Microstructure evolution and thermomechanical fatigue of solder materials

The microelectronics industry is confronted with the new challenge to produce joints with lead-free solder materials replacing classical tin-lead solders in devices used in many fields (e.g. consumer electronics, road transport, aviation, space-crafts, telecommunication). In service, solder materials experience a complex thermomechanical load which may result in microstructure evolution, and strain localization. These phenomena may lead to the formation of macroscopic cracks causing premature failure of components and functional loss of devices. Tin crystals are anisotropic, both mechanically and thermally, the effects of which are compensated in tin-lead solders by the presence of the relatively soft isotropic lead (Pb). Sn is the main constituent in the proposed lead-free alloys (e.g. Sn- Ag, Sn-Cu, Sn-Ag-Cu, Sn-Bi, Sn-Zn, Sn-Zn-Bi, Sn-Ag-Bi). For the safe use of any of these alloys, a thorough understanding of their behavior is required. With this in mind this thesis addresses the microstructure evolution and thermo-mechanical fatigue of eutectic Sn-Pb and Pb-free alternatives employing a variety of microscopic techniques and numerical simulation. The coarsening of Pb-rich aPb domains in eutectic Sn-Pb solder during isothermal annealing has been studied in detail. The importance of anisotropy and of coalescence events and the occurrence of a dynamic scaling regime are analyzed. Orientation imaging microscopy revealed the presence of distinct crystallographic orientations between aPb and ßSn lamellae in quenched eutectic Sn-Pb solder. The domain size distribution function is found to approach a dynamic scaling regime and coalescence of domains is shown to be the dominant mechanism for the growth of domains larger than the mean domain size. Strain field localization and its evolution were measured in a number of Sn-based Pb-free solder interconnections which were mechanically shear loaded. The local strain was found to differ significantly from the applied global strain. Strain localization was shown to depend on the geometry of the samples as well as on the microstructure (at a grain level) of the solder. Strain field localization parallel to the solder-Cu interface was evident and failure typically occurred along these regions. The exact location of damage however was not at the intermetallic layer-solder interface, but rather within the solder itself. Cracks also formed along grain boundaries irrespective of the solder type, indicating the importance of microstructure in damage initiation. The junctions of grain boundaries with the interface are the typical locations of strain concentration in the examined Pb-free solder. A good correlation has been established between the calculated strain fields and observed failures. Next, the effects of the intrinsic thermal anisotropy of Sn were studied in mechanically unconstrained SAC alloy under thermal fatigue. Damage was localized mainly along high angle tilt Sn grain boundaries. It has been demonstrated from a combination of Orientation Imaging Microscopy and Finite Element Modelling that encountered fatigue damage and stresses resulting from the thermal anisotropy of Sn are highly correlated. Microstructure evolution in a Pb-free SAC solder alloy was studied during low cycle mechanical fatigue. Digital Image Correlation was employed to measure the strain-field localization during fatigue. Fatigue damage is correlated well with the measured localized strains. The effect of the elastic (i.e. mechanical) anisotropy on the onset of microscopic slip was found to be small (as shown by elasticity-based finite element calculations). Grain boundaries were not particularly highly stressed and thereby no sign of grain-boundary decohesion or sliding was observed. Plastic anisotropy strongly influences the initiation of microscopic glide during fatigue. Plastic deformation was localized in grains with favorably oriented slip systems with respect to the stress state. On these preferred slip systems the evolution of Persistent Slip Bands was revealed in the Sn dendrites. Microcrack formation was exhibited near the interfaces between these persistent slip bands and hard eutectic regions in the SAC. In practice, a combination of extrinsic and intrinsic thermal mismatches are crucial factors controlling fatigue damage in solder joints. In this context, fatigue damage evolution in SAC solder interconnections was investigated in detail under thermomechanical fatigue. SAC joints subjected to thermomechanical loads showed a combination of the microstructural phenomena encountered in purely thermal and mechanical fatigue. The stress distribution inside the soldered joints shows localization of high stresses at the solder-Cu interface, along high angle tilt grain boundaries, resulting from the differences in thermal expansion coefficients on a sample scale (since different materials are involved) and on a grain scale (determined by the Snanisotropy). The damage encountered in thermomechanical fatigue is correlated with the locations of high stress. The fatigue damage within solder exhibits damage initiation at grain boundaries (intrinsic thermal fatigue contribution) as well as the formation of Persistent Slip Bands (mechanical fatigue contribution). In addition, the correlation between the observed damage and the calculated stress fields provides evidence that three crucial factors: thermal mismatch between Cu and solder, intrinsic thermal mismatches caused by Sn anisotropy and the mechanical constraints posed by the Cu on the soldered joint determine the location and severeness of fatigue damage in solder joints. The effects of thermal, mechanical, and thermomechanical fatigue on the microstructural evolution of SAC solder have been analysed in detail, using microscopic and numerical techniques. The thermal anisotropy of tin has been shown to have a significant influence in fatigue damage initiation in lead-free solders. In the replacement of tin-lead solders by lead-free alternatives this is a crucial aspect. The impact of this effect on practical industrial applications is yet to be investigated by the microelectronics industry.