1997 TMS Annual Meeting: Tuesday Session

DESIGN AND RELIABILITY OF SOLDERS AND SOLDER INTERCONNECTION: Session III: Design and Reliability of Lead-Free Solders and Solder Interconnects

Session Chairpersons: Judy Glazer, Hewlett-Packard Co., Electronic Assembly Development Center, Mail Stop 4U-3, 1501 Page Mill Road, Palo Alto, CA 94304; James F. Maguire, Boeing Co., P. O. Box 3999, Defense & Space Systems Group, Mail Stop 3W-97, Seattle, WA 98124

8:30 am INVITED

DESIGNING WITH Pb-FREE SOLDERS: Walter L. Winterbottom, Reliability Consultant, 30106 Pipers Lane Court, Farmington Hill, MI 48331

As the global competition in the transportation industry intensifies, the focus on quality and time-to-market have become major driving forces in product design and dominant forces in the market place. Although the current design/development process as practiced in the industry provides a time tested path for new products, leadership in these areas seems to be elusive. A York Times article (March, 1991) notes that Consumers Report ranks the reliability of new American models at about the level of the average Japanese models. In 1988, the Material Systems Reliability Department was organized in the Materials Research Laboratory of Ford Research with the mission to evaluate existing, and develop if necessary, design methodologies capable of the task of providing world leadership in both quality and time-to-market. It was apparent from the outset that a life-cycle design approach must be used to contain reliability, cost, and environmental considerations in a time efficient manner. Further, it became increasingly apparent from the Department's failure mode/root cause identification and testing assistance activities that these issues have to be designed into a product rather than being assured by extensive prototype testing programs. A Design-for-Reliability methodology based upon the development of analytical prototypes which has the capability to fulfill the requirements for world leadership. The methodology is based upon the definition of system failure modes and mechanisms, system model development with verification capable of reliability prediction early in the design process prior to the building and testing of prototype hardware. This 'preventive' approach is based upon quantitative reliability estimates as a key design metric for use in assessing design alternatives. In simplest terms, the approach emphasizes the need to consider products as material systems whose reliability must be thoroughly understood early in the design and development process. The traditional 'build, test, and fix' design approach must be replaced with an approach that relies upon analytical prototypes leading to a single, optimized, prototype hardware build. In the presentation, the Design-for-Reliability approach will be illustrated using electronic packaging design with lead-free solder interconnects as example.

8:55 am INVITED

DESIGN OF NEW SOLDER ALLOYS THROUGH MICROSTRUCTURE CONTROL: Sungho Jin, Lucent Technologies, Bell Laboratories, Room 1A-123, 700 Mountain Avenue, Murray Hill, NJ 07974

The mechanical properties of solder alloys and reliability of solder joints are significantly affected by microstructural features such as grain size, phase distribution, and precipitate morphology. A finer grain size, smaller precipi tates, and a uniformity in phase, precipitate and grain size distribution are often beneficial for improving the strength and ductility as well as the resistance in fatigue and creep failures. The control of microstructure is accomplished by modifications in alloy chemistry and processing conditions. Several examples of microstructural control in lead-free solder systems will be discussed. More forward-looking approaches for further control of microstructure in solder alloys and composites, e.g., by distribution of nano-scale dispersoid particles will also be discussed.

9:20 am INVITED

LEAD-FREE SOLDERS FOR ELECTRONIC ASSEMBLY: Fay Hua, Judy Glazer, Hewlett-Packard Co., Electronic Assembly Development Center, Mail Stop 4U-3, 1501 Page Mill Road, Palo Alto, CA 94304

This paper gives an up dated literature review on the metallurgy of lead-free solders for electronic assembly. The review includes newly reported technical data on existing binary Sn-based eutectic alloys: Sn-Ag, Sn-Bi, Sn-Cu and Sn-In. It also reviews the newly developed multi-component solder alloys in following systems: Sn-Zn-In, Sn-Zn-Bi, Sn-Zn-Sb, Sn-Ag-Zn, Sn-Ag-Cu(-Sb), Sn-Ag-Bi, Sn-Ag-In(-Sb), Sn-Bi-In, and Sn-Bi-Sb. The characteristics of these alloys described are chemical composition, physical properties, microstructure, and mechanical properties. The manufacturability issues and applications of these lead-free solders in electronic assembly will be presented.

9:45 am INVITED

SUMMARY OF RECENT STUDIES OF THE EFFECT OF PROCESSING ON MICROSTRUCTURE OF SOME SOLDER ALLOYS: James Clum, E. Cotts, N. Jiang, Mechanical Engineering, Physics Department, State University of New York at Binghamton, Binghamton, NY 13902

A variety of solder alloys selected from the Sn-based, and In-based systems, have been examined to test for the interaction of composition with processing conditions in controlling alloy microstructure. Rate of solidification, amount of compressive strain, annealing temperature and time were the independent variable studied. Grain size, phase fraction and microhardness have been monitored as measures of microstructural changes. A simple factorially designed experimental plan has been used to conduct and interpret the tests. A major effect of solidification rate and a secondary effect of the interaction between deformation and annealing was observed for most alloys. The effect of alloy composition is reflected primarily in terms of the role of Tambient/Tm on microstructural stability. Some other processing related characteristics of these solder alloys will also be discussed such as creep and stress relaxation behavior. The role of microstructure in behaviors such as grain boundary sliding will be illustrated.

10:10 am

EVALUATION OF ALTERNATIVES TO LEAD SOLDERS FOR PRINTED WIRING APPLICATIONS: James F. Maguire, Boeing Co., P. O. Box 3999, Defense & Space Systems Group, Mail Stop 3W-97, Seattle, WA 98124

This paper describes the first phase of a three year program undertaken by Boeing to evaluate "no lead" materials as a potential replacement for solder in printed wiring assembly applications. This program is looking at a number of different potential replacements including both conductive adhesives and "no lead" solder materials. In addition, the impact of other environmentally driven process changes such as "no clean" soldering and lead free PWB finishes are being tested for compatibility with "no lead" attachment materials. The phases of this program are: Phase I: Initial materials evaluations, processability, stability of electrical performance in environmental exposure, sequential exposure to temperature/humidity cycling and thermal shock environments, compatibility with "standard" finishes (reflowed solder and "de-oxidized" copper), down Selection for Phase II testing; Phase II: detailed material performance testing, stability of contact and bulk resistance during environmental exposure, compatibility with high speed/RF applications, * Compatibility with existing PWA processes such as - pre-conformal coat cleaning, rework/repair, down selection for Phase III testing; Phase III: Development of design allowables and manufacturing processes. This paper will discuss the results of Phase I of this program which evaluated a total of 21 conductive adhesives and 8 commercially available solder alloys with control samples fabricated with Sn63 solder.

10:30 am

CREEP AND MECHANICAL PROPERTIES OF Sn-5%Sb SOLDER: Rao K. Mahidhara, K. Linga Murty and Fahmy M. Haggag, Tessera Inc., 3099 Orchard Drive, San Jose, CA 95134; North Carolina State University, Raleigh, NC 27695; Advanced Technology Corporation, Oak Ridge, TN 37830

A knowledge of the mechanical and creep properties of solder materials is required both for alloy development and life-prediction. We report here tensile and creep properties of Sn-5%Sb which is one of the candidate materials for replacing lead-based solders in electronic packaging. The temperature and stress dependencies of the strain-rate are evaluated using both the tensile and creep tests. In addition, the recently developed Stress-Strain Microprobe (SSM) is used to evaluate the strain-rate dependence of stress through automated ball indentation (ABI) tests at ambient. An excellent correlation is noted between creep and ABI data. While creep tests covered low stresses, ABI corresponded to high stresses which revealed breakdown of the Norton-law noted at lower stresses. While creep tests took 5 months time, the ABI tests were completed in a matter of hours although they covered 3 orders of magnitude strain-rates. The utility of ABI technique in solder joints is clearly pointed out and such tests are planned on real joints. ABI tests at elevated temperatures were not performed to-date. Such a facility is now under development and future plans include testing at elevated temperatures on bulk as well as solder joints.

10:50 am BREAK

11:00 am

CHARACTERIZATION OF 58Bi-42Sn SOLDERED ON Sn-Pb COATED SURFACE: Zequn Mei, Fay Hua, Judy Glazer, Hewlett-Packard Co., Electronic Assembly Development Center, Mail Stop 4U-3, 1501 Page Mill Road, Palo Alto, CA 94304

As reported in our previous work, solder joints of 58Bi-Sn on 80Sn-20Pb coated surface failed prematurely in thermal cycles between -40°C to 100°C. The failure mechanism was identified to be the dissolving of Pb atoms into the 58Bi-Sn solder joint during soldering, which caused formation of the ternary eutectic 52Bi-30Pb-18Sn with melting point of 95°C. In this paper, study of three types of solder joints will be reported: (1) 58Bi-42Sn soldered on Cu surface, (2) 58Bi-42Sn solder on 80Sn-20Pb coated surface, and (3) 57.5Bi-41.5Sn-1Pb soldered on Cu surface. These solder joints were aged at 80°C and 110°C. The evolution of the grain or phase sizes and the degradation of mechanical properties will be characterized as function of aging time. Early results indicate that the grains grew much faster in the solder joints containing Pb atoms (types 2 and 3 above), which reduced mechanical strength and ductility, than the solder joints without Pb atoms (type 1). Detailed results will be reported in the meeting.

11:20 am

MICROSTRUCTURAL STABILITY AND MECHANICAL PROPERTIES OF TIN-SILVER-COPPER SOLDER JOINTS: Iver E. Anderson, Ozer Unal, Ames Laboratory, Iowa State University, 122 Metals Development Bldg., Ames, IA 50011

The properties of a Pb-free ternary eutectic solder alloy, Sn-4.7Ag-1.7Cu (wt.%), which was discovered in our laboratory have been evaluated. This patented alloy has a melting point of 217°C, a fine 3-phase eutectic microstructure, and a very good solderability. The primary goal of this investigation has been to enhance its microstructural stability for high temperature environments, greater than 125°C, and to improve its overall mechanical properties, especially for extended service. Thus, minor modifications were made in the base eutectic alloy to control microstructural aging without degrading solderability. Alloying effects on solder/Cu joints in the as-soldered and aged conditions were studied. Tensile strength and shear strength measurements were made using butt and lap shear joint configurations, respectively, under various loading conditions. The results will be presented and their implications will be discussed. Support from the DOE-BES-DMS under Contract No. W-7405-Eng-82 is gratefully acknowledged.

11:40 am

THERMAL AGING AND IN-BOARD PEEL TESTING OF PB-FREE SOLDERS FOR UNDERHOOD APPLICATIONS: Martin W. Weiser, Julie A. Kern, Celeste A. Drewien, Frederick G. Yost, Johnson Matthey Electronics, Spokane Assembly Products Group, 15128 E. Euclid Avenue, Spokane, WA 99216; University of New Mexico, Mechanical Engineering Department, Albuquerque, NM 87131; Sandia National Laboratories, Materials and Process Sciences Center, Albuquerque, NM 87185

The thermal aging and post aging joint strength of four Pb-free solder alloys on a Ag-Pt thick film metalization were studied as the last part of a larger evaluation of solder/flux/metalization systems for use in high temperature under hood automotive application. The solders were the Sn-Ag and Sn-Ag-Cu eutectics and a Bi modified version of each. Solder pastes were reflowed on metalized alumina in a commercial inert atmosphere belt furnace. Aging of both sessile drops and in-board peel test samples was conducted at 134 to 190°C for up to 1000 hours. Addition of Bi and/or Cu increased the intermetallic growth rate and decreased the in-board peel strength.

12:00 am

RATE-CONTROLLING MECHANISM DURING PLASTIC DEFORMATION OF 95.5Sn4Cu0.5Ag SOLDER JOINTS AT HIGH HOMOLOGOUS TEMPERATURES: Hans Conrad, Z. Guo and Y. H. Pao, Materials Science and Engineering Department, North Carolina State University, Raleigh, NC 27695; Ford Scientific Research Laboratory, Materials Systems and Reliability Department, 20000 Rotunda Drive, Dearborn, MI 48121

The effects of stress and temperature on the creep rate of 95.5Sn4Cu0.5Ag solder joints at 22°C to 168°C were investigated employing stress-change and temperature-change tests. The resulting plastic deformation kinetics are in accord with those obtained previously in constant strain rate tests. They are better described by an obstacle-controlled dislocation glide kinetics equation than the usual Dorn equation for diffusion-controlled mechanism. The Helmholtz free energy for overcoming the obstacles is 0.28µb3, where µ is the shear modulus and b the Burgers vector. The activation volume is of the order of 1000 b3. The plastic deformation kinetics are in reasonable accord with the intersection of dislocations as the rate-controlling mechanism. However, alternative mechanisms are not ruled out.

12:20 pm

ALLOY DESIGN OF Sn-Zn-X (X=In, Bi) SOLDER SYSTEM THROUGH PHASE EQUILIBRIA CALCULATIONS: Sueng Wook Yoon, Jeong Ryong Soh, Byeong-Joo Lee, Hyuck Mo Lee, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Kusung-Dong 373-1, Yusung-Gu, Taejon, Korea 305-701; Materials Evaluation Center, Korea Research Institute of Standards and Science, P.O. Box 3, Taedok Science Town, Taejon, Korea 305-606

Thermodynamic studies of the Sn-Zn-X (X=In, Bi) system have been carried out in terms of phase equilibria to design Pb-free solder alloys which are drop-in replacement for Sn-37Pb alloy. Based on the result of phase equilibria calculations, several selected alloys were chosen and analyzed by DSC, XRD and EPMA techniques. Microstructures of as-cast and heat-treated alloys were examined as well as the interface between solder joint and Cu substrate by optical microscopy and SEM. Spreading area test and preliminary mechanical test were performed to investigate the possibility as an alternative of eutectic Pb-Sn-solder.

12:40 pm

ALLOY DESIGN OF Sn-Ag-In-Bi-Sb SOLDER SYSTEM USING THERMODYNAMIC CALCULATIONS: Byeong-Joo Lee and Hyuck Mo Lee, Materials Evaluation Center, Korea Research Institute of Standards and Science, Yusong P.O. Box 102, Taejon 305-600, Korea; Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Kusong-Dong 373-1, Yusong-gu, Taejon 307-701, Korea