This paper studies the solder joints of an Sn–Ag lead-free solder system with pure copper wires. The study focuses upon the interrelationships, which exist between the adhesive strength of the joint, its shear strength, the formation of interfacial intermetallic compounds (IMC) and the fractographic morphology. Additionally, the paper determines how these characteristics, and the relationships between them, are influenced by the storage duration and the storage temperature. Experimental results show that both the adhesive strength and the shear strength of the solder joints decrease significantly following short-term thermal storage. As the storage time is increased, it is noted that both the thickness and the roughness of the interfacial IMC layers increase. Regarding the fracture of the solder joints, fractographic observation reveals that fracture morphology under adhesive loading are similar to those observed under shear loading conditions. In the as-soldered condition, the fracture surface appears to be flat, and some broken Cu6Sn5 and residual solder pieces are evident. When the total thickness of the IMC layer lies within the range 1–10 μm, it is observed that the fracture morphology gradually becomes a dimple-like structure. This phenomenon may be attributed to the residual stresses caused by phase transformation, and by the increasing roughness of the IMC layers which causes an increase in the stress concentration within the Cu6Sn5 layer, and which ultimately results in fracturing of this layer. When the total thickness of the interfacial IMC layers exceeds 10 μm, the roughness of the IMC layers and the residual stress between them and the solder both continue to increase. Eventually this results in a fracture being initiated and propagated within the Cu6Sn5 layer. Fractographic observation shows the fracture to have a cleavage-like morphology.

Influence of interfacial intermetallic compound on fracture behavior of solder joints

https://www.eng.buffalo.edu/Research/packaging/publication/Paper2001-2002/Basaran2002b.pdf

Measuring intrinsic elastic modulus of Pb/Sn solder alloys

Mechanical degradation of microelectronics solder joints under current stressing

Mechanics of Pb40/Sn60 near-eutectic solder alloys subjected to vibrations

Due to the ongoing miniaturization in modern microelectronics reliability and quality control of microelectronics devices will also depend on a detailed understanding of the complex mechanical and thermal of solder joints. Therefore the question of the occurrence of size effects or dimensionally induced constraints, which could change the mechanical properties of solder joints in small dimensions dramatically, came into focus of investigation. Tensile tests were performed to investigate the influence of joint size on the tensile strength and fracture strain. Strains across the solder joint were measured using a non-contacting laser speckle sensor. Scanning electron microscopy (SEM) was used to analyze the complex modes of fracture and crack propagation in the solder interconnect. The variation of the gap size influenced also the crack growth behavior. The observed behavior can be interpreted in terms of an existing theory for brazed joints to complement Finite Element Analysis that is usually used for a description of these phenomena.

Size effects in small scaled lead-free solder joints

The data obtained from bulk test specimens are often used in deformation models to predict the mechanical responses of parts that are very different in size and shape, such as solder joints in VLSI surface mount components. Such predictions are valid only if the size of the solder joints exceeds the representative deformation volume of the bulk specimens. In the present study, the measurement of the representative volume was performed for a single-phase solder alloy (98Pb-2Sn wt.%) deformed in both tension and cyclic loading. Deformation of 98Pb-2Sn alloy at room temperature was shown to be extremely inhomogeneous in both tensile and cyclic loading, and strain localization was observed at the grain boundaries. Strain distribution is more inhomogeneous in the specimen tested in fatigue than the specimen tested in uniaxial tension. This may be due to fatigue inhomogeneity in strain distribution or a small degree of buckling. The number of grains required for a representative volume of the 98Pb-2Sn alloy specimens tested is quite large. >

N.R. Bonda

Papers

Effect of the specimen size in predicting the mechanical properties of PbSn solder alloys