Thermomechanical fatigue failure of interfaces in lead-free solders

M.E. Erinc

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The European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS) banned lead from electronic systems from July 1, 2006 onwards, which has led to much interest in leadfree solders in the past years. Among several lead-free solder alternatives, SnAgCu is a widely accepted replacement due to its better creep-fatigue resistance and microstructural stability. SnAgCu has been extensively studied in the past decade, however, there are still issues to be resolved concerning solder reliability, the underlying mechanisms of thermo-mechanical fatigue failure, fatigue life predictions and the overall effect of decreasing component size, driven by the ongoing miniaturization trend. This thesis aims to scientifically contribute to this subject by a coupled experimental-numerical approach. In solder joint reliability, the bump/pad interface has a crucial role, the quality of which is determined by the metallization and interfacial defects. Solder balls, solder paste and cast eutectic SnAgCu are reflowed on Cu, Ni/Au and Cu/Ni(V)/Au metallization layers and the substrate influence on the bulk and interfacial metallurgy is examined. The damage propagation at SnAgCu soldered joints on Cu and Ni/Au substrates are investigated and microstructure related damage localization is identified as the dominant failure mechanism. Therefore, continuum damage approaches are believed to be inadequate for solder joint reliability predictions. Nano-indentation and tensile testing is used for the mechanical characterization of SnAgCu. An assessment on indentation parameters for solders is conducted and the influence of the Ag content on material properties of SnAgCu is presented. One of the main causes of ball grid array (BGA) failure is thermo-mechanical fatigue crack propagation in the solder, which is almost always observed at the bump/pad junction. Motivated by this fact, a combined experimental-numerical study on the cyclic mechanical response of SnAgCu/Ni-Au interface is conducted. In this study, damage evolution at the bond/pad interface is characterized by dedicated fatigue tests. Local deformations leading to crack propagation are simulated by separation of interfaces through a cohesive zone approach. Solder joints are tested under cyclic shear and cyclic tension for different specimen sizes and strain amplitudes. Two different damagemechanisms are observed: local deformations in the bulk and at the bonding interface. The interfacial failure mode is typically favored at a high initial stress, and a small solder volume. Crack propagation is simulated by an irreversible linear traction-separation cohesive zone law accompanied by a non-linear interfacial damage parameter. Later, tensile and shear experiments are used to characterize the cohesive zone parameters for the normal and the tangential opening, respectively. Interfacial fatigue damage in BGA solders is caused by the difference in coefficient of thermal expansion (CTE) of the materials in the package. Apart from this thermal incompatibility in the package, Sn based solders are themselves prone to thermal fatigue damage due to the intrinsic thermal anisotropy of the ß-Sn phase. Thermal fatigue causes local deformations especially at the grain boundaries. Hence, the thermal fatigue response of bulk SnAgCu is investigated as well. Bulk SnAgCu specimens are thermally cycled between -40 and 125¿C and mechanically tested afterwards in order to quantify the thermal fatigue damage. A size dependent cyclic softening behavior is observed. Test specimens are individually modeled including the microstructure and local crystallographic orientations, on the basis of orientation imaging scans (OIM). Both thermal cycling and tensile testing are imposed as boundary conditions. Reproducing the experimental results in the simulations, parameters of a cohesive zone based intergranular fatigue damagemodel are identified. Finally, the intergranular damage law characterized in this study is combined with the bump/pad interfacial damage law, and a 2Dmicrostructure-incorporated fatigue life prediction tool is established. Using this tool, it is shown that the failure mode of a soldered joint depends extensively on its geometry. The model presented above is extended to 3D for a more complete description of the problem. To provide the microstructural input, a database containing OIM scans of several SnAgCu solder balls is constructed. A missing constituent in the model so far, interfacial defects, i.e. voids, are examined statistically using newly manufactured BGA packages, revealing information on their size, position and frequency. Combining all the data collected, i.e. material properties, microstructure, defects, local damage laws, a 3D slice model from a BGA package is constructed. The slice model contains a single solder ball connecting the board and the chip. A series of case studies is created using experimental input such as different microstructures and initial defects allowing a statistical analysis. Fatigue life of these models are predicted and the results are validated by failure distribution analyses of BGA packages provided by the industry. Here the critical solder ball assumption is made: if a solder ball fails, the electrical circuit of the BGA package is open, thus the package fails. Setting a critical damage value for the interfaces accumulating fatigue damage, a good agreement with the experiments and simulations is obtained. It is seen that microstructural modeling allows to predict and understand the scatter in the solder ball fatigue life observed in reality. Finally, the effect of solder ball size and geometry on interconnect reliability is dis cussed on the basis of numerical analyses. For this purpose, a geometry factor and a microstructure factor is defined, and their influence on damage evolution is discussed
Originele taal-2Engels
KwalificatieDoctor in de Filosofie
Toekennende instantie
  • Mechanical Engineering
Begeleider(s)/adviseur
  • Geers, Marc G.D., Promotor
  • Schreurs, Piet, Co-Promotor
Datum van toekenning3 dec. 2007
Plaats van publicatieEindhoven
Uitgever
Gedrukte ISBN's978-90-386-1149-5
DOI's
StatusGepubliceerd - 2007

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