A study on the reliability of stencil printed solder bumps

The reliability of solder bumps formed using stencil printing and reflow was investigated for flip chip applications. Eutectic solder paste was printed through a metal mask and then reflowed repeatedly. The shear strength of the solder bumps and the process capability C_p were measured as a function of the number of solder reflows. Cu ₆Sn₅ was the only intermetallic compound observed in the interface after reflow. The thickness of the intermetallic compound rapidly increased with the number of reflow cycles, saturating after the third reflow. The intermetallic compound became scallop shaped after the third reflow, changing to a spheroid shape after the fifth reflow and was separated from the interface. The Cu was completely consumed by Cu-Sn reaction after the fifth reflow. The shear strength of the solder bumps increased with the number of reflow cycles until the fifth reflow, after which it decreased. The initial increase in shear strength is attributed to the reduction of pores in the solder bumps. The decrease of shear strength after 5 cycles of reflow is attributed to the direct contact between the solder and nonwettable metal