brought about brittle fracture of a solder joint.

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.