Low-cycle fatigue characteristics of Sn-based solder joints |
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Authors: | K O Lee Jin Yu T S Park S B Lee |
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Affiliation: | (1) Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 305-701 Daejon, South Korea;(2) Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, South Korea |
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Abstract: | Low-cycle, lap-shear fatigue behavior of Sn-based, Pb-free solder alloys, Sn-3.5Ag, Sn-3.5Ag-Cu, Sn-3.5Ag-Bi, and Sn-0.7Cu,
were studied at room temperature using specimens with printed circuit board (PCB)/solder/PCB structure under total displacement
of ±10 μm, 12 μm, 15 μm, and 20 μm. The fatigue lives of various solder joint materials, defined as 50% load drop, were correlated
with the fracture paths and analyzed using the Coffin-Manson relation, Morrow’s plastic-energy dissipation model, and Solomon’s
load-drop parameter. The Sn-3.5Ag, Sn-0.7Cu eutectics, and Sn-3.5Ag-Cu ternary alloys showed the same level of fatigue resistance,
while Bi-containing alloys showed substantially worse fatigue properties. Cross-sectional fractography revealed cracks initiated
at the solder wedge near the solder mask and subsequently propagated into the solder matrix in the former group of alloys,
in contrast with the crack propagation along the solder/under bump metallurgy (UBM) interfaces in the Sn-3.5Ag-Bi alloys.
Inferior fatigue resistance of Bi-containing alloys was ascribed to high matrix hardness, high stiffness, possible Bi segregation
to the interface, and high residual stress in the interfacial area. |
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Keywords: | Low-cycle fatigue lead-free solder Coffin-Manson relationship Morrow energy mode Solomon parameter |
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