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About the 1996 TMS Annual Meeting: Monday Afternoon Sessions (February 5)

February 4-8 · 1996 TMS ANNUAL MEETING ·  Anaheim, California


Sponsored by: EMPMD Electronic Packaging and Interconnection Materials Committee

Program Organizers: M. McCormack, AT&T Bell Laboratories, Rm lA- 123, 600 Mountain Ave., Murray Hill, NJ 07974; S. K. Kang, IBM, T. J. Watson Research Center, P. O. Box 218, Yorktown Heights, NY 10598; M. R. Notis, Department of Materials Science and Engineering, Lehigh University, 5 East Packer Ave., Whitaker Laboratory, Bethlehem, PA 18015- 3195; K. N. Tu, Department of Materials Science and Engineering, UCLA, Los Angeles, CA 90095-1595

Monday, PM Room: Grand K

February 5, 1996 Location: Anaheim Marriott Hotel

Session Chairperson: S. K. Kang, IBM, T. J. Watson Research Center, P. O. Box 218, Yorktown Heights, NY 10598

2:00 pm Invited


Lower temperature processing in manufacturing allows less expensive materials with greater temperature sensitivities to be used and lower overall production costs. Several new Pb- free and Pb- bearing solder alloys have been developed with the goal of meeting various material property requirements. The metallurgical design of these new solders will be discussed in terms of their strength, creep- resistance, strain- rate sensitivity, ductility, and fatigue properties. In addition, the wetting behavior of these new, alternative alloys will be discussed in terms of wetting balance data and area of spread tests. The effects of different surface finnishes, atmospheres, and fluxes on the wetting behavior of the new alloys will also be discussed.

2:25 pm Invited

SOLDER INTERACTIONS WITH MULTICOMPONENT SUBSTRATE CHEMISTRIES: P. T.Vianco, R. Grant, Sandia National Laboratories, Albuquerque, NM 87185

Traditional solder joints are comprised of a tin- based solder in contact with copper, or in some cases, with the nickel finish of a low expansion alloy I/O. The simple tin/copper metal couple, and the interface reactions which characterize this binary couple, have been found to adequately describe such cases. On the other hand, complex, multicomponent substrates are found in thick film conductors used on hybrid microcircuits. These systems present an opportunity to examine the growth kinetics and chemistry hierarchy of interfacial phases which develop between the solder and substrate materials or complex chemistries. A study was performed to investigate the system of 63Sn37Pb solder on 76Au- 21Pt- 3Pd (wt.%) alloy. Time dependent solid state growth kinetics were evaluated for aging temperatures of 55deg.C - 170deg.C and time periods of up to 5000 hours. The interface phase compositions were determined by electron microprobe analysis. A parallel study was performed on the 63Sn- 37Pb/lOOAu system to serve as a baseline to the 63Sn- 37Pb/Au- Pt- Pd work. The experimental results will be used to assess the engineering of solder/substate systems which minimize such interactions. This work was performed at Sandia National Laboratories and supported by the U.S. Dept. of Energy under contract DE- AC04- 94AL85000.

2:50 pm Invited


The suitability of noble metal printed wiring board surface finishes was assessed for new technology applications thal incorporate both direct chip attachment (wire bonding) and peripheral leaded device attachment (soldering) on the same product vehicle. The robustness of the surface finishes to temperature and humidity exposures was evaluated first for screening purposes. The performance of potential candidates was then assessed relative to wire bondability and solder joint integrity. Gold and aluminum ball/wedge bonding characteristcs were monitored as a function of assembly processes. Solder joint integrity was determined as a function of temperature cycling and thermal shock.

3:15 pm Invited

COMPARISON OF SN- PB AND SN- AG SOLDER JOINTS SUBJECTED TO VARIOUS THERMAL CYCLING PROFILES: Y.- H. Pao, S. Badgley, E. Jih, R. Govila Research Laboratory, Ford Motor Company Dearborn, MI 48121- 2053

The thermal cyclic and fatigue behavior of 63Sn- 37Pb, 90Pb- lOSn, 63Sn- 0.7Sb- lAg- lIn- Pb (SS- 100) and 96.5Sn3.5Ag solder alloys are studied using a modified bi- material specimen. The specimen is subjected to temperature cycling with two different profiles, one is between - 40[[ring]]C and 125deg.C with a two hour and 15 minute period, and the other has a 40- minute period between 40[[ring]]C and 140[[ring]]C with a 10 minute ramp rate. The shear stress and strain in the joint during thermal cycling are measured with the aid of four high temperature strain gages. The steady state creep parameters of these solders are determined based on the stress relaxation data. Viscoplastic constitutive equations are assumed for the solder alloys and are implemented in a finite element program ABAQUS to model the test for each solder alloy. The fracture surface as a result of thermal cycling is examined with scanning electron microscopy to identify the dominant failure mode and associated failure mechanisms. In addition, a comparison of the evolution of hysterics loops is also made for the above solder alloys, and the results are used to develop a damage parameter in the constitutive equations and to establish an accelerated thermal cycling factor or the solder joint. The results serve as part of the effort at Ford to assess Pb- free solders versus some conventional Pb- containing solders.

3:40 pm BREAK

3:50 pm

MICROSTRUCTURAL EVOLUTION DURING NATURAL AGEING OF A Sn- Ag SOLDER: A. Gibson, T R. Bieler, K.N. Subramanian, Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824- 1226

The need for lead- free solders in automotive electronics in the next few years requires an investigation of how mechanical properties are affected by the microstructure and microstructural changes that occur in an automotive environment. For automotive applications, solders must withstand thermal fatigue resulting from termperature chages from - 40deg.C to 150deg.C as well as mechanical fatigue from vibration. Lead- tin solders have shown improved fatigue resistance when the size of microstructural features can be kept small by pinning strategies that prevent coarsening of the microstructure. Consequently the natural ageing behavior of Sn- Ag solders is investigated. In the soldering process used in production, the heating and cooling history, along with the size of the joint, will affect the microstructural scale within the solder joint. Several specimen configurations having different geometry and size that are convenient for laboratory testing of creep and fatigue properties will be presented, as well as comparisons of the microstructure with realistic solder joints. In each of these, the evolution of microstructure and the hardness is monitored with time. These observations will be compared with feature sizes and cooling rates following solidification.

4:10 pm

KINETIC ANALYSIS OF THE SOLDERING REACTION BETWEEN EUTECTIC Sn- Pb AND Cu: H.K Kim, K.N. Tu, Department of Materials Science and Engineering, UCLA, Los Angeles, CA 90095-1595

Owing to the advance in VLSI technology, there are increasing concerns about the functionality and reliability of input- output connections using solder joints. The reliability of such connections is, however, degraded by intermetallic compounds formation between the solder and metal substrate. The morphology and kinetics of Cu- Sn intermetallic compound formation in the eutectic Sn- Pb/Cu soldering reaction have been studied. At the interface, Cu- Sn compounds do not form layered structures, rather the Cu6Sn5 phase grows as scallop- like grains and ripening occurs between the grains. It is a complicated growth process. We have analyzed the growth by a combination of two fluxes: the flux of interfacial reaction and the flux of ripening. The mathematical model will be presented. Also we have measured the rate of consumption of Cu substrate in the soldering reaction.

4:30 pm

ASSEMBLY YIELD PREDICTION USING SURFACE EVOLVER WITH SOLDER PROCESS VARIABLES AS INPUTS: D. Lewis, M. R. Notis, Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015- 3195; G. C. Munie, AT&T Network Systems, Naperville, IL 60566; D. Noctor, AT&T Bell Laboratories, Allentown, PA 18103

Surface Evolver,a public domain program, was used to predict solder joint geometry using component and assembly parameters that effect solderability. To verify modeling results, as assembly trial was completed using different combinations of solder volume, component mechanical tolerance, component aging condition, and solder alloy. This study was developed to correlate manufacturing yields to theoretical modeling, and the final application is intended to provide a simple method for surface mount assemblers to predict solderability and yield. Application examples from Surface Evolver modeling will be provided to highlight results from the assembly trial.

4:50 pm

COMPARISON OF Sn- Ag AND Sn- Ag- Bi LEAD- FREE SOLDERS ON ENTEK COATED PCBs: A. Grusd, Heraeus Inc., Cermalloy Division, West Conshohocken, PA 19428

In addition to eliminating lead, many manufacturers desire to develop and use higher melting point solders as compared to eutectic tin- lead in order to meet demands of higher operating temperatures. Solder alloys of eutectic Sn- Ag (Tm=221deg.C) and Sn- Ag- Bi (188deg.C to 218C) were made into pastes, using both no- clean and RMA fluxes, and printed onto Entek coated PCBs, and then reflowed. The Sn- Ag- Bi exhibited improved wettability over Sn- Ag in air reflow on Entek. The wetting characteristics of the solder pastes on chip capacitors with various end termination metallurgies including Sn/Pb, Ag, Ag/Pd, and Ag/Pd/Pt will also be discussed.

5:10 pm

THE DESIGN OF PB- SN BASED COMPOSITES FOR INTERCONNECT APPLICATION IN MICROELECTRONICS: R.K Mahidhara, S. Hariprasad, High Performance Materials Inc. (HPM), Campus Box 1087; S. Bayhnam, S.M.L. Sastry, K.L. Jerina, Mechanical Engineering Department, Campus Box 1185, Washington University, St. Louis, MO 63130

Solders are critical materials for preserving the integrity of components in microelectronic packaging. Current tinlead solders lack shear strength, resistance to creep and resistance to thermal- mechanical fatigue damage. Pb- Sn based in- situ composites were developed by rapid solidification processing (RSP) following additions of suitable ternary alloying elements to Pb- Sn eutectic. The fine microstructure due to RSP has culminated into the development of two kinds of in- situ composite solders for application in microelectronics as solder paste. One kind of solder exhibits higher tensile strength, creep and high fatigue resistance compared to commercially available Pb- Sn eutectic solders. The other kind exhibits higher ductility and low cycle fatigue resistance. Tensile, creep and fatigue tests were conducted on single lap shear joints made with each solder composition to assess the reliability of the composite solders. The performance of HPM's composite solders exceed those for commercially available lead-tin solders.

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