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| Investigate influence of solder thickness |
Abstract
solder joint thickness has an important effect on the fatigue failure behavior of solder joint. In this study, fatigue behavior of solder joints with different solder thickness (0.1mm, 0.2mm, 0.3mm) were studied under 398K. The failure path and rupture morphology were observed utilizing scanning electron microscopy (SEM) in order to reveal the failure mechanism.
Moreover, fatigue life was evaluated using the Coffin-Masson model and the influence of solder thickness on the fatigue exponent and coefficient was analyzed. The result shows that fatigue life of solder joint increases with the solder thickness.
Solder joins of lowest thickness has the fast maximum load drop rate. Besides, the fatigue ductility exponent in Coffin-Masson model decrease with the solder thickness, so, the higher solder thickness means super fatigue resistance. On the other hand, fatigue crack is initiated at the corner of the solder joint, and propagated in the solder matrix with a direction paralleling to the loading direction.
A deformation concentrated zone along the diagonal of solder joint is formed with increasing the solder thickness, inducing a step-like fracture morphology and all samples show a ductility rupture mode.
Keywords : solder joint, thickness, fatigue failure, fatigue life, high temperature
INTRODUCTION
Minimization and portability are the development trend for manufacture of electronic product. Correspondingly, the size of solder joint in the electronic component becomes smaller and smaller, unfortunately, this will induce more seriously reliability problem.
Generally, electronic components during service will experience thermal cycles and inevitably induce solder joint deformation due to the mismatch of coefficient of thermal expansion between different parts. During the deformation of the solder joint, an inhomogeneous three dimensional stress state will develop owing to the presence of the elastic substrates that prevent lateral shrinkage of solder near the interface, and this introduce a significant apparent hardening, which is usually called constraining effect.
Therefore, solder joint size has an important influence on the solder joint property. Many scholars have studied the effect of solder joint size on the property of solder joint using experimental or finite element simulation method. Amalu investigated the influence of stand-off thickness on the fatigue life of solder joint utilizing simulation method and indicated that the fatigue life of the assemblies decrease exponentially as the component stand-off thickness decreases in a cubic function, a similar result was found by Ekpu. Cugnoni investigated the stress- strain relation of solder joints with different gap width and reported that the ultimate stress of thinner solder joint improved 35% compared to the thick joint due solely to the plastic constraints.
Zimprich conducted a series of tests to study the constrain effect on lead free solder joints and concluded that the tensile strength of solder joint increased with decreasing gap size, however, the ductility has the opposite trend. In addition, the stress relaxation is more easily occur in solder joint with a larger size, therefore, residual stress may be higher for a thin solder joint and more damage was accumulated.
Generally, the ultimate tensile strength of solder joint seems to always increase with decreasing the gap size. However, Yin obtained a non-consistent result after performed tensile tests to solder joints with different dimensions and found that the joint’s strength increases with decreasing joint volume.
From the researches described above, the present studies about size effect on solder joint are mainly focus on the tensile property and the research on fatigue behavior mainly utilize the simulation method. Therefore, in this particle, single lap- shear solder joints were fabricated and fatigue tests were conducted in order to investigate the effect of solder thickness (0.1mm, 0.2mm, 0.3mm) on fatigue behavior under 398K. After fatigue test, the failure path and fracture morphology were observed using SEM in order to analyze the failure mechanism. Moreover, the fatigue life was also evaluated.
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