Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

• Published in: Journal of electronic materials 2001-10, Vol.30 (10), p.1323-1331
• Main Authors: SHIN, C. K, BAIK, Y.-J, HUH, J. Y
• Format: Article
• Language: English
• Subjects: Applied sciences; Electronic equipment and fabrication. Passive components, printed wiring boards, connectics; Electronics; Exact sciences and technology
• ISSN: 0361-5235; 1543-186X
• DOI: 10.1007/s11664-001-0119-7

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 deg C and aging at 150 deg C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMG 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 Cu sub 6 Sn sub 5 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 mu m for all solders. The bulk solder strength decreased rapidly as the fine Cu sub 6 Sn sub 5 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 deg 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 solder joints, especially during system use at high temperatures.