Microstructure and Performance of Micro CU Pillars Assemblies

Microstructure and Performance of Micro CU Pillars Assemblies
Correlations between the shear strength and microstructure of Cu pillars were examined for different solder compositions, and for different aging times.
Analysis Lab


Authored By:

Mohammed Genanu, Eric J.Cotts, Ph.D.
Physics and Materials Science Binghamton University, Binghamton, NY

Francis Mutuku
Universal Instruments Corporation, Conklin, NY

Eric Perfecto
IBM Corporation, Fishkill, NY, USA (now GlobalFoundries)

Scott Pollard, Aric Shorey
Corning Inc. Corning, NY, USA

Babak Arfaei, Ph.D.
Ford Motor Company, Ann Arbor, MI, USA


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, such as Si or 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 compositions, and for different aging times. Microstructure analysis (e.g. 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 bumps were examined. The shear strength performance of micro Cu pillars with three different bump diameters (30μm, 50μm, 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. ATC test results for two different interposers (Si and Glass with High CTE) will be discussed.


In the current study, relations between processing, microstructure and reliability of assemblies enabled through Cu pillar/interposer technology were examined. Both Si/micro Cu pillar/solder cap/glass and Si/micro Cu pillar/solder cap/Si assemblies with a large number of I/Os were examined in a configuration that allowed monitoring of electrical continuity during test. Significant variation in Ag3Sn precipitate morphology was observed under nominally identical fabrication conditions. These were correlated with relatively large variations in mechanical behavior, for instance in measured values of shear strength. Large variations in Ag3Sn precipitate size and number were also observed with changes in composition and upon aging, as would be expected. Cu pillar assemblies revealed small, but continuous solder layers. After failure during ATC, cracks were found to have propagated through these continuous solder layers. Even though the Si to Si joint have no CTE mis-match there were still solder fails and so underfill is recommended for all structures, both Si-Si and Si-glass will benefit.

Initially Published in the SMTA Proceedings


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