1997 TMS Annual Meeting: Monday Abstracts

MATERIALS & TECHNOLOGIES FOR MICROELECTRONICS: Session II: An Issue for Lead-Free Solders (NCMS)

Session Chairperson: TBA

FILLET-LIFTING IN PLATED-THROUGH-HOLE EOMETRICS: Carol Handwerker, Ursula Kattner, Bill Boettinger, NIST, 223/A153, Gaithersburg, MD 20899; Chris Bailey, University of Greenwich, Wellington Street, Woolwich, London SE18 6PF

Separation of the solder fillet from the circuit board pad along the solder-intermetallic interface has been observed for a wide range of lead-free solder alloys. The tendency for such separation depends on a number of factors, including solder composition, the freezing range (also known as the pasty range), the mechanical properties of the solder, and the interfacial toughness. The processes leading to fillet-lifting will be described and the potential for eliminating this problem in lead-free alloys will be discussed.

2:30 pm INVITED

CONCLUSIONS OF THE NCMS LEAD-FREE SOLDER DEVELOPMENT PROJECT: Duane Napp, NCMS, 3025 Boardwalk, Ann Arbor, MI 48108-3266

A summary of the completed 4 year study of lead free solder alloys will be presented. In addition to toxicological considerations it was critical to define and agree on the factors and the methodology used in the down selection of 79 alloys to 7 promising alloys. It was necessary to conduct a manufacturing evaluation of compatible surface finishes and generate manufacturing process parameters necessary to assemble test hardware. The results of the extensive reliability evaluations and the modeling of the final down selected alloys will be presented. Follow-on NCMS material interconnect projects will be discussed.

3:00 pm INVITED

HIGH TEMPERATURE FATIGUE RESISTANT SOLDER - A NCMS PROJECT: Duane Napp, NCMS, 3025 Boardwalk, Ann Arbor, MI 48108-3266

To address the critical issue of placing electronic assemblies in extreme temperature environments that exceed the operating temperatures of commonly used eutectic Sn-Pb solder or high-Pb content alloys, National Center for Manufacturing Sciences (NCMS) initiated a national consortium in 1996 titled "High Temperature Fatigue Resistant Solder Project". The project objective is to demonstrate and deploy the technology to substantially improve the performance of solder joints at elevated temperatures under thermal cycling conditions. Two types of solder are to be developed: one with a performance objective of up to 160C, "Alloy 160", for organic substrate; and a second alloy with a performance objective of 205C, "Alloy 205", for ceramic substrate, copper die bonding substrates, and hybrid technology. The goal is to demonstrate a thermal fatigue resistance that, in the respective temperature ranges, is equal to or better than that of conventional solders. The need for these new alloys is being driven by the automotive, telecommunications, avionics, and military industries. The project is organized into four task groups with five phases of work, and extends over a period of 33 months. The participants include automotive and military companies, solder suppliers, academic institutions, and national laboratories.

3:30 pm BREAK

3:50 pm INVITED

MICROSTRUCTURE AND PHASE EQUILIBRIA IN THE Ag-Cu-Sn TERNARY SYSTEM: M.R. Notis, Department of Materials Science and Engineering, Lehigh, University, 5 East Packer Ave., Bethlehem, PA 18015

The Ag-Cu-Sn system is of interest for use in Pb-free solder alloys, brazing alloys, and in alloys for dental applications. The nature of the invariant reactions in the Sn-rich corner remain in question, and little is known about the effect of Cu addition on the width of the Ag3Sn phase field and its adjoining two- and three-phase fields at lower temperatures. A series of alloys were prepared and studied by metallography and by thermal analysis in order to elucidate these aspects of the Ag-Cu-Sn system.

4:20 pm INVITED

TAPE AUTOMATED BONDING (TAB) TECHNOLOGY FOR CHIP INTERCONNECTIONS: Sung K. Kang, IBM T.J.Watson Research Center, P.O.Box 218, Rm 37-250, Yorktown Heights, NY 10598

Tape automated bonding (TAB) technology has been widely used in low-cost consumer products as well as in high-performance multichip modules. TAB technology facilitates the chip-on-board (COB) assembly scheme in most applications. In this talk, two key issues of TAB technology, wafer bumping and inner lead bonding, are reviewed in conjunction with COB applications. Bumping refers to the process of adding raised metal contacts to bond pads in order to provide both the necessary bonding metallurgy for chip-to-lead bonding and a physical standoff to prevent lead-chip shorting. This is done on a wafer or on a TAB tape. Various bumping processes in use are discussed in terms of bump materials/processing, structure, geometry, bump properties and behavior during bonding.

4:50 pm

MONTE CARLO SIMULATION OF STEP COVERAGE OF COPPER: Y.G.Wang, X.W.Zhou, R.A.Johnson, H.N.G.Wadley, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903

A new Monte Carlo procedure for atomistically simulating physical vapor deposition processes is developed and used to model the two-dimensional physical vapor deposition of the step coverage of copper. The method consists of an implant approximation for the initial adatom adsorption on a surface and a multipath diffusion analysis to simulate subsequent surface morphology and interior atomic structure evolution. An embedded atom method is used to determine the activation energies for each of the many available diffusion paths. The method has been used to predict the morphology/structure evolution of copper films over the length and time scales encountered in practical deposition processes. The modeling approach has enabled determination of the effect of vapor processing variables such as flux orientation, deposition rate, substrate temperature, kinetic energy and aspect ratio on deposit morphology/microstructure and step coverage behavior.

5:10 pm

PEEL STRENGTH IN A Cu/Cr POLYIMIDE SYSTEM: I. S. Park, Jun Yu, Department of Materials Science and Engineering, KAIST, P.O. Box 201, Cheongryang, Seoul, 130-650, Korea

The adhesion strength of a metal film on a substrate is usually measured by the Peel Test. This test is not a direct measure of the interfacial energy because of the severe plastic deformation of the metal film accompanying the test. In the present analysis, using the Cu/Cr/polyimide system, the surface energy was deduced from the peel strength measurements and corresponding elasto-plastic analysis of the peel test. The metal/polyimide interface energy was varied by changing the pretreatment condition of the polyimide surface in Ar(+) RF plasma, and the adhered film thickness and yield stress were varied by changing the electroplating conditions. After the peel test, the chemistry of the Cr/polyimide interfaces were studied by AES and XPS.X-ray measurements of the residual strain in the peeled films were very useful in estimating the plastic work expenditure accompanying peeling. Results indicate that the deduced interfacial fracture energy is usually much smaller than the peel strength and is influenced by the RF pretreatment, but the interfacial energy is independent of the mechanical properties of the metal film, as expected.