P.J.G. Schreurs

Residual stresses in microelectronics induced by thermoset packaging materials during cure

This paper presents a constitutive model for predicting the stresses in thermosetting resins during cure. An overview is given of the experimental techniques used for determining the parameters in this model. The model is validated by comparing its predictions to additional measurements, which have not been used for the actual parameter estimation. This validation showed that the model is capable of giving fair predictions of the measured stresses. The model is implemented in a commercially available finite element package and its use is demonstrated by applying it to the study of a flip-chip underfilling process.

Microstructure evolution of tin-lead solder

Microstructural length scales are relatively large in typical soldered connections. The microstructure which is continuously evolving is known to have a strong influence on damage initiation and propagation. In order to make accurate lifetime predictions by numerical simulations, it is therefore necessary to take the microstructural evolution into account. In this work, this is accomplished by using a diffuse interface model based on a strongly nonlocal variable. It can be seen as an extension of the Cahn-Hilliard model, which is weakly nonlocal since it depends on higher-order gradients which are by definition confined to the infinitesimal neighborhood of the considered material point. Next to introducing a truly nonlocal measure in the free energy, this nonlocal formulation has the advantage that it is numerically more efficient. Additionally, the model is extended to include elastically stored energy as a driving force for diffusion, after which the entire system is solved using the finite element approach. The model results in a computational efficient algorithm which is capable of simulating the phase separation and coarsening of a solder material caused by combined thermal and mechanical loading.

A coupled numerical and experimental study on thermo-mechanical fatigue failure in SnAgCu solder joints

In ball grid array (BGA) packages, solder balls are exposed to cyclic thermo-mechanical strains arising from the thermal mismatch between package components. Since fatigue cracks in solder balls are observed generally at the chip side junction, dedicated fatigue experiments are conducted using eutectic SnAgCu- Ni/Au specimens in order to mechanically characterize the bonding interface. Sn based solders are prone to thermal fatigue due to the intrinsic thermal anisotropy of the beta-Sn phase. Bulk SnAgCu specimens are thermally cycled and mechanical tests are conducted to quantify the thermal fatigue damage. In both damage schemes a strong size effect is observed. Experimental results are used to develop a cohesive zone based fatigue damage evolution law. Fatigue crack propagation is predicted by an irreversible linear traction-separation cohesive zone law accompanied by a non-linear damage variable. Finally, bulk damage in SnAgCu due to thermal fatigue and the interfacial fatigue failure in BGA balls are combined to simulate a BGA solder ball exposed to thermo- mechanical fatigue in 2D. This combined approach gives a more realistic outcome when determining the overall mechanical response, since the microstructural entities and the solder ball itself are on the same size scale and thus the solder ball cannot be treated as a continuum.

Impact of miniaturisation of solder joint reliability

The ongoing miniaturisation trend in the microelectronics industry enforces component sizes to get smaller, and possibly their geometrical shapes to change. In this study, the thermomechanical fatigue life of lead-free solder joints is investigated with respect to solder size and geometry. In ball grid array packages the pitch size is correlated to the solder ball diameter and the stand-off height. In this study, pitch size is also correlated to the contact angle between the solder and the substrate. Finite element models are constructed for various contact angles with different stand-off heights. Bump/pad interfaces and the bulk solder are analysed by determining plastic strain accumulation and equivalent Von Mises stresses resulting from thermal loading. Geometrical and microstructural criticalities are quantified by describing a geometry factor and a microstructure factor. The geometry factor is related to the shape of the solder, and the microstructure factor is related to the grain boundary area and the mismatch of local crystallographic orientations at the grain boundaries. The influence of these two factors on solder reliability is discussed.

Advances in Delamination Modeling of Metal/Polymer Systems: Continuum Aspects

Adhesion and delamination have been pervasive problems hampering the performance and reliability of micro- and nano-electronic devices. In order to understand, predict, and ultimately prevent interface failure in electronic devices, development of accurate, robust, and efficient delamination testing and prediction methods is crucial. Adhesion is essentially a multi-scale phenomenon: at the smallest scale possible, it is defined by the thermodynamic work of adhesion. At larger scales, additional dissipative mechanisms may be active which results in enhanced adhesion at the macroscopic scale and are the main cause for the mode angle dependency of the interface toughness. Undoubtedly, the macroscopic adhesion properties are a complex function of all dissipation mechanisms across the scales. Thorough understanding of the significance of each of these dissipative mechanisms is of utmost importance in order to establish physically correct, unambiguous values of the adhesion properties, which can only be achieved by proper multi-scale techniques.

Reliability of SnAgCu solder balls in packaging

SnAgCu ternary alloy is a commonly used lead-free alternative for SnPb solder. However, subjects regarding its thermo-mechanical response, microstructural evolution, material degradation and deformation mechanisms need further investigation. In BGA solder balls it is seen that deformation and damage propagation is strongly dependent on the microstructure in bulk as well as the bump/pad interfaces. The initial microstructure and material properties are collected through E-SEM observations and nano-indentation experiments. Critical interfaces under certain loading conditions are evaluated by inspecting fracture surfaces of SnAgCu solder paste reflowed on different substrates. These experimental results are supported by thermal cycling of commercial BGA packages with SnAgCu solder balls. A finite element model of a SnAgCu solder ball with Cu/Ni/Au metallization is made in 2-D. A cycling mechanical load is applied. Important microstructural constituents are also placed in the finite element model, and interfacial failure is analyzed using a cohesive zone model.