Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints |
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Authors: | C K Shin Y J Baik J Y Huh |
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Affiliation: | (1) Division of Materials Science and Engineering, Korea University, 136-701 Seoul, Korea;(2) Thin Film Research Center, Korea Institute of Science and Technology, 130-650 Seoul, Korea |
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Abstract: | The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5,
and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic
(IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth
and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from
the pad during reflow soldering resulted in a fine dispersion of Cu6Sn5 particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation
hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to
shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders.
The bulk solder strength decreased rapidly as the fine Cu6Sn5 precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders,
the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged
joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect
on the shear strength of BGA solderjoints, especially during system use at high temperatures. |
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Keywords: | BGA solder joint ball shear strength Sn-Cu solder intermetallic compound fracture mode |
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