SAT-200 The Effect of the Roughness of Cu Surface on the Grain Growth of Lead-Free Solder

Saturday, October 13, 2012: 7:00 PM
Hall 4E/F (WSCC)
Kenny Lo , Material Science Engineering, University of California, Berkeley, Berkeley, CA
John Morris, ScD , Material Science Engineering, University of California, Berkeley, Berkeley, CA
Environmentally friendly, lead-free solders are widely used for microchips and electronic

devices. The efficiency of an electronic device is highly dependent on the solder: the failure rate

of lead-free solders has been found to correlate to its orientation relative to current flow. Leadfree

solders, such as the tin-based solder SnAg-Cu (SAC 305), have a body-centered tetragonal

(BCT) structure with the c-axis being the shortest. When the c-axis is oriented parallel to the

current flow, Cu-Sn intermetallics grow faster in the solder joint, leading to failure. The

reliability of the device can thus be increased by controlling the crystallinity and grain

orientation of the soldered joints. This project studies the effects of copper surfaces on the

crystallinity and orientation of SAC solder joints. The SAC solder was mounted between Cu

pads with surface finishes ranging from 78 μm to 7 μm. The orientations of the Sn and Cu were

examined using electron backscatter diffraction analysis (EBSD). This study has concluded that

increased roughness of the copper surface refines the grain size and strengthens the SAC solders.

No clear correlation was found between the orientation of the Sn to the Cu surface. Additional

studies are being conducted to investigate whether a relationship between orientation and

crystallization of Sn on Cu surfaces can be established.