Pb free solder is fast becoming a reality in electronic manufacturing due to marketing and legislative pressures. The industry has pretty much concluded that various versions of SnAgCu solder alloy offer the best alternative for eutectic Sn/Pb solder currently in use. With the current trend of cheaper, faster, and better electronic equipment, it has become increasingly important to evaluate the package and system performance very early in the design cycle using simulation tools. This requires life prediction models for new solder alloy systems so that the package-to-board interconnect reliability can be predicted for various environmental and field conditions. This paper describes in detail the life prediction models for SnAgCu solder joints. The models are based on published constitutive equations for this alloy and thermal cycle fatigue data on actual components. The approach uses advanced finite element modeling and analysis techniques and is based on mechanics of deformation. Both accumulated creep strain and creep strain energy density based models are developed. The model has been correlated with a number of data points and predicts life within 25% in most cases. The framework of modeling and prediction methodology described here is fully compatible with the framework used for SnPb solder previously.

Accumulated creep strain and energy density based thermal fatigue life prediction models for SnAgCu solder joints

A constitutive equation for a typical SnPb eutectic or near-eutectic solder joint is developed, based on empirical data in shear and generalized to three dimensions. Three strain components are considered: elastic, time independent plastic, and steady-state creep. A continuum mechanics rather than a metallurgical approach is taken with emphasis on the formulation of an equation useful for predicting solder behavior under a variety of conditions. Solutions of the constitutive equation predict the experimental hysteresis data for the loading histories available. Solutions of the equation for the test conditions of three independent sets of solder fatigue data show that the equation, together with the matrix creep failure indicator, can give an estimate of fatigue life. >

Constitutive relation and creep-fatigue life model for eutectic tin-lead solder

Solder joint reliability has received a renewed interest since the inception of BGA and CSP type packages. While traditionally board level reliability during thermal and power cycling is considered important for most applications, solder joint failures due to board bending have also become a major reliability concern for portable applications. With the current trend of cheaper, faster, and better electronic equipment, it has become increasingly important to evaluate the package and system performance very early in the design cycle using simulation tools. Although a number of life prediction models for solder joint under thermal cycle conditions have been proposed in the literature, not enough work has been reported in extending these models to power and bend cycle simulations. The accuracy of life prediction tool has also become critically important, as the designs need to be evaluated and improved with high degree of confidence not through relative comparison but by providing absolute numbers. This paper describes in detail the life prediction model for solder joints for thermal cycle conditions and its extension to power and bend, cycle conditions. The approach uses advance finite element modeling and analysis techniques such as constraint equations and sub-structuring and is based on mechanics of deformation. The model has been correlated with more than 60 data points and predicts life within 25% in most cases. The framework of modeling and prediction methodology described here is fully applicable for developing life prediction models for Pb free solder also, once the deformation mechanisms are identified.

Predicting solder joint reliability for thermal, power, and bend cycle within 25% accuracy

A life prediction approach for solder joints under thermal fatigue, based on fracture mechanics and assuming that the thermal fatigue crack propagation in solder joints is primarily controlled by the C* and the J Integrals, is presented. The approach is applied to model experiments in which surface mounted electronic components were mounted on substrates with eutectic Pb/Sn solder joints and subjected to thermal cycling. The constitutive equation used for eutectic solder takes into account elasticity, time-independent plasticity, and power law secondary creep. Shear stress, strain components, and crack growth history in the solder joint are solved numerically by the Runge-Kutta method. The results are compared with both experimental data and predictions based on a modified Manson-Coffin equation. Good agreement is found between the present results and the experimental data, while the Manson-Coffin predictions are inconsistent with either the present ones or the experimental data. Examples illustrate how to apply the approach in order to design accelerated thermal cycling tests. >

A fracture mechanics approach to thermal fatigue life prediction of solder joints

Viscoplastic finite-element simulation methodologies were utilized to predict solder joint reliability for a same die size, stacked, chip scale, ball grid array package under accelerated temperature cycling conditions (-40C to +125C, 15 min ramps/15 min dwells). The effects of multiple die attach material configurations were investigated along with the thickness of the mold cap and spacer die. The solder structures accommodate the bulk of the plastic strain that is generated during accelerated temperature cycling due to the thermal expansion mismatch between the various materials that encompass the stacked die package. Since plastic strain is a dominant parameter that influences low-cycle fatigue, it was used as a basis for evaluation of solder joint structural integrity. The paper discusses the analysis methodologies as implemented in the ANSYS finite element simulation software tool and the corresponding results for the solder joint fatigue life.

Finite element based solder joint fatigue life predictions for a same die size-stacked-chip scale-ball grid array package

An analytical framework to predict failure of solders under creep conditions is proposed. A creep-rupture model for two-phase eutectic solders, based on both micromechanics and fracture mechanics, has been developed. The general agreement between the model predictions and reported creep-rupture data in the literature for lead/tin eutectic solder indicates that the mode and mechanisms proposed in the model may control the solder creep-rupture process. >

A creep-rupture model for two-phase eutectic solders

The White River Bridge will become a key link extending the regional Foothills Trail, which when complete will run more than 48-km through communities and scenic landscapes in the shadow of Mount Rainier. The three-span arch footbridge will become a destination where users can linger over the beautiful White River. It will also provide a much-needed secondary route for emergency vehicles.The total bridge length of 174m is divided over three network arch spans. Network arches create a very transparent structure that is very stiff. The crisscrossing hanger pattern of the structure can carry heavy point loads from a fire truck without distressing members. Generous lookouts at the central pier will provide an area for users to linger and take in views of the river.Key elements of the design included selection of the preferred geometry of the arch spans, optimization of the hanger pattern, fabrication and construction considerations, and seismic design approach using base isolation of the superstructure atop the supporting piers. The erection sequence of the large spans has been an up-front consideration with every effort to minimize disturbance to the wooded site. User experience and aesthetics are a focus of design, but with equal emphasis on economy and efficiency. This footbridge showcases how all these aspirations can be achieved with the right design approach.

D.E. Helling

Papers

A creep-rupture model for two-phase eutectic solders

Connecting a Region: Foothills Trail White River Bridge