Effect of Processing Variables on the Mechanical Reliability of Copper Pillar SnAg Solder Joints

The effect of the variation of processing parameters such as thermal history, or the composition of Sn-Ag-Cu solder joints, on microstructure and reliability performance depends strongly on joint geometry, in particular length scale. The advent of 2.5/3D packaging technologies in microelectronics has further decreased joint length scales and changed interconnect aspect ratios to reduce I/O pitch. While fifty to one-hundred-micron diameter joints are becoming more common, the diameter of some SnAg based solder joints has been reduced to as little as ten microns, affecting the solidification microstructure and the formation of intermetallic compounds at solder/metallization interfaces. This study investigates these effects and correlates them with shear strength and reliability performance in accelerated thermal cycle test. Comparisons with results from solder joint studies at much larger length scales (ball grid array) are reported. Copper pillars having Sn caps with controlled variations of Ag content, height, and diameter were joined to silicon interposer substrates with Cu based metallizations. The microstructures of such Cu pillar to interposer solder joints, as well as those of reflowed (but unjoined) copper pillar structures were examined using optical and electron microscopy. The size and number density of Ag3Sn precipitates were quantified for a number of different processing conditions. The shear strength of solder caps as-reflowed on Cu pillars was measured. Accelerated thermal cycling test was the primary means of reliability testing for the interposer joint structures.