Influence of the Substrate on the Creep of SN Solder Joints

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TRODUCTION

CONCERN over the toxicity of lead has led to the introduction of Pb-free solders in microelectronic devices. The most widely used are high-Sn alloys with small additions of Ag or Cu. The properties of the Sn matrix ordinarily dominate the mechanical behavior of these high-Sn alloys. Since Sn has a complex crystal structure and anisotropic mechanical properties that have not been thoroughly explored, the reliability of Snrich solder joints is of some concern. The creep behavior of high-Sn joints is particularly important, since the low melting temperatures of solder alloys have the consequence that creep is the dominant deformation mechanism, even at room temperature.[1] Given the technological need, there is an increasing body of published research on the creep of high-Sn alloys.[2–7] However, most of this work describes the creep of the alloys in bulk form. Solders in microelectronic joints may have microstructures and, hence, mechanical properties that differ significantly from those found in bulk samples.[8] The manufacture of fine-pitch solder joints ordinarily includes processes such as rapid solidification and reactions between the solder and the substrate that may create compositions and textures that are very different from those encountered in bulk samples.[9,10] K.-O. LEE, formerly Ph.D. Candidate, with the Department of Materials Science and Engineering, University of California, is Packaging Engineer, Substrate Pathfinding, Assembly and Test Technology Development, Intel Corporation, Chandler, AZ 85226. Contact e-mail: [email protected] J.W. MORRIS, Jr., Professor of Metallurgy, is with the Department of Materials Science and Engineering, University of California, Berkeley, CA 94720. F. HUA, Senior Packaging Engineer, is with Components Research, Intel Corporation, Sunnyvale, CA 95054. Manuscript submitted June 24, 2007. Article published online April 21, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A

Solder joints also contain interfacial intermetallic layers that may influence creep behavior. The present work was undertaken to study the influence of the substrate metallization on the creep of Sn-rich solder joints. To simplify the interpretation of the results, we used pure Sn as the solder rather than conventional ‘‘Pb-free’’ solders, which include small additions of Ag and Cu. The substrate pairs were CuCu, Cu-Ni, and Ni-Ni, all of which are commonly found in microelectronic devices.

II.

EXPERIMENTAL PROCEDURE

The geometry of the creep specimens used in this study is shown in Figure 1. Flat OFHC Cu plates were overlapped into a single-lap shear configuration and joined by a 3 9 3 pattern of 9 solder joints. The solder joints were located by making small, raised lands on the surfaces of the Cu plates. These were either used as-made for Cu substrates or plated with ~4-lm electroless Ni coated with 0.13-lm Au to form the Ni substrates. The solder was made in the form of pure Sn foils that were melted, cast in Cu blocks and homogenized for 48 hours at 443 K (170 C) before being rolled to 180 l

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