Investigation on impact strength of the as-soldered Sn37Pb and Sn3.8Ag0.7Cu solder joints

Charpy impact specimens of eutectic Sn37Pb and Sn3.8Ag0.7Cu solder joints with U-type notch were prepared to investigate the joint impact strength. The gap sizes of the butt joint were selected at 0.3 and 0.8 mm. Compared with the values of 0.3 mm joint gap, the impact absorbed energies of two solder joints were increased at the joint gap of 0.8 mm. The impact strengths of Sn37Pb joints were higher than those of Sn3.8Ag0.7Cu joints in both cases. From the macrographic observation of the fracture path, when the gap was 0.3 mm, the crack initiation of two solder joints located at the root of U-type notch then propagated along one interface of the joint. For the Sn37Pb joints, the fracture path was not changed at 0.8 mm gap size. However, the fracture path of Sn3.8Ag0.7Cu joint was totally changed and the fracture occurred not at the root of pre-U notch but from one side of the solder/Cu interfaces. From the micrographic observation, the crack of the Sn37Pb joints was concentrated on the Pb-rich layer in the vicinity of interfacial intermetallic (IMC) layer and the fracture morphology mainly appeared to be a ductile-like structure. Meanwhile, the fracture of Sn3.8Ag0.7Cu joints propagated along either the interface of IMC/solder or within the IMC layer and showed a brittle failure mode.     

Microstructural effects on the behavior of Sn–Ag solder joints under repeated reverse stressing

Double shear lap joints made with eutectic Sn–Ag solder were cooled from 270 °C at different rates ranging from air-cooling to iced-brine quenching. These joints were subjected to reversed stressing with constant shear strain amplitude at room temperature. Shear strain amplitudes between 0.75 and 1 were imposed during the course of this investigation. Role of microstructural features on the surface damage accumulation and stress–strain behavior was investigated as a function of shear strain amplitude and number of cycles of repeated stressing. Shear banding was found to be along Sn dendrites in air-cooled specimens, while it cut across the small Sn grains present in the quenched specimens.     

Comparison of drop performance between the Sn37Pb and the Sn3.8Ag0.7Cu solder joints subjected to drop test

This study was concerned with the drop performance between the Sn37Pb and the Sn3.8Ag0.7Cu (wt. %) solder joints when the specimens were subjected to drop test after soldering process. The U-notch butt-jointed specimen was adopted and a lab-designed drop tester was employed. Meanwhile, the electrical resistance values of two kinds of solder joints were measured and recorded after certain drop tests, and finally drop number versus resistance curves were plotted and compared. From the resistance variation with the drop number, it was observed that the Sn37Pb joints presented significantly higher drop performance than the Sn3.8Ag0.7Cu ones. For the Sn3.8Ag0.7Cu specimens, the average drop number before failure was approximately 15-18 and then the resistance values sharply increased. However, the average drop number of the Sn37Pb joints was over 110 and the increasing rate of the electrical resistance was smooth, which is consistent with the results of the board-level drop test. Moreover, one specimen of each kind was picked out and the microstructural observation was carried out to investigate the joint deformation behavior in the dynamic load. It was obvious that the plastic deformation capacity of the Sn37Pb joints was remarkably higher than the one of the Sn3.8Ag0.7Cu joints, proving that most of SnAgCu-based solders presented low deformation compatibility and low energy absorption.     

Effect of the cooling rate on the mechanical and electrical behaviour of a 63Sn/37Pb solder bump on a metallized Si substrate

Flip-chip bonding technology, with solder bumps deposited on metal terminals on the chip, provides the highest functional densities of all the present bonding techniques. In this study, tin-lead solder bumps were screen printed onto a metallized Si substrate. Various cooling rates were employed after reflowing the solder bumps. The effect of the cooling rate on the microhardness, the adhesion strength and the electrical resistance of solder joints was studied and the mechanism explored. It was found that intermetallic compounds play important roles in both the mechanical and the electrical behaviour of the solder bumps.     

Degradation of Sn37Pb and Sn3.5Ag0.5Cu solder joints between Au/Ni (P)/Cu pads stressed with moderate current density

Sn37Pb (SP) and Sn3.5Ag0.5Cu (SAC) ball grid array (BGA) solder joints between Au/Ni (P)/Cu pads were stressed with a moderate current density of 6.0 × 10² A/cm² at an ambient temperature of 125°C up to 600 h. The solder joint reliability was evaluated in terms of temperature measurement, microstructural analysis and mechanical strength test. It was confirmed that no obvious electromigration occurred with this moderate current density. However, the local temperature of solder joints rose considerably due to massive Joule heating, which degraded the solder joint reliability seriously. Phase coarsening was observed for both solders and it was particularly apparent in the SP solder joints. Compared to the SP, the SAC was found to be more reactive and hence a thicker intermetallic compound (IMC) was developed during the current stressing. Nevertheless, the IMC thickening was not as remarkable as expected with current stressing at high temperature. It exhibited a sub-parabolic growth manner that was mainly controlled by grain boundary diffusion. However, a sufficiently thick IMC layer initially formed during reflow soldering and the low diffusivity of the Ni atoms retarded the growth. The shear strength of the solder joints was found to decrease severely with the current stressing time. This degradation was attributed to the large stresses arising from localized thermal mismatch, phase coarsening, volume shrinkage of IMC evolution, Ni–P layer crystallization and the pad cracking during current stressing.     

Electrochemical migration behaviour of Cu, Sn, Ag and Sn63/Pb37

The Electrochemical Migration (ECM) behaviour of leaded surface finishes was compared to other surface finishes that are applied in the electronics manufacturing. The studied surface finishes were as follows: bare copper (bCu), immersion tin (iSn), immersion silver (iAg) and Sn63Pb prepared by HASL (Hot Air Solder Leveling). The results were evaluated by Water drop test with the calculation of the Mean Time to Failure (MTTF) and by the investigation of the composition of the dendrites. The results have shown some contradictions relating bare copper and HASL compared to the ECM ranking published previously (Harsányi and Inzelt in Microelectron Reliab 41:229–237, 2001; Yu et al. in J Mater Sci: Mater Electron 17(3):229–241, 2006). The copper and the HASL can change their places in the ECM ranking depending on the technological circumstances of the investigations. This impact can be caused by the composition of lead alloys (eutectic or not), the solubility parameters of the metal hydroxides; the oxidation state of the copper surface, etc. Further and theoretical explanations and the necessary fine adjustment of the migration models are discussed in the paper.     

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.     

Diffusion and intermetallics formation between Pd/Ag metallization and Sn/Pb/Ag solder in surface mount solder joints

Interdiffusion and intermetallics formation between metallization conductor Pd–Ag and solder 62Sn-36Pb-2Ag have been studied. Silver and palladium dot mapping images of solder joints demonstrate that the Pd–Ag metallization layer gradually disappears after 11 days of ageing. It is observed that silver-rich areas exist in the bulk of the 62Sn-36Pb-2Ag solder after ageing but palladium-rich areas are not evident. The diffusion coefficient of the silver in the solder joints for this material system was measured. The activation energy and pre-exponential factor for the silver diffusion were found to be about 0.475 eV and 0.56×10^-10 m² s⁻¹, respectively for the configuration of surface mount thick film solder joints studied. X-ray diffraction results reveal the formation of the intermetallic compounds Ag₅Sn, Ag₃Sn, Pd₃Sn_2, Pd₂Sn_, PdSn₂, PdSn_4, PdSn_, PbPd₃, and Pb₃Pd₅.     

Critical studies to improve service reliability of Sn–Ag–Cu solder joints by thermal treatments

Lead-free electronic packages intended for use in applications such as aerospace, military, and other highly demanding service conditions, necessitate exceptional mechanical reliability of lead-free electronic solder joints under realistic service conditions. Most current design strategies employed for improving the reliability of lead-free electronic solder joints are aimed at developing suitable alloying additions and reinforcements to the solder itself. At present there exists no suitable methodology to minimize the effects of service conditions while the solder joint is in service. Since thermomechanical fatigue reliability of electronic solder joints is closely related to the crack nucleation that occurs during very early stages of repeated thermal excursions, this study is based on subjecting solder joints to a limited number of thermal shock (TS) cycles in a chosen temperature regime to nucleate cracks, then evaluating their effectiveness in improving reliability when the solder joints are subjected to additional TS cycles in a different temperature regime. This study is a preliminary investigation, aimed at developing suitable methodology to minimize the effects of damage to lead-free solder joint specimens subjected to repeated thermal excursions during service, by imposing appropriate thermal treatments. These thermal treatments can be automatically implemented at programmed intervals during the service life of the electronic packages. Methods employed in these studies may also be useful to enhance long-term service reliability and to obtain a conservative estimate of long-term service reliability.     

Experiments and simulations of uniaxial ratchetting deformation of Sn–3Ag–0.5Cu and Sn–37Pb solder alloys

This paper discusses uniaxial ratchetting deformation of lead-free solder alloy Sn–3Ag–0.5Cu and lead-containing solder alloy Sn–37Pb, which were subjected to tension–compression loading with several stress amplitudes and stress ratios, minimum stress over maximum stress. First the uniaxial ratchetting tests were conducted with three maximum stresses and four stress ratios. All tests were conducted using cylindrical bulk specimens of the solder alloys at 313 K. The test results show that there are differences in the ratchetting deformation behavior of the two solder alloys; the larger ratchetting strain occurs in the lead-containing solder alloy than in the lead-free solder alloy. The ratchetting deformation was simulated by the dislocation based constitutive model proposed by Estrin et al. (J Eng Mater Technol 118:441, 1996). The evolution equation of the back stress employed in the constitutive model was modified considering a dynamic recovery term. The effect of the modification of the back stress evolution is discussed by comparing the simulations with the corresponding experimental results. The simulations suggest that the recovery term in the kinematic hardening rule plays an important role in fitting the simulation to the experimental results of the ratchetting deformation of the solder alloys.     

Failure mechanisms of solder interconnects under current stressing in advanced electronic packages

The pursuit of greater performance in microelectronic devices has led to a shrinkage of bump size and a significant increase in electrical current. This has resulted in a high current density and accompanying Joule heating in solder interconnects, which places great challenges on the reliability of advanced electronic packages. A review of current stressing-induced failures of solder interconnects is thus timely. This review is devoted to five types of physical failure mechanisms occurring in high current density applications, which include electromigration (EM), Joule heating-induced failures, interfacial reactions, stress-related damage, and thermomigration (TM). In practice, some of these failure mechanisms are mixed together so that the real root cause cannot be easily detected and understood. Reliability designers need to be well informed to evaluate the electrical characteristics, thermal characteristics and mechanical strength for solder interconnects in advance. This review summarizes recent progress and presents a critical overview of the basis of atomic transport, diffusion kinetics, morphological evolution, and numerical simulation. Special emphasis is on the understanding of the interactions of EM with other failure mechanisms. Aside from the review of the current status of knowledge, the remaining challenges as well as future directions are also discussed.     

Correlation between displacement rate and shear force in shear test of Sn–Pb and lead free solder joints

Abstract

An experimental investigation was combined with a non-linear finite element analysis using an elastic–viscoplastic constitutive model to study the effect of ball shear speed on the shear forces of ball grid array (BGA) solder joints. Two solder compositions were examined in the present work: Sn–37Pb and Sn–3·5Ag. Within the Sn–3·5Ag solder, Ag₃Sn intermetallic compound particles were found. For both types of solder used, a Ni₃Sn₄ intermetallic compound layer was observed at the interface between the nickel plated layer and solder, while a gold layer appeared to have dissolved into the liquid solder leaving no observable gold at the interface. The shear force was observed to increase linearly with increasing shear speed and reached a maximum value at the highest shear speed in both the experimental and the computational results. All test specimens fractured in a ductile mode. The failure mechanisms are discussed in terms of von Mises stresses and plastic strain energy density distributions.