1997 TMS Annual Meeting: Tuesday Session

DESIGN AND RELIABILITY OF SOLDERS AND SOLDER INTERCONNECTION: Session IV: Reliability of Solder Interconnects

Session Chairpersons: Paul T. Vianco, Sandia National Laboratories, Department 1831, Mail Stop 0340, P.O. Box 5800, Albuquerque, NM 87185; James A. Warren, Metallurgy Division, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899

2:00 pm INVITED

THE NIST SOLDER INTERCONNECT DESIGN TEAM PROGRAM: James A. Warren, Carol A. Handwerker, Metallurgy Division, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899

The NIST Solder Interconnect Design Team has been found to address several pressing issues in the design and fabrication of circuit board. Having met frequently over the past three years, in partnership with academic and industrial researchers, the Team has established an agenda for solving solder joint shape, and the consequential thermal/mechanical properties of the formed joint. Our ultimate goal is to provide the industrial community with a suite of useful software tools for solder interconnect design, and to provide solved test problems (available electronically on the World Wide Web), that can be modified to suit the needs of the particular user. With this in mind we are actively supporting the development of software which will interface the public domain program Surface Evolver, which has been shown to be quite capable at computing equilibrium meniscus shapes. A discussion of the outstanding problems, as well as the software tools under development will be presented.

2:25 pm INVITED

MODELING NON-ISOTHERMAL INTERMETALLIC LAYER GROWTH IN THE 63Sn-37Pb SYSTEM: Paul T. Vianco, P.L. Hopkins, K.L. Erickson, D.R. Frear, R. Davidson, Department 1831, Mail Stop 0340, P.O. Box 5800, Sandia National Laboratories, Albuquerque, NM 87185

The reliability of mechanical and electronic systems can be acutely dependent on the integrity of the soldered joints used in their assembly. During product manufacture, intermetallic layers from in reaction zones between the dissimilar materials of solder joints. Thermal cycling during service can cause further growth of the intermetallic layer, which may jeopardize the mechanical integrity of the joint as well as its capacity for rework or repair later on. Models describing service-related changes to solder joint microstructure are essential to understanding and predicting the long-term mechanical reliability and serviceability of these interconnects. In previously published work (J. Electronic Materials, v. 23, No. 8, 1994, pp. 721-729) a model describing the diffusion-controlled growth of multiple intermetallic layers and the displacement of the interfaces between layers was developed and implemented in a one-dimensional computer code based on the method-of-lines. The model can accommodate cases involving: (1) finite initial layer thickness, (2) rate-limiting interfacial reactions, (3) multiple and variable diffusion coefficients, and (4) finite material boundaries. Additionally, the effects of nucleation can be modeled empirically. A transformation of spatial coordinate circumvented the need to remesh the growing and (or) shrinking layers. Results from the one-dimensional code were verified by comparing the numerical output with analytical solutions for simple systems involving two, three, and five layers. The computer code was then applied for analysis of intermetallic layer growth from solder aging experiments performed with 100Sn and 63Sn-37Pb solders. The analysis indicated that intermetallic layer growth was consistent with bulk diffusion mechanism involving Cu and (or) Sn and variable diffusion coefficients that reflect some enhanced diffusion during early growth. In this work, non-isothermal solder-aging experiments were done with the 63Sn-37Pb/Cu system using two temperature histories: a low frequency history consisting of 4 cycles per day between -50 and 170°C, and a high frequency history consisting of 72 cycles per day and the same limits. Thicknesses of both the Cu3Sn and Cu6Sn5 intermetallic layers were determined as a function of time for both temperature histories. An enhanced version of the previously developed model was used to predict the non-isothermal intermetallic layer growth for both temperature histories. Arrhenius expressions for diffusion coefficients in both the Cu3Sn and Cu6Sn5 layers were determined using experimental data from previous isothermal studies. This paper describes the non-isothermal experiments and a comparison of calculated and observed layer growth as a function of time. This work was performed at Sandia National Laboratories and supported by the U.S. Department of Energy under contract DE-AC04-94AL8500.

2:50 pm INVITED

SOLDER JOINT FORMATION, SIMULATION AND RELIABILITY PREDICTION: Xiaohua Wu, Kai Hu, Xinyu Dou, Gary Mui, Chao-pin Yeh, and Karl Wyatt, Applied Simulation and Modeling Research (ASMR), Corporate Software Center (CSC), Motorola Inc., 1303 E. Algonquin Road, Mail Stop: IL01/ANX2, Schaumburg, IL 60196

The fatigue-induced solder joint failure of surface mounted electronic devices has become one of the most critical reliability issues in electronic packaging industry. Solder joint reliability performance has been found to be highly dependent on the solder joint configuration, which, in turn, is governed by bond pad size, component weight, alloy material, and leadframe structure, as well as solder reflow characteristics. The objective of this work is to develop numerical models: 1) to simulate the solder joint formation during the reflow process; 2) to determine the stress/strain distribution within the joint; and 3) further predict the reliability (fatigue life) of the solder joints. The solder joint formation process during solidification stage can be simulated using the Surface Evolver software tool developed by University of Minnesota. The thermomechanical stress-strain analysis can then be carried out using ANSYS to study selected critical design/manufacturing parameters such as leadframe geometry, pad size and dimensions, solder paste volume, leadframe placement misalignment, etc. This effort also involves the development of interface linking Surface Evolver and ANSYS.

3:15 pm BREAK

3:25 pm INVITED

COMPUTATIONAL CONTINUUM MODELING OF SOLDER JOINT INTERCONNECTS: Steve Burchett, M.K. Neilsen, D.R. Frear, J.J. Stephens, Department 9117, Mail Stop 0443, P.O. Box 5800, Sandia National Laboratories, Albuquerque, NM 87185

The most commonly used solder for electrical interconnects in electronic packages is the near eutectic 60Sn-40Pb alloy. This alloy has a number of processing advantages (suitable melting point of 183°C and good wetting behavior). However, under conditions of cyclic strain and temperature (thermomechanical fatigue) the microstructure of this alloy undergoes a heterogeneous coarsening and failure process that makes prediction of solder joint lifetime complex. A visco-plastic, microstructural dependent, constitutive model for solder has been developed and implemented into a finite element code. With this computational capability, the thermomechanical response of solder interconnects, including microstructural evolution, can be predicted. This capability was applied to predict the thermomechanical response of various solder interconnects to determine the effects of variations in geometry and loading. In this paper, the constitutive model will be briefly discussed and response predicted by the constitutive model will be compared to material test results. Finally, the results of computational studies to determine the effect of geometry and loading variations will be presented. This work was performed at Sandia National Laboratories, and supported by the U. S. Department of Energy under Contract No. DE-AC04-94AL8500.

3:50 pm INVITED

AN ELASTOPLASTIC BEAM MODEL FOR COLUMN-GRID-ARRAY (CGA) SOLDER INTERCONNECTS: Steven M. Heinrich*, J.A. Swanson*, and P.S. Lee**; *Rockwell Automation, Allen-Bradley Co., Milwaukee, WI 53233; **Department of Civil and Environmental Engineering, Marquette University, Milwaukee, WI 53233

A semi-analytical model is developed and implemented to analyze the deformation of solder columns in column-grid-array (CGA) assemblies. Each solder column is modeled as a prismatic beam of circular cross-section, subjected to end shearing deflections caused by thermal mismatch between the module and the circuit board. The solder is idealized as an elastic-perfectly plastic material whose yielding is governed by the Von Mises criterion. Since the columns are relatively short, transverse shear deformation has been incorporated into the beam model. The results generated with the model indicate the following: (a) yielding is governed by bending for slenderness ratios (height-to-diameter) of h/d1/3; (b) the nonlinear stiffness relationship for a sheared column, presented in dimensionless form, reduces to a single curve which is valid for arbitrary values of slenderness ratio (1/3) and material parameters; (c) the dimensionless relationship between maximum shear strain (in the Tresca sense) and the relative end deflection depends on Poisson's ratio but is independent of the other material parameters and the slenderness ratio. The nonlinear stiffness results presented in the paper may be used to create more efficient finite element models of entire assemblies by replacing each column with a single nonlinear spring element. When used in conjunction with an appropriate Coffin-Manson relationship, the maximum shear strain curves presented herein may be utilized to estimate column fatigue life.

4:15 pm INVITED

EFFECT OF ASSEMBLY STIFFNESS AND TEMPERATURE CYCLE PARAMETERS ON THERMOMECHANICAL RESPONSE OF SOLDER JOINTS: Ahmer Syed, Delco Electronics Corporation, M/S T100-26, 700 East Firman Drive, Kokomo, IN 46904

Grain boundary sliding and matrix creep are recognized as the main contributors to creep behavior of eutectic Sn/Pb solder joints in thermomechanical loading. The relative contribution and dominance of these mechanisms in the total response is governed by the external (temperature, assembly stiffness, CTE mismatch) as well as the internal shape (shape, size and thickness of the solder joint) factors. This paper examines the effect of external factors on the grain boundary and matrix creep response of solder joints. Experimental as well as numerical techniques are employed to compare which mechanism dominates under several different assembly stiffness and temperature profile conditions. A fatigue life prediction model which accounts for different mechanisms is used to determine the impact of grain boundary sliding and matrix creep on solder joint fatigue life.

4:40 pm INVITED

BEHAVIOR OF SOLDER JOINTS UNDER COMPLEX DISPLACEMENT LOADING: Matthew G. Bevan, Manfred Wutting, The John Hopkins University, Applied Physics Laboratory, John Hopkins Road, Laurel, MD 20723

In many applications, solder is exposed to a complex fatigue spectrum composed of thermal effects, shock and vibration. In the laboratory; however, solder is usually tested using simple wave forms generated by mechanical testers or by thermal cycling. In order to apply laboratory results to predict the real world fatigue, several crucial steps are necessary. One of these steps is understanding how to combine the effects of simple displacement (or stress) waves to predict the effects of complex waves. One method of combining waves is to use a linear damage accumulation model employing rainflow analysis. Briefly, rainflow analysis establishes an envelope to strain (or stress) range of the waveform, and then measures the reversals within the range. Damage from the envelope is added to the damage from the individual reversals within the envelope to calculate the damage of the whole cycle. The linear damage accumulation model is not valid for all loading conditions. Damage is accumulated in a linear fashion, assuming there is no wave-to-wave interaction. For Mode I (tensile) loading, this assumption is known to be false. Six mechanisms for-wave-to-wave interaction are known. The wave-to-wave interaction can significantly accelerate or delay fatigue crack propagation. In contrast, Mode III (tearing) loading has shown no wave-to-wave interaction. Very little research has been done to investigate damage accumulation in solder. The paper looks at single solder joints in shear. First the fatigue behavior of the solder joints is characterized using sine waves under displacement control. Then by looking at sums of waves with a constant displacement envelope, the damage from minor waves may be separated from damage from the major, envelope wave. The results show that the linear damage accumulation model (using rainflow analysis) is accurate as long as the minor waves remain in the elastic range. Once the minor waves begin to generate plastic deformation, the damage accumulates more rapidly than predicted.

5:05 pm

THERMOMECHNICAL FATIGUE TESTING OF SOLDER ALLOYS: Mark A. Palmer, P.E. Redmond, R.W. Messler, Jr., Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180

Thermomechanical fatigue (TMF) is one of the most common sources of failure in solder joints. TMF occurs due to thermal cycling, as a cyclic stress is induced on the solder joint due to thermal expansion mismatch. Due to the complicated nature of thermomechanical fatigue, directly testing a material's resistance to TMF is not straight forward. A novel test apparatus has been developed which allows the direct measurement of stress due to thermal cycling. The TMF behavior of three alloys, eutectic Sn-Bi, Sn-Pb, and Sn-Ag as examined by this apparatus will be presented. The data will be compared with that generated by other test methods.

5:25 pm INVITED

EVALUATION OF INTERMETALLIC PHASE FORMATION AND CONCURRENT DISSOLUTION OF INTERMETALLIC DURING REFLOW SOLDERING: M. Schaefer, W. Laub, R.A. Fournelle and J. Liang, Materials Science Program, Marquette University, 1515 W. Wisconsin Ave., Milwaukee, WI 53201-1881; Allen-Bradley Company, 1201 S. Second St., Milwaukee, WI, 53204