Cu In Solder Research Articles

Microstructure, shear properties and corrosion resistance of Cu/Cu-In/Cu solder joints

Transient liquid phase sintering can successfully fabricate reliable high-temperature solder joints at a relatively low joining temperature by forming intermetallic compounds. In this work, microstructure, shear properties and corrosion resistance of Cu/Cu-In/Cu solder joints prepared by TLPS process were investigated. The shear strength of the Cu-In solder joints significantly increased during the initial aging period and then slightly decreased due to the increase of pores after continuous 24 h aging. The corresponding microstructure is that part of the Cu2In in the Cu-In solder joints transforms to Cu7In3 in the meanwhile. After continuous aging, the interfacial microstructure remained stable and contributed to good shear strength that exceeds 20 MPa after 96 h aging at 250 °C and 350 °C, which was 20.8 MPa and 23.4 MPa, respectively. Regarding corrosion resistance, the Cu-In interfacial layer (−0.24 V) has similar corrosion resistance to Cu substrate (−0.22 V) and shows superior to the commonly used Sn63Pb37 solder (-0.40 V) and Sn95.5Ag3.8Cu0.7 solder (−0.42 V).

Simulation for Cu Atom Diffusion Leading to Fluctuations in Solder Properties and Cu6Sn5 Growth during Multiple Reflows

The multiple reflows process is widely used in 3D packaging in the field of electronic packaging. The growth behavior of interfacial intermetallic compound (IMC) is more important to the reliability of solder joints. In this paper, experimental measurement combined with simulation calculation were preformed to investigate the evolution of Cu concentration in solders during multiple reflows, as well as its effects on the growth behavior of IMC and solder properties. The concentration of Cu in solder fluctuated, increasing with the increase of reflow times, which led to the fluctuation in the growth rate of the IMC. Furthermore, the Vickers hardness and melting point of the solder fluctuated during the multiple reflow processes due to the fluctuation in the Cu concentration. The data generated during this study could help to develop machine learning tools in relation to the study of interfacial microstructure evolution during multiple reflows.

Effect of initial Cu concentration on the IMC size and grain aspect ratio in Sn–xCu solders during multiple reflows

The effect of initial Cu concentration in Sn–xCu solders, on grain size and morphology of interfacial intermetallic compound (IMC) during multiple reflow process, was investigated in this work. It was found that the initial Cu content in solders is closely related to the IMC nucleation and Cu migration behavior, which can govern the IMC crystal growth as well as grain aspect ratio evolution. In general, higher initial Cu in solder would lead to an increase in thickness of IMC, regardless of the reflow number. For Sn, Sn0.5Cu, Sn1.5Cu and Sn2.0Cu, the grain aspect ratios show decreased values with rising initial Cu percentage in solder; but for Sn0.7Cu, it has an even smaller aspect ratio than pure Sn. Moreover, Sn0.5Cu, Sn1.5Cu and Sn2.0Cu present a pronounced suppression of grain aspect ratio with reflow number, whereas this is not observed in context of pure Sn and Sn0.7Cu solders. Fine Cu6Sn5 nanoparticles on the surface of interfacial IMC grains in Sn0.7Cu alloy were observed in both isothermal heating and cooling, which may have served as the reason for its unique behavior of grain aspect ratio evolution. The relationship between Cu addition and IMC grain growth can be utilized in the reliability design of Pb-free solder materials.

Electromigration and Thermomigration in Pb-Free Flip-Chip Solder Joints

Pb-free solders have replaced Pb-containing SnPb solders in the electronic packaging industry due to environmental concerns. Both electromigration (EM) and thermomigration (TM) have serious reliability issues for fine-pitch Pb-free solder bumps in the flip-chip technology used in consumer electronic products. We review the unique features of EM and TM in flip-chip solder bumps, emphasizing the effects of current crowding and Joule heating. In addition, the challenges to a better understanding of EM and TM in Pb-free solders are discussed. For example, the anisotropic nature of Sn microstructure in Pb-free solders can enhance the dissolution rates of Ni and Cu in solders driven by EM and TM.

Effect of indium on the microstructure of the interface between Sn3.13Ag0.74CuIn solder and Cu substrate

Influence of indium in Sn3.13Ag0.74Cu solder containing 4, 15, 30, 50 and 75 at.% In on the microstructure at the solder/Cu interface after wetting at 523 K for 1800 s was studied. The scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX), standard and spatially resolved X-ray diffraction (XRD) techniques were used to determine the phases present at the solder/Cu interface. It was found that for In concentration up to 30 at.% the interface is formed by Cu 6Sn 5 phase. For higher In content (50 and 75 at.% In) interface consists of copper rich Cu 41Sn 11 phase.

Cross-interaction between Au and Cu in Au/Sn/Cu ternary diffusion couples

Both Au and Cu are so-called fast diffusers in Sn, and can diffuse very long distances in Sn in a relatively short time. In this study, the cross-interaction between Au and Cu across a layer of Sn was investigated through the use of the Au/Sn/Cu ternary diffusion couples. A 7-µm Au layer and a 100-µm Sn layer were electroplated over Cu foils to produce the Au/Sn/Cu diffusion couples. Aging at 200°C revealed that cross-interaction could occur in as short a time as 10 min. Evidence of this cross-interaction included the formation of (Cu1−xAux)6Sn5 on the Au side of the diffusion couples as well as on the Cu side. The reaction products on the Au side included the Au-Sn binary compounds. Between the Au-Sn compounds and the Sn was (Cu1−xAux)6Sn5. The reaction products on the Cu side initially was only (Cu1−xAux)6Sn5, but a layer of Aufree Cu3Sn eventually formed between (Cu1−xAux)6Sn5 and Cu. A detailed atomic flux analysis showed that the Cu flux through the Sn layer was about 2–3 times higher than the Au flux at any moment. The results of this study show that the cross-interaction of Au and Cu in solders is extremely rapid, and cannot be ignored in those solder joints that have both elements present.

Influence of Cu addition to interface microstructure between Sn-Ag solder and Au/Ni-6P plating

The formation and growth of intermetallics at the interface between Sn-Ag-(Cu) alloy balls and Au/Ni-6P plating were experimentally examined as a function of soldering period. Joint strengths were also evaluated by a ball pull test. For the joint with Sn-3.5Ag, the primary reaction product of Ni3Sn4 exhibits growth and shrinkage in thickness repeatedly with a passage of reaction time up to 30 min, while the Ni3SnP reaction layer monotonously increases its thickness without fluctuation. In the cases of the joints with Cu bearing solder, Sn-3Ag-0.5Cu and Sn-3.5Ag-0.8Cu, a single η-(Cu,Ni)6Sn5 interface layer grows by fast Cu segregation from liquid solder to the interface layer on soldering. For all the soldered joints, a P-rich layer appears at the surface region of a Ni-6P plating layer by Ni depletion to form those intermetallic compounds at interfaces. The growth rate of a P-rich layer for Sn-3.5Ag is faster by about 4–8 times than those of the Sn-Ag-Cu. The presence of Cu in solder enhances the formation of the Cu6Sn5 intermetallic layer at the interface resulting in prevention of Ni diffusion to liquid solder. For all the soldered joints, coarsened reaction interfaces decrease the joint strengths.