Materials Week '97: Monday PM 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 afternoon, September 15.

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

Session Chair: S. Jin, Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974

ENERGETICS AND KINETICS OF DISSOLUTIVE WETTING PROCESSES: F.G. Yost, P.A. Sackinger, E.J. O'Toole, Sandia National Laboratories, Albuquerque, NM 87185-1411

Dissolutive wetting occurs when a liquid wets and spreads over a solid surface with simultaneous dissolution of the solid into the liquid. It is shown that this process initially yields a metastable equilibrium and a compact model for the kinetics of approach to this metastable state is described. The technique for constructing these kinetics stems from the early work of Onsager and begins with a relationship for the entropy production. From this, a coupled set of nonlinear, ordinary differential equations can be written directly. The equations are solved numerically for the wetted area and compared with experimental data. The model captures many of the subtle complexities of dissolutive wetting such as multiple metastable states. The main weaknesses of the model are the assumptions of fully-stirred liquid and a spherical solid/liquid interface which were made to keep the analysis simple. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy Contract number DE-AC04-94AL85000.

2:25 pm INVITED

THROUGH-HOLE SOLDER JOINT FILLET LIFTING PHENOMENON AND ITS POSSIBLE REMEDIES: C. Handwerker, NIST, Gaithersburg, MD 20899; Y. Zhu, Delco Electronics, Kokomo, IN 46904-9005; T.-Y. Pan, H.D. Blair, J.M. Nicholson, Ford Motor Company, Dearborn, MI 48121-2053

Fillet lifting failure, observed immediately after wave soldering and hand soldering with certain tin-silver based lead-free alloys, is characterized as separation between the copper pad and intermetallic layer of the solder joint. This observation raised serious concerns of usability of these lead-free solder alloys. Considerable efforts have been made to investigate the root cause of the failure and the possible remedy for the problem. A study of using Design of Experiment (DOE) was conducted to determine the effect of alloy composition, board thickness, pad size, and surface finish on the cracking problem. Results from this DOE study will be discussed.

2:50 pm INVITED

EFFECT OF SURFACE FINISH ON SOLDER FLOW OVER FINE LINE FEATURES: F.M. Hosking and C.L. Hernandez, Sandia National Laboratories, P.O. Box 5800, MS1411, Albuquerque, NM 87185

The rapid advancement of interconnect technology has necessitated the development of new surface finishes to replace the standard, technology-limiting copper and Sn-Pb solder coatings. As part of the effort, new solderability test methods have been developed to measure the effectiveness of the new metallic and organic coatings. Of particular interest is how solder flow over fine line features can be "engineered" through a science-based understanding of the wetting process. Sandia National Laboratories' Center for Solder Science and Technology has designed a capillary flow test geometry that addresses this surface flow issue. The presentation will examine the effects of circuit geometry, surface finish, aging, and solder processing on solder flow over the test surfaces. The results, and their significance to advanced interconnect technology, will be discussed.

3:15 pm INVITED

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF ULTRAFINE-GRAINED SOLDERS: H. Mavoori, S. Jin, and T.H. Tiefel, Bell Labs, Lucent Technologies, Murray Hill, NJ 07974

A fine grain size is desirable in solders due to the improved fatigue resistance, strength and possible superplastic behavior. This leads to better reliability, durability during handling, and accommodation of strains without damage accumulation. In this work, ultrafine-grained lead-free and lead-containing solders have been obtained through the use of non-reacting, non-coarsening refractory dispersions. The processing involves powder metallurgy techniques followed by plastic deformation. Grain sizes were of the order of 2000Å and the microstructure was seen to be quite stable. This paper discusses the mechanical properties and microstructures of these solders with different dispersed particles such as Al₂O₃ and TiO₂ and contrasts them with a dispersion-free solder subjected to the same processing and testing conditions. The dispersions are seen to have a significant effect on the mechanical deformation characteristics with respect to tensile, fatigue and creep behavior of the solders and can be used to develop new solders with more desirable properties for various electronic packaging applications.

3:40 pm BREAK

4:00 pm INVITED

INVESTIGATION OF DEFORMATION MECHANISMS IN Sn-Ag AND Sn-Sb SOLDER ALLOYS USING TENSILE, CREEP AND ABI TESTS: K.L. Murty, H. Yang, P. Deane, P. Magill, G. Rinne, North Carolina State University, Raleigh, NC 2795; MCNC Electronic and Information Technologies, RTP, NC 27709

Excellent thermal fatigue characteristics of Sn-Ag and Sn-Sb solder materials made these alloys candidates for electronic applications to replace leaded solders such as Pb-5Sn and Pb-63Sn. A thorough understanding of these alloys is needed for developing reliable life prediction methodologies. Tensile and creep tests were performed on bulk Sn-3.5Ag and Sn-5Sb samples. In addition, automated ball indentation (ABI) tests were performed on the shoulder portions of the creep samples using a Stress-Strain Microprobe (SSM) at different indentation speeds at ambient temperature. Long-term creep data correlated with those derived using short-term ABI tests. Transitions in deformation mechanisms were observed with distinct values for the stress exponent, and in Sn-3.5Ag, the low-temperature dislocation climb due to dislocation core diffusion was identified at high stases. In addition, single lap shear tests were performed on 3333 solder bump array of Sn-3.5Ag which exhibited relatively more scatter.

4:25 pm INVITED

INFLUENCE OF INITIAL LAYER THICKNESS ON THE GROWTH OF Cu-Sn INTERMETALLIC LAYERS IN SOLDER-COPPER DIFFUSION COUPLES: M. Schaefer, R. Fournelle, Materials Science Program, Marquette University, Milwaukee, WI 53201-1881; J. Liang, Rockwell Automation, 1201 South Second Street, Milwaukee, WI 53204-2496

An experimental study was done to evaluate the growth kinetics of Cu-Sn intermetallic compound layers. Samples of solder (62Sn-36Pb-2Ag) on copper were created by reflow soldering for various times such that resulting intermetallic layers were 1, 2, 3, 6, and 12 microns in thickness. Samples were subsequently aged at 125 or 165°C for times ranging from 1 to 69 days. The growth rate of relatively thin layers was faster than for thick layers. For a 1 micron initial layer, growth followed a t^0.33 dependence on time, t. For increasing initial layer thicknesses, growth shifted toward a t^0.5 dependence on time. This behavior may be explained in terms of a growth model which assumes that growth of thin layers is related to grain boundary diffusion and growth of thick layers is controlled by volume diffusion through the intermetallic layer. Samples with an initially thin layer exhibited more rapid development of a Pb-rich zone adjacent to the intermetallic.

4:50 pm

MICROSTRUCTURES AND FATIGUE PROPERTIES OF Bi-CONTAINING SOLDERS: C. Zhang, and J.K. Shang, Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801

Effects of Bi-addition on fatigue properties of Sn-based solder joints were examined by introducing different amounts of Bi in the bulk solder or on surfaces of copper substrates. Flexural peel specimens were made by reflowing Bi-modified solder alloys between cleaned copper substrates and by reflowing unmodified Sn-Pb solder between Bi-coated copper substrates. Microstructures and fatigue properties of the Bi-modified solder joints were studied by optical and electron microscopy, and by conducting fatigue crack propagation along solder interfaces. The results were compared to the previous work on unmodified Sn-Pb/Cu joint. The relationship between interfacial microstructure and properties will be discussed.