1997 TMS Annual Meeting: Monday Session

DESIGN AND RELIABILITY OF SOLDERS AND SOLDER INTERCONNECTS: Session II: Design and Reliability of Lead-Tin Solders

Session Chairpersons: Sungho Jin, Lucent Technologies, Bell Laboratories, Room 7G-326, 700 Mountain Avenue, NJ 07974; Iver E. Anderson, Ames Laboratory, Iowa State University, 122 Metals Development Bldg., Ames, IA 50011

2:00 pm INVITED

THE INFLUENCE OF MICROSTRUCTURE ON THE FAILURE OF EUTECTIC SOLDERS: John W. Morris, Jr., H. Reynolds, Department of Materials Science and Mineral Engineering, University of California, Berkeley, and Center for Advanced Materials, Lawrence Berkeley Laboratory, CA 94720

There are three key mechanisms of failure during the life of microelectronic solder joints: overload failure during handling, thermal fatigue failure during service, and, particularly in the case of joints for optoelectronic devices, dimensional changes during service. Each of these failure modes is strongly influenced by microstructure of the solder, which is, in turn, affected by the composition of the solder, the chemical nature of the substrate, and the manufacturing process that is used to create the joint. The present talk will discuss the varieties of microstructure that are found in common solder joints, their influence on lifetime and failure mode, and the metallurgical techniques that can be used to control microstructure and modify the nature and kinetics of joint failure.

2:25 pm INVITED

DIFFUSION PROCESSES IN LEAD BASED SOLDERS USED IN MICROELECTRONIC APPLICATIONS--AN OVERVIEW: Devendra Gupta1, J. M. Oberschmidt2, K. Vieregge3, 1IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598; 2IBM Semiconductor Research and Development Center, Rt. 52-AP1, Hopewell Junction, NY 12533; 3Hoogovens Aluminium GmbH, 5400 Koblenz, Germany

Lead based solders are main stay of the microelectronic industry. Besides their use in printed boards, they are employed in interconnections of Si chip-to-substrates such as the multi-chip-modules, the thermal-conduction-module, the tape-automated bonding etc. During their service, the solder interconnections, commonly known as C-4 joints, are subjected to temperatures of the order of 100°C which translate to homologous temperatures of T/Tm 0.5 where Tm is the melting temperature in Kelvin. At these temperatures, a host of diffusion and related phenomena become operative notably the diffusion in the lattice and grain boundaries, creep and fatigue which eventually determine reliability of the device packaging. We have carried out extensive diffusion studies on the Pb-In and Pb-Sn solders over a range of composition using radioactive tracer techniques. We will discuss therein the various diffusion process, their modifications through ternary solute additions such Au and Cu and the role of unstable microstructure particularly in the eutectic solders.

2:50 pm INVITED

DEFORMATION, FATIGUE CRACKING AND COARSENING IN A LEAD-TIN EUTECTIC: T. Plookphol1, Donald S. Stone1 and S.-M. Lee2, 1Materials Science and Engineering Dept., University of Wisconsin, Madison, WI; 2Samsung Electronics Co., Suwon, Korea

Over the last several years, we have experimented with lead-tin eutectic carefully controlled microstructure in order to learn about creep, microstructural evolution, and fatigue cracking in this alloy. The purpose of those experiments has been to formulate a mechanistic basis for modeling the creep-fatigue interaction. In this paper we review the work. Creep tests, tensile tests, and load relaxation tests combined with microstructural characterization and theoretical modeling have helped to provide insight into the roles of dislocations and colony boundary sliding during creep. Colony boundary sliding impacts upon cracking and coarsening during fatigue. Work hardening and dynamic recovery during creep affect subsequent coarsening. Fatigue experiments examining the effects of frequency and strain range on fatigue crack growth have revealed transitions in mode of crack growth accompanying the onset of colony boundary sliding at low strain rates. Experimental studies of the effect of prior deformation on subsequent coarsening reveal that the factor by which coarsening accelerates depends on the rate of deformation. *Supported by the Wisconsin Alumni Research Foundation.

3:15 pm BREAK

3:25 pm

A NEW METHODOLOGY TO MEASURE DAMAGING STRAINS IN LEAD/TIN SOLDERS: Abbas I. Attarwala1 and Juan M. Sanchez2, 1Hewlett-Packard Company, IC Business Development Division, Palo Alto, CA 94304; 2Center for Materials Science, The University of Texas at Austin, TX 78712

A new methodology was developed to isolate and measure the damaging strains in lead/tin solders. The new methodology is based on load controlling cycling as opposed to strain controlled cycling. Under load controlled cycling conditions it is easy to separate out the effects of the different strain components. It was determined that the damaging strains were primarily creep strains, even at a cyclic rate of 0.5 Hz at a temperature of -40°C. Fractographic analysis of the fractured specimens confirmed that failure even at -40°C occurs by creep processes. The envelope strain curve generated from load controlled cycling data is a direct measure of the damaging strains stored in the material. A new parameter, D was identified. D is the damaging strain stored per cycle. The damaging strains per cycle measured for various solder compositions tested at varying frequencies, temperatures and microstructures could all be correlated by simple linear relationships. It was observed that the damaging strain stored during the secondary creep region of the envelope strain curve ranges from 2% strain to 18% strain depending on the test conditions.

3:45 pm

CREEP CRACK PROPAGATION OF 63Sn/37Pb WITH EMPHASIS ON COLONY BOUNDARY DEFORMATION AND RUPTURE: Scott A. Schroeder1 and M. R. Mitchell1 and A. G. Evans2, 1Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360; 2Division of Applied Sciences, Harvard University, Pierce Hall, 29 Oxford Street, MA 02138

Experiments on deformation and rupture of eutectic Sn/Pb solder will be discussed. These involve in-situ field emission SEM observations made during tensile and thin-walled torsional shear testing at room and elevated temperatures. Deformation concentrates on colony boundaries, leading to cavitation, cracking, and tertiary creep. Cavity formation also induces a large anelastic (time-dependent recoverable strain) effect. Progressive colony boundary damage has been mapped from sequential in-situ images. Subsequent strain field mapping and animations, highlight and quantify the relative deformations. High magnification image sequences characterize cavity growth and coalescence mechanisms. Upon combining with calculations of stress distribution, creep induced cavitation and crack propagation have been quantified.

4:05 pm

MECHANICAL PROPERTIES OF Pb/Sn SOLDERS AT THE TEMPERATURE RANGE OF -200°C TO 150°C: W. Kinzy Jones, Yanqing Liu, Marc A. Zamino, Gerardo L. Gonzales, Department of Mechanical Engineering, Florida International University, Miami, FL 33199

The mechanical properties (E, 0.2, UTS and %) of five Pb-Sn solder alloys (63Sn/37Pb, 62Sn/36Pb/2Ag, 96Sn/4Ag, 95Pb/5Sn, 90Pb/10Sn) commonly used in electronic packaging have been determined over the temperature range of -200°C to 150°C using uniaxial tensile test, dynamic mechanical analysis, and acoustic pulse method. The following results have been found: (1) the elastic moduli decreases linearly with increasing temperature until 70°C, then rapidly drop for the temperature 100°C and above; the strength (0.2, UTS) decreases with increasing temperature with the exception that for the solid solution solder (95Pb/5Sn, 90Sn/10Sn) which remain approximately constant; (3) the ductility change is complex: for lead matrix solid solution solders; the total elongation (r) increasing slowly with increasing temperature, the uniform elongation (u) is relatively high (>20%), and decreases slowly with increasing temperature, and the neck elongation (n) increases sharply with increasing temperature; whereas, for the eutectic solders (63Sn/37Pb, 62Pb/36Ag/2Ag, 96Sn/4Ag), the super-plasticity occurs at high temperature (>100°C), while at low temperature (-150°C) the brittle fracture occurs, the higher uniform elongation occurs at about -100°C.

4:25 pm

A MODEL OF REACTIVE WETTING FOR THIN DROPS: James A. Warren1, W. J. Boettinger1, A. R. Roosen2, 1Metallurgy Division, 2Ceramic Division, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899

When a liquid alloy spreads on a substrate, interdiffusion may result in melting of the substrate of the substrate and/or formation of intermetallic phases. We investigate the former case and describe the evolution of the non-planar interface on the lower surface of the drop. An approximate diffusion analysis is performed for a drop whose height is much smaller than its width. The coupling between the melting process and the rate of advance of the triple junction is explored for a variety of contact angle conditions. Numerical solution of the governing equation are performed and compared to experimental results for Bi-Sn alloys spreading of a droplet on a grain boundary.

4:45 pm

EFFECT OF COMPOSITE STRENGTHENING STRATEGIES ON THE MICROSTRUCTURAL EVOLUTION IN A TIN-SILVER SOLDER: A.W. Gibson, S.L. Choi, J.L. McDougall, T.R. Bieler, K.N. Subramanian, Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824

Pending Federal regulations provide the impetus for using Pb-free solders. Automotive electronics and solders are exposed to thermal cycles in the range of -40°C to 150°C under cyclic and quasi-static conditions, and they experience low and high frequency mechanical fatigue vibrations. Since coarsening of microstructural features is known to affect fatigue resistance, the effects of aging behavior is investigated using eutectic Sn-Ag solder as a model system, with and without intentionally added intermetallic strengthening phases. Small single shear lap specimens with a size similar to joints in microelectronic applications are used to obtain microstructures that are obtained in real solder joints. Eutectic Sn-Ag solder joint microstructures coarsen when aged between 40 and 150°C for as little as a week. The microstructural evolution and its effect on mechanical properties is monitored with ageing temperature and time, and the kinetics of ageing is determined. The effects of adding composite intermetallic phases on the solder interface and ageing behavior are compared to the model Sn-Ag system.

5:05 pm

FORCED DIFFUSION THERMOGRAPHY FOR NONDESTRUCTIVE EVALUATION OF MICROSTRUCTURES: David A. Jahnke, Bela I. Sandor, Nuclear Engineering and Engineering Physics Dept., University of Wisconsin, 153 Engineering Research Building, Madison, WI 53704

Forced Diffusion Thermography (FDT), differential temperature detection across a material flaw while providing a controlled AC heat flux input, has been developed recently for structural integrity assessment in large structures. This paper presents FDT to identify microscopic flaws by both finite element analysis and experimentation. The technical difficulties are the production of a microscopic width heat flux line pattern and the spatial and temperature resolution of the infrared camera. A finite element model one millimeter square and one-third millimeter deep of tin solder material with various surface crack sizes specimens with known microscopic flaws were performed using an infrared camera with a temperature resolution of one mK. A seven mW helium-neon laser provided an AC heat input area of 250 micron wide by five millimeters long to the specimen's surface. The results show that this method can be applied to detect flaws in small specimens of solder and similar materials.

5:25 pm

TiN-BASED, ACTIVE METAL CONTAINING SOLDERS FOR JOINING OF ALUMINA: Tim Schwilm, O.T. Inal and Frederick G. Yost, Materials and Metallurgical Engineering Department, New Mexico Tech, Soccoro, NM 87801; Sandia National Laboratories, Albuquerque, NM 87815