Materials Week '97: Tuesday AM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Tuesday morning, September 16.

ALLOY DESIGN AND SOLDERING TECHNOLOGIES FOR LEAD-FREE AND LEAD-BEARING SOLDERS: Session III

Session Chair: D.R. Frear, Sandia National Laboratories, Albuquerque, NM 87185

RIPENING-LIMITED INTERMETALLIC COMPOUND FORMATION IN SOLDERING REACTIONS: K.N. Tu, Dept. of Materials Science & Engineering, UCLA, Los Angeles, CA 90095; H.K. Kim, Samsung Corporation, Seoul, Korea; P.A. Totta, IBM East Fishkill Facility, Hopewell Junction, NY 12533

In interfacial reactions, not all intermetallic compounds grow in layers as it is assumed in the diffusion-limited or interfacial-reaction-limited mode of growth. During chip-joint in electronic packaging, the compound formed between molten SnPb solder and the Cu thin film on a Si chip is not layer-like, rather it is scallop-like. The growth of these scallops is limited by ripening. The ripening can lead to spalling of the compounds from the chip surface; it weakens the solder joint and is a serious device reliability issue. In this talk, the ripening morphology and kinetics will be presented.

8:55 am INVITED

MECHANICAL BEHAVIOR OF SOLDER MATERIALS IN FIBER-OPTICS STRUCTURES: E. Suhir, Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974

Mechanical behavior of solder materials and joints, used in fiber-optics engineering, is discussed, as well as some general considerations underlying the reliability requirements for such materials and joints. Several analytical stress models for the evaluation of thermal and mechanical stresses in optical fibers soldered into ferrules are suggested: 1) evaluation of the thermally induced stresses in an optical glass fiber soldered into a ferrule; 2) evaluation of the "global" and "local" thermal mismatch stresses in a fiber whose ends are soldered onto a ferrule (capillary); 3) evaluation of the "mechanical" interfacial shearing stress in a fiber soldered into a ferrule and subjected to tension; 4) predicted curvatures and stresses in a fiber-optic interconnect subjected to bending; 5) evaluation of the role of the bending stress in, and the appropriate length of, a glass fiber specimen soldered into a ferrule and subjected to a pull-test force; and 6) predicted "mechanical" and thermal stresses in, and optimization of the configuration of, an optical fiber interconnect in a laser package design. The developed analytical stress models, calculation procedures, and recommendations can be helpful in the materials selection, and in the analysis and design of solder joints in fiber optics.

9:20 am INVITED

PROPERTIES OF SEVERAL DIE ATTACH SOLDERS AS A FUNCTION OF TEMPERATURE: M.W. Weiser, Johnson Matthey Electronics, Spokane, WA 99216

The coefficient of thermal expansion, elastic, and shear properties of several solder alloys that are used for die attach in power devices were measured as a function of temperature ranging from as low as -65C to just below melting point. The solders included high Pb alloys like Pb₅Sn, Pb₅In_2.5Ag, and Pb_2.5Ag₂Sn; semi-hard alloys like Sn₂₅Ag₁₀Sb (alloy J) and Sn_8.5Sb; and a some lower melting alloys such as Sn_3.5Ag and Sn₃₇Pb for comparison to literature data. The elastic properties were evaluated by measuring the longitudinal and shear wave velocities in the ultrasonic region. The shear properties were evaluated in a TO-247 package with a bare Cu lead frame where the Si die was replaced with a Ni plated Mo tab.

9:45 am INVITED

STUDY OF DIE BOND RELIABILITY IN MICROELECTRONICS POWER DEVICES USING SURFACE ANALYSIS TOOLS: A. Scandurra, Laboratorio Superficied Interfaci-Consorzio Catania Ricerche, c/o SGS Thomson Microelectronics, 95100 Catania, Italy

Die bond reliability is one of the most important quality parameter in microelectronics power devices. A variety of phenomena occurring during thermomechanical fatigue in Pb-Sn2-Ag2.5 soft die bond in microelectronics power devices are discussed. In addition reliability of the die bond is correlated with surface chemical composition of the base metals, soft solder microstructure evolution and composition of fracture surfaces developed under fatigue. The usefulness of surface analysis tools like Secondary Ion Mass Spectrometry (SIMS) and X-ray Photoelectron Spectroscopy (XPS) in these applications is discussed.

10:10 am BREAK

10:30 am INVITED

CHARACTERIZATION OF INTERFACIAL MICROSTRUCTURES OF LEAD-TIN SOLDER JOINTS: G. Ghosh, Dept. of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108

Various metallization schemes, such as Pd/Ni/Cu, Pd/Ag/Cu etc., are used in electronic packaging. During soldering metallized layer(s) react with the liquid solder and form various intermetallics both at the solder/substrate interface and in the solder. Diffusion of atomic species in the solid-state also lead to the formation and growth of intermetallic(s). In order to improve the reliability of electronic packaging, it is necessary to understand and control the diffusion path(s) and reaction mechanisms between the metallization layers and lead-tin solders. In this study we have used a variety of electron microscopy techniques, such as SEM, TEM and AEM to characterize the interfacial microstructures. The complex interfacial microstructures formed, on various substrates, due to interdiffusion will be discussed in detail.

10:55 am INVITED

MICROSTRUCTURE AND PROPERTIES OF SOME Sn-Zn-(X,Y) ALLOYS: R. Hwang, N. Jiang, J. Clum, E. Cotts^*, Mechanical Engineering, ^*Physics Department, State University of New York at Binghamton, Binghamton, NY 13902

Multicomponent (ternary and quaternary) alloys based on the Sn-Zn eutectic (198C) system have been examined to test for the interaction of composition with processing conditions in controlling microstructure and properties (microhardness, load relaxation, spreading reaction, etching reaction). Alloying additions have been tested with the objective of maintaining the eutectic melting behavior of the base alloy. The process conditions include rate of solidification, percent compressive strain, aging temperature and time. A simple factorially designed experimental plan was used to conduct and interpret the tests. In addition to the effect of alloying composition, a major effect of solidification rate and a secondary effect of the interaction between deformation and annealing was observed. Alloying additions have been seen to refine the structure of the base Sn-Zn eutectic as well as alter the stability of the microstructure.

11:20 am

THE DEVELOPMENT OF A LEAD FREE, IN-SITU COMPOSITE SOLDER ALLOY CONTAINING Ni OR Fe-BASED COMPOSITE PHASES: S. Choi, A.W. Gibson, J.L. McDougall, T.R. Bieler, K.N. Subramanian, Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824

The implementation of lead-free solder is impeded due to the lack of experience, material property data base, and reliability criteria for lead-free solder joints. Microstructural instability from thermomechanical cycling of solder is often detrimental to the performance of solder joints. Elemental Ni or Fe and additional Sn were added to eutectic Sn/Ag alloy to obtain about 20 Vol % of small in-situ composite intermetallic particles. Since coarsening of microstructural features during deformation is known to affect creep-fatigue resistance, the effect of the composite phase on aging behavior was investigated. Small single shear lap joints with a size similar to joints in microelectronic applications were fabricated using a melt reflow process similar to industrial practice. The effects of composite phase on microstructural evolution were monitored with aging temperature and time to determine the kinetics of aging processes. The effect of the composite phase on mechanical properties of the solder is also discussed.

11:40 am

MECHANICAL FATIGUE AND FATIGUE/CREEP OF SOLDERS: A.W. Gibson, K.N. Subramanian, T.R. Bieler, J.P. Lucas, Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824-1226

Automotive electronics and solders are exposed to relatively harsh environments, especially in under-the-hood applications. The solders are exposed to high and low frequency mechanical fatigue, as well as thermal fatigue/creep caused by temperatures fluctuating from -40 to +150C. To combat the detrimental effects of fatigue and/or creep, composite strengthening strategies are considered. Single shear lap solder joints, approximating the size used in microelectronics, are fatigue tested in cantilever bending mode utilizing a shaker table. Specimens are tested at room temperature and elevated temperatures to simulate under-the-hood temperature variations. Effects on microstructural coarsening, and crack initiation and growth are compared in similarly tested composite and non-composite solders.