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A viscoplastic theory for braze alloys

Description: A new viscoplastic theory for CusilABA and other braze alloys has been developed. Like previous viscoplastic theories,this new theory uses a hyperbolic sine function of effective stress in its kinetic equation for the inelastic strain rate. This new theory has an internal state variable which accounts for isotropic hardening and recovery and a second-order, internal state tensor which accounts for kinematic hardening and recovery. Unlike previous theories, the new theory uses evolution equations for the state variables which describe competing mechanisms of power law hardening and static recovery. The evolution equations used in previous theories describe competing mechanisms of linear hardening, dynamic and static recovery. The new viscoplastic theory was implemented in several finite element codes and used in several metal-to-ceramic brazing simulations. Two approaches for obtaining material parameters for the new viscoplastic theory were developed.
Date: April 1, 1996
Creator: Neilsen, M.K.; Burchett, S.N.; Stone, C.M. & Stephens, J.J.
Partner: UNT Libraries Government Documents Department

Microstructurally based finite element simulation of solder joint behavior

Description: 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 183C 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 the prediction of solder joint lifetime complex. A finite element simulation methodology to predict solder joint mechanical behavior, that includes microstructural evolution, has been developed. The mechanical constitutive behavior was incorporated into the time dependent internal state variable viscoplastic model through experimental creep tests. The microstructural evolution is incorporated through a series of mathematical relations that describe mass flow in a temperature/strain environment. The model has been found to simulate observed thermomechanical fatigue behavior in solder joints.
Date: January 1, 1996
Creator: Frear, D.R.; Burchett, S.N.; Neilsen, M.K. & Stephens, J.J.
Partner: UNT Libraries Government Documents Department

Validating theoretical calculations of thermomechanical stress and deformation using the ATC4.1 flip-chip test vehicle

Description: Two closed form analytical solutions for tri-material thermomechanical stress and deformation, along with one-quarter section finite element model (FEM), were validated using an in-situ CMOS piezoresistive stress measurement test chip that has been repatterened into a fine pitch area array flip-chip. A special printed circuit board substrate for the test chip was designed at Sandia and fabricated by the Hadco Corp. The flip-chip solder attach (FCA) and underfill was performed by a SEMATECH member company. The measured incremental stresses produced by the underfill are reported and discussed for two underfill materials used in this experiment. Detailed comparisons between theory and experiment are presented and discussed.
Date: August 1, 1997
Creator: Peterson, D.W.; Sweet, J.N. & Burchett, S.N.
Partner: UNT Libraries Government Documents Department

A comparison of LBW and GTAW processes in miniature closure welds

Description: When small electronic components with glass-to-metal seals are closure welded, residual stresses developed in the glass are of concern. If these stresses exceed allowable tensile levels` the resulting weld-induced seal failure may cause the entire component to be scrapped or reworked at substantial cost. Conventional wisdom says the best welding process for these applications is that which provides the least heat input, and that Laser Beam Welding (LBW) provides less heat input than Gas Tungsten Arc Welding. (GTAW); however, other concerns such as weld fit-up, part variability, and material weldability can modify the final choice of a welding process. In this paper we compare the characteristic levels of heat input and the residual stresses generated in the glass seals for the two processes (as calculated by 3D Finite Element Analysis) as a function of heat input and travel speed, and contrast some of the other manufacturing decisions that must be made to choose a production process. The geometry chosen is a standing edge corner weld in a cylindrical container about 20 mm diameter by 35 mm tall. Four metal pins are glassed into the part lid. The stresses calculated to result from continuous wave C0{sub 2} LBW are compared with those that result from GTAW. The total energy required by the laser weld is significantly less than for the equivalent size GTA weld. The energy input required for a given size weld is inversely proportional to the travel speed, but approaches a saturation level as the travel speed increases. LBW travel speeds ranging from 10 mm/sec to 50 mm/sec were examined.
Date: July 1, 1995
Creator: Knorovsky, G.A.; Fuerschbach, P.W.; Gianoulakis, S.E. & Burchett, S.N.
Partner: UNT Libraries Government Documents Department

Computational continuum modeling of solder interconnects

Description: The most commonly used solder for electrical interconnections 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 viscoplastic, microstructure dependent, constitutive model for solder which is currently in development was 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 leadless chip carrier solder interconnects to determine the effects of variations in geometry and loading. In this paper, the constitutive model will first be briefly discussed. The results of computational studies to determine the effect of geometry and loading variations on leadless chip carrier solder interconnects then will be presented.
Date: March 1, 1997
Creator: Burchett, S.N.; Neilsen, M.K.; Frear, D.R. & Stephens, J.J.
Partner: UNT Libraries Government Documents Department

Computational continuum modeling of solder interconnects: Applications

Description: The most commonly used solder for electrical interconnections in electronic packages is the near eutectic 60Sn-40Fb alloy. This alloy has a number of processing advantages (suitable melting point of 183C 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 the prediction of solder joint lifetime complex. A viscoplastic, microstructure dependent, constitutive model for solder, which is currently under development, was 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 a mini ball grid array solder interconnect. In this paper, the constitutive model will first be briefly discussed. The results of computational studies to determine the thermomechanical response of a mini ball grid array solder interconnects then will be presented.
Date: April 1, 1997
Creator: Burchett, S.N.; Neilsen, M.K. & Frear, D.R.
Partner: UNT Libraries Government Documents Department

Life prediction modeling of solder interconnects for electronic systems

Description: A microstructurally-based computational simulation is presented that predicts the behavior and lifetime of solder interconnects for electronic applications. This finite element simulation is based on an internal state variable constitutive model that captures both creep and plasticity, and accounts for microstructural evolution. The basis of the microstructural evolution is a simple model that captures the grain size and microstructural defects in the solder. The mechanical behavior of the solder is incorporated into the model in the form of time-dependent viscoplastic equations derived from experimental creep tests. The unique aspect of this methodology is that the constants in the constitutive relations of the model are determined from experimental tests. This paper presents the constitutive relations and the experimental means by which the constants in the equations are determined. The fatigue lifetime of the solder interconnects is predicted using a damage parameter (or grain size) that is an output of the computer simulation. This damage parameter methodology is discussed and experimentally validated.
Date: February 1, 1997
Creator: Frear, D.R.; Burchett, S.N. & Neilsen, M.K.
Partner: UNT Libraries Government Documents Department

The effect of travel speed on thermal response in CO{sub 2} laser welding of small electronic components

Description: A comprehensive three-dimensional numerical investigation of the effect of beat source travel speed on temperatures and resulting thermal stresses was performed for CO{sub 2}-laser welding. The test specimen was a small thermal battery header containing several stress-sensitive glass-to-metal seals surrounding the electrical connections and a temperature sensitive ignitor located under the header near the center. Predictions of the thermal stresses and temperatures in the battery header were made for several travel speeds of the laser. The travel speeds examined ranged from 10mm/sec to 50mm/sec. The results indicate that faster weld speeds result in lower temperatures and stresses for the same size weld. This is because the higher speed welds are more efficient, requiring less energy to produce a given weld. Less energy absorbed by the workpiece results in lower temperatures, which results in lower stresses.
Date: February 1, 1995
Creator: Gianoulakis, S.E.; Burchett, S.N.; Fuerschbach, P.W. & Knorovsky, G.A.
Partner: UNT Libraries Government Documents Department

Effects of microstructural heterogeneity on BGA reliability

Description: The near eutectic 60Sn-40Pb alloy is the most commonly used solder for electrical interconnections in electronic packages. 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 the prediction of solder joint lifetime complex. A viscoplastic constitutive model for solder with an internal state variable that tracks microstructural evolution is currently under development. This constitutive model was implemented in to several finite element codes. 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 a ball grid array (BGA) solder interconnect. BGAs with both homogeneous and heterogeneous initial microstructures were evaluated. In this paper, the constitutive model used to describe the solder will first be briefly discussed. The results of computational studies to determine the thermomechanical response of BGA solder interconnects will then be presented.
Date: July 1, 1998
Creator: Neilsen, M.K.; Burchett, S.N.; Fang, H.E. & Vianco, P.T.
Partner: UNT Libraries Government Documents Department

Computer simulation of solder joint failure

Description: The thermomechanical fatigue failure of solder joints is increasingly becoming an important reliability issue for electronic packages. The purpose of this Laboratory Directed Research and Development (LDRD) project was to develop computational tools for simulating the behavior of solder joints under strain and temperature cycling, taking into account the microstructural heterogeneities that exist in as-solidified near eutectic Sn-Pb joints, as well as subsequent microstructural evolution. The authors present two computational constitutive models, a two-phase model and a single-phase model, that were developed to predict the behavior of near eutectic Sn-Pb solder joints under fatigue conditions. Unique metallurgical tests provide the fundamental input for the constitutive relations. The two-phase model mathematically predicts the heterogeneous coarsening behavior of near eutectic Sn-Pb solder. The finite element simulations with this model agree qualitatively with experimental thermomechanical fatigue tests. The simulations show that the presence of an initial heterogeneity in the solder microstructure could significantly degrade the fatigue lifetime. The single-phase model was developed to predict solder joint behavior using materials data for constitutive relation constants that could be determined through straightforward metallurgical experiments. Special thermomechanical fatigue tests were developed to give fundamental materials input to the models, and an in situ SEM thermomechanical fatigue test system was developed to characterize microstructural evolution and the mechanical behavior of solder joints during the test. A shear/torsion test sample was developed to impose strain in two different orientations. Materials constants were derived from these tests. The simulation results from the two-phase model showed good fit to the experimental test results.
Date: April 1997
Creator: Burchett, S. N.; Frear, D. R. & Rashid, M. M.
Partner: UNT Libraries Government Documents Department

Stresses from flip-chip assembly and underfill; measurements with the ATC4.1 assembly test chip and analysis by finite element method

Description: The authors report the first measurements of in-situ flip-chip assembly mechanical stresses using a CMOS piezoresistive test chip repatterned with a fine pitch full area array. A special printed circuit board substrate was designed at Sandia and fabricated by the Hadco Corp. The flip-chip solder attach (FCA) and underfill was performed by a SEMATECH member company. The measured incremental stresses produced by the underfill are reported and discussed for several underfill materials used in this experiment. A FEM of a one-quarter section of the square assembly has been developed to compare with the measured as-assembled and underfill die surface stresses. The initial model utilized linear elastic constitutive models for the Si, solder, underfill, and PC board components. Detailed comparisons between theory and experiment are presented and discussed.
Date: December 31, 1996
Creator: Peterson, D.W.; Sweet, J.N.; Burchett, S.N. & Hsia, A.
Partner: UNT Libraries Government Documents Department

Coarsening of the Sn-Pb Solder Microstructure in Constitutive Model-Based Predictions of Solder Joint Thermal Mechanical Fatigue

Description: Thermal mechanical fatigue (TMF) is an important damage mechanism for solder joints exposed to cyclic temperature environments. Predicting the service reliability of solder joints exposed to such conditions requires two knowledge bases: first, the extent of fatigue damage incurred by the solder microstructure leading up to fatigue crack initiation, must be quantified in both time and space domains. Secondly, fatigue crack initiation and growth must be predicted since this metric determines, explicitly, the loss of solder joint functionality as it pertains to its mechanical fastening as well as electrical continuity roles. This paper will describe recent progress in a research effort to establish a microstructurally-based, constitutive model that predicts TMF deformation to 63Sn-37Pb solder in electronic solder joints up to the crack initiation step. The model is implemented using a finite element setting; therefore, the effects of both global and local thermal expansion mismatch conditions in the joint that would arise from temperature cycling.
Date: April 12, 1999
Creator: Vianco, P.T.; Burchett, S.N.; Neilsen, M.K.; Rejent, J.A. & Frear, D.R.
Partner: UNT Libraries Government Documents Department

Calculation and Validation of Thermomechanical Stresses in Flip Chip BGA Using the ATC4.2 Test Vehicle

Description: We report the first in situ measurements of thermomechanical stresses in a 1000 I/O 250 {micro}m pitch piezoresistive flip chip test chip assembled to a 755 I/O 1.0 mm pitch 35 mm Ball Grid Array (BGA). The BGA substrates employed build-up dielectric layers containing micro-vias over conventional fiberglass laminate cores. Experimental data, which include in situ stress and die bending measurements, were correlated to closed form and Finite Element Method (FEM) calculations. Cracking and delamination were observed in some of the experimental groups undergoing temperature cycling. Through use of bounding conditions in the FEM simulations, these failures were associated with debonding of the underfill fillet from the die edge that caused stresses to shift to weaker areas of the package.
Date: March 9, 1999
Creator: Burchett, S.N.; Mitchell, R.T.; Nguyen, L.; Peterson, D.W. & Sweet, J.N.
Partner: UNT Libraries Government Documents Department