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In situ tensile tests on Sn-3.5Ag lead-free solder were performed with different rates of 10 μm/s and 300 μm/s at − 198 °C liquid nitrogen temperature. Microstructure evolution and deformation behavior of Sn-3.5Ag solder were observed by optical metallographic microscopy. The morphological influence of the second-phase particles Ag3Sn on fracture behaviors was investigated by scanning electron microscopy. The results showed that plastic deformation occurred at the low tensile rate, while no fracture toughness was observed at the high tensile rate. Several fracture modes were observed, including intergranular fracture and transgranular fracture. The morphology and distribution of the second-phase particles had a significant impact on the fracture behavior of solder. The long rod-shaped second-phase particles perpendicular to the tensile direction were difficult to fracture, while the parallel second-phase particles were broken into segments. The smaller second-phase particles barely deformed, and the voids nearby provided a favorable pathway for fracture. [ABSTRACT FROM AUTHOR]

The effects of temperature and ultrasonic-assisted time on the microstructures and mechanical properties of the electroless nickel-immersion gold (ENIG)/Sn-3.5Ag/ENIG sandwich structure in ultrasonic chip bonding were investigated. Ultrasonic-assisted solder bonding was performed for 0.5–4 s at 25, 150, and 250 °C. The bonding was successful under all conditions. At 25 °C, Au–Sn intermetallic compounds (IMCs) were formed at the upper interface, and AuSn4 was formed on Ni–Sn IMCs at the lower interface. As the bonding time increased, spalling of AuSn4 was increasingly observed at the upper interface. At 150 °C, a trend similar to that at 25 °C was observed, but as the time increased, the thickness of the joint decreased, Au–Sn IMCs at the upper interface disappeared, and thin Ni–Sn IMCs and Cu6Sn5 were formed. The results observed at 250 °C differed from those observed at 25 and 150 °C because ultrasonic-assisted solder bonding was performed with the solder in a molten state. Ni–Sn IMCs were formed at the upper and lower interfaces, residual Sn and Au–Sn IMCs were present inside, and the thickness of the joint was very small. At 250 °C, the shear strength was high regardless of the bonding time. Observations of the top views and cross-sections of the fracture surfaces showed that, at 25 and 150 °C, the fractures initially occurred at the lower interface but moved to the upper interface as the bonding time increased. At 250 °C, the fractures did not significantly change, regardless of the ultrasonic-assisted time.

A method of decreasing the ductile–brittle transition temperature (DBTT) of Sn-3.5Ag solder by aging treatment has been found to allow the solder to serve at lower temperatures. Charpy impact tests under various temperatures (from −150°C to 20°C) were performed on Sn-3.5Ag solder after 150°C aging treatment, and the morphology of second-phase particles was also acquired, compared with the in situ aging test to analyze the influence of second-phase particles. The results show that with the increase of aging time, the DBTT of the impact curve first decreases and then increases, and the DBTT of impact curve is the lowest at 48 h. Combined with the in situ aging morphology of second-phase particles, precipitation, fusion, and coarsening of Ag3Sn particles are the reason for the change in DBTT. In summary, aging treatment can decrease the DBTT value of Sn-3.5Ag solder, so that the solder can have a greater toughness area. That is, the solder after aging treatment will be able to achieve service at lower temperatures. [ABSTRACT FROM AUTHOR]

The anomalous precipitation behavior of Ag3Sn phase in Sn-3.5Ag lead-free solder joints during liquid–solid electromigration at 1.0 × 104 A/cm2 has been observed using a real-time imaging method with synchrotron radiation. Large Ag3Sn phases precipitated at the anode instead of cathode side in the Ag-containing solder joints during cooling, due to directional diffusion of Ag atoms towards the anode under current stressing. The electromigration-induced directional diffusion behavior of Ag atoms can mainly be attributed to their negative effective charge number (ZAg*) of −0.48. Electromigration induced a localized Ag-rich area and thereby a large number of Ag3Sn-based clusters at the anode. This resulted in preferential nucleation and anomalous precipitation of Ag3Sn phases at the anode side in Ag-containing solder joints. The rapid growth of the Ag3Sn plates can be explained in terms of the net atomic flux driven by the electromigration (Jem) and chemical potential (Jchem) effects. [ABSTRACT FROM AUTHOR]

The mechanical reliability of Sn-3.5 wt.%Ag solder joints with four different electroless Ni plating layers [Ni-1B, Ni-3B, Ni-7P, and Ni-10P (in wt.%)] was investigated as a function of aging time up to 60 days at 150 °C. The ultimate shear stresses for fracture were higher in the ball shear tests when using Ni-B samples than those with Ni-P metallization if the aging treatment at 150 °C was shorter than 15 days, and vice versa when the aging time was higher than 45 days. In all the joints, Ni3Sn4 intermetallic compounds (IMCs) were formed at the interfaces. The thickness of the IMC layer increased with decreasing B or P content, i.e., increasing Ni content. The reaction rate between the Sn-Ag solder and Ni-P was slower than that between the Sn-Ag solder and Ni-B. In the shear test, the failure mode switched from a bulk-related failure (ductile fracture) to an interfacerelated failure (brittle fracture), depending on the aging time. After prolonged aging treatment, weak solder/Ni3Sn4 interfaces led to a failure mode of brittle fracture for all the solder joints, due to the formation of thick Ni3Sn4 IMCs. The failure for the Sn-Ag/Ni-B joints was more abrupt and brittle due to the formation of the thick, interfacial Ni3Sn4 IMC. The results demonstrated that the Sn-Ag/Ni-P joint was more reliable than the Sn-Ag/Ni-B joint from the viewpoints of interfacial IMC thickness and long-term mechanical reliability. [ABSTRACT FROM AUTHOR]

An investigation has been carried out to compare the dissolution kinetics of the Cu pad of the ball grid array (BGA) substrate into the molten conventional Sn–Pb solder and Sn–3.5Ag solder. A fixed volume of the BGA solder ball (760 μm diameter) was used on the 15–18 μm thick Cu pad having a circular area with a diameter of 650 μm. The dissolution measurement was carried out by measuring the change of Cu pad thickness as a function of time and temperature. A scanning electron microscope (SEM) was used to observe the microstructure of the solder joint and to measure the consumed thickness of Cu. A fast dissolution of the substrate occurred in the beginning for the molten solder/solid Cu reaction couple. But the dissolution in Sn–Ag was much more higher than that in Sn–Pb solder. The rates of Cu dissolution were measured for different soldering temperatures ranging from 225 to 240 °C and activation energies of 54 and 116 kJ/mol were found for the dissolution reaction in Sn–Ag and Sn–Pb solder, respectively. [Copyright &y& Elsevier]

The growth kinetics of intermetallic compound layers formed between Sn-3.5Ag solder and Cu substrate were investigated as a consequence of solid-state isothermal aging. Isothermal aging was carried out in a temperature range between 70°C and 200°C for 0 to 60 days. A quantitative analysis of the intermetallic compound layer thickness as a function of time and temperature was performed. The diffusion couples showed a composite intermetallic layer comprised of Cu6Sn5 and Cu3Sn. The growth of intermetallic compounds followed diffusion-controlled kinetics and the layer thickness reached only 9 μm after 60 day of aging at 150°C. The apparent activation energies were calculated for the growth of the total intermetallic compound (Cu6Sn5+Cu3Sn); Cu6Sn5 and Cu3Sn intermetallic are 65.4, 55.4 and 75.7 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]

Ball-grid array (BGA) samples were aged at 155°C up to 45 days. The formation and the growth of the intermetallic phases at the solder joints were investigated. The alloy compositions of solder balls included Sn-3.5Ag-0.7Cu, Sn-1.0Ag-0.7Cu, and 63Sn-37Pb. The solder-ball pads were a copper substrate with an Au/Ni surface finish. Microstructural analysis was carried out by electron microprobe. The results show that a ternary phase, (Au,Ni)Sn4, formed with Ni3Sn4 in the 63Sn-37Pb solder alloy and that a quaternary intermetallic phase, (Au,Ni)2Cu3Sn5, formed in the Sn-Ag-Cu solder alloys. The formation mechanism of intermetallic phases was associated with the driving force for Au and Cu atoms to migrate toward the interface during aging. [ABSTRACT FROM AUTHOR]

A variety of Pb-free solders and under bump metallurgies (UBMs) was investigated for flip chip packaging applications. The result shows that the Sn-0.7Cu eutectic alloy has the best fatigue life and it possess the most desirable failure mechanism in both thermal and isothermal mechanical tests regardless of UBM type. Although the electroless Ni-P UBM has a much slower reaction rate with solders than the Cu UBM, room temperature mechanical fatigue is worse than on the Cu UBM when coupled with either Sn-3.8Ag-0.7Cu or Sn-3.5Ag solder. The Sn-37Pb solder consumes less Cu UBM than all other Pb-free solders during reflow. However, Sn-37Pb consumes more Cu after solid state annealing. Studies on aging, tensile, and shear mechanical properties show that the Sn-0.7Cu alloy is the most favorable Pb-free solder for flip chip applications. When coupled with underfill encapsulation in a direct chip attach (DCA) test device, the Sn-0.7Cu bump with Cu UBM exhibits a characteristic life or 5322 cycles under -55°C/+150°C air-to-air thermal cycling condition. [ABSTRACT FROM PUBLISHER]

Power diodes in an alternator convert alternating current, generated by the spinning magnetic field, to direct current to be used by the battery and all the automotive electrical/electronic components. The diodes are press-fit into aluminum heatsinks to quickly and efficiently dissipate the heat from the silicon dies in the diode body. The diodes are soldered to a rectifier circuit board through the diode leads by a wave soldering process using a Pb-free, eutectic Sn-3.5Ag solder. A set of positive diodes reside on a different substrate than the set of negative diodes, resulting in differences in the lengths of the diode leads. The distance from the diode body to the solder joint on the leads of the positive diodes is 7 mm less than those of the negative diodes. Solderability, cross-section micrographs, and thermal-cycling fatigue reliability studies were compared between the positive and negative diodes and between diode designs from different suppliers. Wetting balance testing showed significant differences in solderability between positive and negative diodes and between the two different diode designs. Combining the diode body and lead together had a more drastic effect on the solderability than the lead alone. It was discovered that, although the nature of the diode design is to dissipate the heat away from the diode quickly and efficiently, there is a large temperature gradient along the lead immediately above the solder bath which can be as much as 100°C just 2 mm from the bath. This large temperature gradient caused some leads to be too “cold” to form good solder fillets. The solder fillets obtained in the laboratory wetting tests matched those observed in the actual alternators. The inadequate solder fillets resulted in a 250% difference in the thermal cycling fatigue reliability between the two diode designs. [ABSTRACT FROM AUTHOR]