SMTA International Conference Proceedings

Effects Of Materials, Design And Process On Intermetallic Formation And Evolution In Solder Microbumps

Authors: K. Schnabl, L. Wentlent, K. Mootoo, E. Perfecto, M. Lu, and P. Borgesen
Company: Binghamton University, IBM Microelectronics, and IBM Research
Date Published: 10/13/2013   Conference: SMTA International

Abstract: Stacking of die on top of each other and/or assembly onto Si or glass interposers usually requires very small contact pad diameters, often on a very fine pitch. Sn based microbumps may seem to offer process compatibility with conventional high speed assembly technologies, but solder volumes are orders of magnitude smaller than what we have experience with even in regular flip chip assembly. The resulting solder microstructure and properties may thus be very different from what we are used to. The intermetallic (IMC) bonds now become a large fraction of the overall joint. Partially contradictory reports in the literature suggest that both initial IMC thicknesses and subsequent growth in solid state aging may depend strongly on the initial solder thickness and Ag content, as well as on the thermocompression/reflow process.

We report results of an ongoing systematic study of the evolution of the intermetallic bond layer and the solder microstructure and resulting properties in microbump based assemblies during reflow or thermocompression bonding and subsequent aging. Pure Sn as well as alloys with two different initial concentrations of Ag were considered. Thicknesses down to 5.5µm were deposited and reflowed for different lengths of time on Cu and Ni pads of different thicknesses and diameters ranging from 153?m to 11?m. Systematic trends are emerging: Initial thicknesses and the subsequent growth of individual IMC layers in aging vary in non-trivial manners with the combination of pad finish, reflow parameters, solder thickness, and solder composition. Different thicknesses and structures were achieved than in SMT assembly. The coupled growth of Cu3Sn and Cu6Sn5 on Cu pads is more complex than the growth of Ni3Sn4 on Ni, but thinner solder generally leads to faster IMC growth.

Key Words: 

Pb-free, microbump solder, Intermetallic (IMC) growth, Effect of Ag, Kirkendall voiding

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