Materials Week '97: Monday 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 Monday morning, September 15.

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

Session Chair: J.W. Morris, Jr., University of California, Berkeley, CA 94720

PHASE COARSENING AND CRACK GROWTH RATE DURING THERMO-MECHANICAL CYCLING OF 63Sn37Pb SOLDER JOINTS: P. Hacke, J. Fahmy, H. Conrad, Materials Science & Engineering Department, North Carolina State University, Raleigh, NC 27695-7907

The change in average phase size and in the crack area growth rate dA_c/dN during thermo-mechanical cycling (-30°C to 130°C) of 63Sn37Pb solder joints in shear were determined. The increase in phase size during the thermo-mechanical cycling was significantly greater than occurred in static isothermal annealing. Further, dA_c/dN increased during the thermo-mechanical cycling concurrent with the increase in phase size. The mechanisms for the phase coarsening and its effect on the crack area growth rate are discussed.

8:55 am INVITED

THE BEHAVIOR OF LOW AND INTERMEDIATE MELTING TEMPERATURE SOLDERS: D.R. Frear, Sandia National Laboratories, Albuquerque, NM 87185

A number of low and intermediate melting temperature solder alloys are used for hierarchical soldering in applications such as multichip modules and radar assemblies. Two alloys that have desirable properties include 50In-50Pb (MP=170°C) and 40Sn-40In-20Pb (MP=121°C). In addition to their relative melting temperatures, these alloys also have excellent stress relaxation properties that are necessary for assembling the multilevel ceramic layers required for radars. This paper will discuss experimental and computational results performed to determine the physical reactions, mechanical behavior, and lifetime reliability of 50In-50Pb and 40Sn-40In-20Pb. The reaction of these alloys with base metallizations such as Au-based thick films on alumina and copper/gold traces on Duroid-like substrates will be described. The time dependent constitutive relations were experimentally determined by compression creep tests. he derived constitutive relations were incorporated into a finite element tool that has been developed for near-eutectic Sn-Pb solders and lifetime predictions under used conditions for the In-Pb and In-Sn-Pb solders will be discussed. Experimental thermomechanical fatigue tests results for these alloys will be presented as validation for the computational simulations. This work was performed at Sandia National Labs which was supported by the United States Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.

9:20 am INVITED

MICROSTRUCTURAL CONTROL FOR IMPROVED SOLDER PROPERTIES: S. Jin, H. Mavoori, Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974

Technical barriers to the high-density circuit and device interconnection and packaging include the absence of reliable solder joint materials that will withstand high strain/stress conditions, for example imposed by severe thermal expansion mismatch. When the solder joint size is reduced in high-density packaging, the shear strain on solder is substantially increased, which tends to cause accelerated microstructural coarsening and deterioration in creep and fatigue resistance. Microstructural control of solder alloys, e.g., in grain size or the morphology, size and distribution of precipitate phases can be effective in improving the mechanical properties and solder joint reliability. In this talk, various approaches for solder alloy processing and microstructural modifications for improved mechanical properties will be discussed.

9:45 am INVITED

SINTERING OF SOLDER JOINTS: M.A. Palmer, C.N. Alexander, R.W. Messler, Jr., W.Z. Misiolek, Rensselaer Polytechnic Institute, Troy, NY 12180-3590

The electronics industry has developed a manufacturing infrastructure based on the properties of eutectic lead-tin solder, which has a liquidus of 183°C. There is interest in alternatives to this work-horse alloy, based both on concerns about the toxicity of lead, and the need for more demanding applications, where temperatures may exceed 150°C or 93% of Tm. The high melting point of such alternative alloys, typically in excess of 220°C, raises manufacturing concerns. Forming solder joints, with the alternative sintering process at lower temperatures is investigated in this work. The objective is to apply the new proposed process using available equipment for today's technology. In this presentation we will examine the sinterability of eutectic tin-lead solder paste, through several proof-of-concept experiments. The results of these experiments, and their implications will be discussed.

10:10 am BREAK

10:30 am INVITED

VOIDING IN SOLDER JOINTS SUBJECTED TO HIGH TEMPERATURE AGING: L.E. Felton, Foster Miller, Inc., 195 Bear Hill Road, Walthem, MA 02154-1196; T.-Y. Pan, H.D. Blair, J.M. Nicholson, Ford Motor Company, P.O. Box 2053, Building R, M/D 3135, Dearborn, MI 48121-2053

Higher application temperatures have been the trend of automotive military electronic packages. A high temperature limit of 160°C has been proposed as the specification for the next generation printed-wiring board assemblies. However, voids have been shown to form in the intermetallic compound layer on electroplated copper when aged at such a temperature. A metallurgical study on samples with 63Sn-37Pb and 96.5Sn-3.5Ag solders aged at 160°C for 30 days has shown that the extent of voiding can be associated with different brighteners and/or surfactants in the plating bath. This voiding is usually confined to the Cu₃Sn layer and may ultimately cause mechanical failure of the solder joints. With time this void formation at elevated temperatures can be as deleterious to joint integrity as thermal cycle fatigue.

10:55 am INVITED

CREEP DEFORMATION OF NON-COMPOSITE Sn-Ag EUTECTIC SOLDERS UNDER ISOTHERMAL AND THERMAL CYCLING CONDITIONS: K.N. Subramanian, J.L. McDougall, A.W. Gibson, S. Choi, T.R. Bieler, Department of Materials Science and Mechanics, Michigan State University, E. Lansing, MI 48824-1226

Under normal operating conditions, soldered components in an automobile experience cyclic stress relaxation due to differential thermal expansion. The resulting strain and damage conditions are similar to creep deformation. Sn-Ag eutectic solder with and without intermetallic reinforcements were subjected to creep testing under isothermal and cyclic temperature conditions. Miniature single shear lap specimens used in this study are similar in size to soldered connections found in automotive circuit boards, and were fabricated using a melt reflow process similar to industrial practice. Strain measurements were made by comparing micrographs taken with a digital camera periodically throughout the experiment. Overall strain and strain heterogenities were quantified. The results for composite solder creep are compared with data for Sn-Ag solder from the literature.

11:20 am

MICROMECHANICAL CHARACTERIZATION OF SOLDER JOINTS: J.P. Lucas, A. Gibson, T. Bieler, K.N. Subramanian, Department of Materials Science, and Mechanics, Michigan State University, East Lansing, MI 48824-1226

Solder joints are in-situ composites consisting of highly distinctive phases. The microstructure of solders evolves almost continuously as solder exists at fairly high homologous temperatures. Moreover, microstructural evolution is enhanced by aging and thermo/mechanical history. The mechanical performance of solder in service depends on the microconstituent phases present in the solder joints. To control microstructural evolution, attempts are being made to stabilize the microstructure in-situ using thermally-stable reinforcement phases. To characterize the mechanical behavior and assess the stability of the solder, nanoindentation testing will be performed. Mechanical properties of the constituent phases, such as the stress, strain rate, creep behavior, and hardness will be determined spatially close to and far from interphase boundaries, and related to the microstructural stability of solder under simulated service conditions.