Microstructure and Performance of Micro Cu Pillars Solder Caps and Assemblies
Authors: Mohammed Genanu, Babak Arfaei, Eric Cotts, Francis Mutuku, Eric Perfecto, Scott Pollard, Aric Shorey Company: Binghamton University, Universal Instruments Corporation, IBM Corporation, and Corning Inc. Date Published: 10/1/2017
Abstract: The desire for smaller, lighter and faster products drives the development of 2.5D/3D integration technologies that can utilize tens of thousands of connections per die. Micro copper (Cu) pillar geometries have been widely adopted because their small size and fine pitch provides high thermal conductivity, higher input/output (I/O) density and resistance to deleterious electromigration effects. In micro Cu-pillars, SnAg solder is electroplated on top of a Cu pillar. Because of the small volume of solder employed, intermetallic compounds comprise a significant fraction of the resulting solder joint, and very fine Ag3Sn precipitate morphologies can occur. Thus, the microstructure of SnAg solder/Cu pillar microstructures varies significantly from that of larger solder joints such as flip chip solder joints. Furthermore, 2.5D applications include interposers of distinctly different materials (Si and High CTE glass). The different properties of these materials such as coefficient of thermal expansion, affect the thermomechanical response of the package to temperature excursions and the lifetime of the package. Thus, behaviors of Cu pillar packages during Accelerated Thermal Cycling (ATC) were examined, Correlations between the shear strength and microstructure of Cu pillars were examined for different solder cap compositions, and for different aging times. Microstructure analysis (Ag3Sn precipitate morphology) was performed with both optical and scanning electron microscopy. The effects of thermal aging on the growth of intermetallic compounds (IMCs), the Ag3Sn precipitate morphology and on the mechanical properties of micro Cu pillar solder caps were examined. The shear strength performance of micro Cu pillars with two different caps diameters (30µm, and 100µm) was also evaluated. Results were considered in terms of variations in the precipitate morphology, and in terms of increases in the thicknesses of intermetallic layers at micro solder/ substrate interfaces. The results show a dramatic decrease in the number of Ag3Sn precipitates in the solder cap after aging for 1000h and 2000h at 125°C. For both solder caps diameters, the average IMC thickness for as received samples are 2.1 µm after aging the thickness increases gradually till reach 4.5 µm after 2000h of aging. This changing in the microstructure results in a decrease in the shear strength measurements. ATC test results for two different interposers (Si and Glass with High CTE) have Ni surface finish and diameter 50 µm, our results indicate that due to the high thermal mismatch between the Si/ High CTE glass interposer assemblies failed faster than Si/Si interposer assemblies.
Cu pillars, Lead-Free Solder Alloys, 2.5D Packaging, Interposers, Shear strength