Fengjiang Wang

Effect of indium addition on interfacial IMC growth and bending properties of eutectic Sn–0.7Cu solder joints

The microstructure and growth behaviors of interfacial IMCs between the Sn–0.7Cu–xIn (x = 0–5.0 wt%) solder and Cu substrate under solid aging were investigated. Bending test was conducted to evaluate the mechanical properties of Cu/Sn–0.7Cu–xIn/Cu solder joints. The needle-like Cu6(Sn,In)5 IMCs formed at interface instead of Cu6Sn5 in as-soldered In-containing solder joints. During solid aging, indium addition had little influence on the growth of Cu6Sn5 IMCs, but strongly suppressed the growth of Cu3Sn IMCs. In Sn–0.7Cu–5.0In/Cu couple, the maximum solubility of indium in Cu6Sn5 and Cu3Sn was about 4.9 and 3.1 at.% respectively which was independent of aging temperature. The average composition of Cu6(Sn,In)5 and Cu3(Sn,In) were given as Cu6(Sn0.89,In0.11)5 and Cu3(Sn0.88,In0.12). After indium addition, the diffusion coefficients were found to be lower than that of Sn–0.7Cu/Cu couple and the activation energy for the growth of Cu3Sn increased with the increase of indium content. Doping indium into Sn–0.7Cu solder improved the bending properties of solder joints. Interfacial cracks were suppressed effectively after long time aging with indium addition.

Interfacial Reaction and Mechanical Properties of Sn-Bi Solder joints

Sn-Bi solder with different Bi content can realize a low-to-medium-to-high soldering process. To obtain the effect of Bi content in Sn-Bi solder on the microstructure of solder, interfacial behaviors in solder joints with Cu and the joints strength, five Sn-Bi solders including Sn-5Bi and Sn-15Bi solid solution, Sn-30Bi and Sn-45Bi hypoeutectic and Sn-58Bi eutectic were selected in this work. The microstructure, interfacial reaction under soldering and subsequent aging and the shear properties of Sn-Bi solder joints were studied. Bi content in Sn-Bi solder had an obvious effect on the microstructure and the distribution of Bi phases. Solid solution Sn-Bi solder was composed of the β-Sn phases embedded with fine Bi particles, while hypoeutectic Sn-Bi solder was composed of the primary β-Sn phases and Sn-Bi eutectic structure from networked Sn and Bi phases, and eutectic Sn-Bi solder was mainly composed of a eutectic structure from short striped Sn and Bi phases. During soldering with Cu, the increase on Bi content in Sn-Bi solder slightly increased the interfacial Cu6Sn5 intermetallic compound (IMC)thickness, gradually flattened the IMC morphology, and promoted the accumulation of more Bi atoms to interfacial Cu6Sn5 IMC. During the subsequent aging, the growth rate of the IMC layer at the interface of Sn-Bi solder/Cu rapidly increased from solid solution Sn-Bi solder to hypoeutectic Sn-Bi solder, and then slightly decreased for Sn-58Bi solder joints. The accumulation of Bi atoms at the interface promoted the rapid growth of interfacial Cu6Sn5 IMC layer in hypoeutectic or eutectic Sn-Bi solder through blocking the formation of Cu6Sn5 in solder matrix and the transition from Cu6Sn5 to Cu3Sn. Ball shear tests on Sn-Bi as-soldered joints showed that the increase of Bi content in Sn-Bi deteriorated the shear strength of solder joints. The addition of Bi into Sn solder was also inclined to produce brittle morphology with interfacial fracture, which suggests that the addition of Bi increased the shear resistance strength of Sn-Bi solder.

Comparative study on the wettability and interfacial structure in Sn–xZn/Cu and Sn/Cu–xZn system

Zn is the common used alloying element in Pb-free solder to depress the growth of interfacial intermetallic compound (IMC) in solder joints, but is also an oxidation preferred element to decrease the wettability of solder. This paper provides a potential replacement on the Zn-contained solder with small amount of Zn alloyed Cu substrate to avoid the deterioration on wettability but keep the depressing effect on growth of IMC through the comparative study on Sn–xZn/Cu and Sn/Cu–xZn. To systematically study the effect of Zn alloying, 0.2–0.8 wt% content of Zn was added into pure Sn solder and 2.29 and 4.89 wt% content of Zn was alloyed into Cu substrate. The wettability, interfacial reaction during soldering and interfacial IMC growth of Sn–xZn/Cu and Sn/Cu–xZn during isothermal aging were studied and compared with Sn/Cu system. Zn alloyed into Cu substrate exhibited a far better wettability than Zn alloyed into solder. Although there was no significant change on the composition, morphology and thickness of interfacial of IMC in Zn-contained system during soldering, Zn alloyed into Cu substrate or Sn solder obviously depressed the formation of Cu3Sn and the growth of IMC layers during isothermal aging compared with Sn/Cu system. The depressing effect and calculated activation energy on the growth of IMC increased with the content of Zn in Cu or in Sn, and can be attributed to the CuZn solid solution existed at the interface because it decreased the driving force on formation of Cu3Sn and the diffusion rate of Cu atoms. Compared Sn/Cu–xZn with Sn–xZn/Cu, Cu–4.89Zn substrate provided a similar depressing effect on IMC growth but an exceeding wettability compared with Sn–Zn solder, and can be offered as an alternative under bump metallurgy (UBM) in solder joints.

Effect of electromigration of Sn-xAg-Cu solder joints on its microstructure and mechanical properties

In this study, effect of electromigration of Sn-0.7Cu, Sn-0.3Ag-0.7Cu and Sn-3.0Ag-0.5Cu solder joints on the microstructure and mechanical properties were investigated. Within a certain range, addition of Ag enhanced the electromigration stressing resistance and the joint strength. Electromigration changed the distribution and morphology of the IMC. In cathode side, the Cu6Sn5 layer reduces with the increasing time, while in anode side, the Cu6Sn5 layer would increased with the increasing time. Electromigration reduced the mechanical properties of linear joints. The drop rate of ultimate tensile strength has a linear relationship with the stressing time. According to the fracture morphology, the fracture mechanism changed from ductile to cleavage fracture.

Reliability of Pb-free BGA solder joints under random vibration

Alternating elastic and plastic strain or even creep was inevitably produced in the solder joints under thermal cycle, vibration or impact circumstance, and finally caused the fatigue failure of solder joints and electronic equipments. The fatigue life and its failure mechanism are always the interests to the reliability of solder joints. Generally, the low-cycle fatigue induced by thermal cycling is the major concern in the reliability of electronic packaging, while dynamic loading effects to solder joint fatigue life have not been thoroughly investigated. Actually, the interaction of the strain induced by dynamic loading and low cycle fatigue is going to accelerate the process of failure. Under certain circumstance, vibration and impact would like to be the main failure reasons. Also, comparing to the fact that alternating thermal strain induced by thermal cycle is just depended on temperature, strain in the joints in vibration circumstance is more depended on the components location in PCB, solder joint location in the array, size of solder balls, constraint positions and the other working circumstance. This paper is concerned with the reliability of lead-free ball grid array (BGA) packaging under random vibration including the effect of the location of BGA on the PCB and holding positions during tests. Special circuit was designed to detect the voltage of the BGA ball circuit and the real failure time of solder joints during tests.

Electromigration behaviors in Sn–58Bi solder joints under different current densities and temperatures

Sn–58Bi eutectic solder is attracted much attention to replace Sn–Ag–Cu Pb-free solder due to its lower melting temperature in recent years. However, its low melting temperature also raised serious electromigration (EM) reliability due to the accelerated atomic migration from the high local temperature induced by Joule heating. This paper mainly studied the EM behaviors occurred in Sn–58Bi solder joints with consideration the effect of different current densities and temperature. Variation on the atomic migration and phase coarsening were observed in Sn–58Bi solder joint, and the threshold current densities to trigger EM damage were estimated. The results showed that Bi-rich layer was produced at anode side during stressing with low current density while caused Cu cathode to dissolve into Sn–Bi solder and formed Cu6Sn5 intermetallic compounds near anode side during stressing with high current density. Accordingly, the corresponding dominant diffusing species in Sn–58Bi solder were Bi atoms at low temperature and Cu and Sn atoms at high current density, respectively. High temperature applied with EM accelerated the atomic migration. The threshold current densities at different temperature were then calculated from the thickness of Bi-rich layer at anode side. There existed a lower threshold current density at higher temperature due to the active atomic mobility. Finally, Bi phase coarsening was observed in Sn–Bi solder under different current densities and temperatures. Higher current density or higher temperature induced faster grain growth, but the applied current had a greater effect on Bi phases coarsening in Sn–Bi solder matrix than the applied temperature. The controlling kinetics on the growth of Bi phases were also suggested for the effect of current densities and temperatures.

Microstructure, interfacial reactions and mechanical properties of Co/Sn/Co and Cu/Sn/Cu joints produced by transient liquid phase bonding

In this paper, the wettability and growth behaviors of interfacial intermetallic compounds (IMCs) in the Sn/Co and Sn/Cu liquid/solid couples were comparatively investigated. In situ tensile tests were conducted to evaluate the mechanical properties of the Co/Sn(IMCs)/Co and Cu/Sn(IMCs)/Cu joints. The wettability of Sn on Cu substrate was better than that of Co substrate. The lath-like CoSn3 IMCs limited the improvement of wettability of Sn on Co substrate, even if the soldering temperature was increased. The Sn/Co couples showed an acceptable wettability for electronic application. During liquid aging, the rapid growing interfacial CoSn3 were controlled by the combination of chemical reaction and atomic diffusion. Grain boundary diffusion should be responsible for the growth of Cu6Sn5 in the Sn/Cu liquid/solid couples. The tensile strength and elongation of the Co/CoSn3/Co joints were basically equal to that of the Cu/IMCs/Cu full IMCs joints. Interfacial IMCs cracks were more likely to occur in the joints fabricated by Co substrate. The Co/CoSn3/Co sandwich structure can be fabricated within a remarkable short period of time at low temperature without pressure.

Electromigration behavior of liquid Sn-58Bi/Cu joints through minor Zn alloying substrates

This study aims to investigate the interfacial reactions of Sn-58Bi/Cu and Sn-58Bi/Cu-Zn solder joints during liquid-solid electromigration (L-S EM). L-S EM behavior of Sn-58Bi/Cu and Sn-58Bi/Cu-2.29Zn solder joints about external microstructure under 1.5 × 104 A/cm2 at room temperature was studied using scanning electron microscopy (SEM). For the two types of solder joints, the thickness of IMC layer at the anode side was much higher than that at the cathode side with the same stressing time, respectively. At the anode side of Sn-58Bi/Cu and Sn-58Bi/Cu-2.29Zn solder joints, the rod-like Cu6Sn5 grew rapidly and showed an obvious “Ripening phenomenon”. It is also found that doping 2.29Zn into Cu substrate reduced the average thickness of IMC layers at both sides. The “Ripening phenomenon” happened in the two types of solder joints, which indicated that the doping Zn did not change the direction of Cu atoms. There was no back-stress in L-S EM, and the flux induced by EM and chemical potential gradient during L-S EM was discussed.

The shear strength and fracture mode of Sn-xBi (x=0, 2.5, 5, 15)/Cu solder joints

In these study, the effect of Bi content on shear strength of Sn-xBi (x=0, 2.5, 5, 15) solder balls were investigated. The Bi addition could improve the shear strength of solder balls, added 2.5wt.% Bi into pure Sn solder alloys had a greater increase in shear strength. The fracture mode always remained the same and the shear strength had no obvious changes with increasing the aging time. The Sn-2.5Bi solder balls had the highest fracture energy due to its large elongation properties and the high shear strength. The shear strength of Sn-5Bi and Sn-15Bi solder balls were bigger than pure Sn solder alloys, when aging time increased to 10 days, the shear strength reached maximum value. Due to the growth of IMCs and the change of fracture mode, the shear strength of Sn-5Bi and Sn-15Bi solder balls had a considerable change. The fracture mode of Sn-5Bi and Sn-15Bi solder balls changed from ductile solder mode to mixed solder/IMC mode, finally converted to brittle IMC mode. Sn-15Bi solder balls had the lowest fracture energy due to its poor elongation properties and lower shear strength.

Electromigration behavior of Sn3.0Ag0.5Cu/Sn58Bi structural composite solder interconnect

The microstructure and electromigration of Bi and Sn in Cu/Sn58Bi/Cu and Cu/Sn58Bi /Sn3.0Ag0.5Cu/ Sn58Bi/Cu structural composite solder joint, were researched under the current density of 1.0×104 A/cm2 at room temperature. Two kinds of solder joints changed their morphology at both anode sides and cathode sides after current stressing. Bi layer was formed in the anode side while Sn layer was formed in the cathode side. The microanalysis indicated that Bi was the major diffusion element from cathode to anode and the migration of atomic Bi was faster than atomic Sn. Furthermore, through the comparison of two kinds of solder joints, it was easy to find that Sn3.0Ag0.5Cu suppressed the migration of atomic Bi and atomic Sn during electromigration.