Improvement of Thermal Fatigue Properties of Sn-Ag-Cu Lead-Free Solder Interconnects on Casio’s Wafer-Level Packages Based on Morphology and Grain Boundary Character

Thermal cycling was imposed on plastic ball grid array (PBGA) packages with a small die, a package design that does not impose a large strain on solder joints. Less cracking was observed after 2500 cycles from 0°C to 100°C (with 10 min dwell times and 10 min ramps) than in a prior study with a higherstress package design, so these samples were thermally cycled (TC) to 6400 cycles to investigate the relationship between cracks, microstructure, and grain crystal orientation. Cracked joint locations within the package were identified using the dye and pry method, indicating that cracks were most often found in joints near the perimeter of the die. Using orientation imaging microscopy (OIM), cracks were observed in many joints having a variety of dominant crystal orientations where the c-axis was between 0°and $50°from the package interface. Continuous recrystallization processes occurred and caused gradual rotations of initial orientations that reduced the angle between the c-axis and the package interface. While cracks were observed in joints with a variety of orientations, cracks were highly correlated with recrystallized grains having the [001] c-axis nearly parallel to the interface (''red'' orientations) in those joints that did not initially have this orientation.

Section: Introductionmentioning

confidence: 99%

Crack Development in a Low-Stress PBGA Package due to Continuous Recrystallization Leading to Formation of Orientations with [001] Parallel to the Interface

Zhou

Bieler

Lee

et al. 2010

“…2b), which is in good agreement with results obtained in previous studies. 11,15,32 This indicates that nucleation of the Sn grains is suppressed because more latent heat of freezing is released during the solidification process due to the high entropy of fusion of tetragonal Sn (14 J/mol K). [33][34][35] However, fast growth of b-Sn dendrites could be achieved with the significant undercooling normally associated with Sn and Sn-rich solder alloys, leading to the few grains present in the solder interconnects.…”

Section: Resultsmentioning

confidence: 99%

“…However, it should be noted that a high-angle grain boundary could be assumed as a potential cracking site (or preferred grain boundary sliding site) due to its high energy. 32 If the high-angle boundary coincides with a high stress concentration area, it will accelerate initiation and propagation of cracks or change the cracking path and corresponding failure mode, leading to fast degradation of solder interconnects. On the contrary, low-energy boundaries (low angle and coincidence site lattice) may contribute to enhance the thermal fatigue properties by suppressing grain boundary sliding due to their low energy.…”

Section: Facilitated Cracking By Anisotropic Thermomechanical Responsmentioning

confidence: 99%

“…5 The observation of localized recrystallization behavior in Pb-free BGA solder interconnects in the present study is in good agreement with previous studies on practical solder interconnects due to the similar size and structures. [5][6][7][8][9][10][24][25][26][27][28]32 Based on what has been observed at the very beginning of the recrystallization, as shown in Fig. 5a-c, the dynamic recrystallization should have been induced by subgrain rotation; i.e., the newly formed recrystallized grains evolved from subgrains by gradual rotation under cyclic stress during the thermomechanical fatigue process.…”

Section: Localized Recrystallization Induced By Subgrain Rotationmentioning

confidence: 99%

See 1 more Smart Citation

Localized Recrystallization Induced by Subgrain Rotation in Sn-3.0Ag-0.5Cu Ball Grid Array Solder Interconnects During Thermal Cycling

Chen

Han

2011

The evolution of microstructures and grain orientations of a Pb-free solder interconnect during thermal cycling significantly affects its mechanical properties and failure modes. Thus, Sn-3.0Ag-0.5Cu ball grid array assemblies were subjected to thermal cycling to study the thermomechanical responses of the solder interconnects. The orientations and microstructures of the solder interconnects were studied by optical microscopy with cross-polarized light and scanning electron microscopy with an electron backscattered diffraction analysis system. Localized recrystallization behavior was observed in Pb-free solder interconnects during thermal cycling. Closer examination of the very early stage of recrystallization in the same solder interconnect revealed that the subgrains appeared before the formation of the recrystallized grains, and the orientations of the small recrystallized grains separated by high-angle grain boundaries evolved from the initial orientations by subgrain rotation. The localized recrystallization produced fine-grained microstructures during thermal cycling, providing an additional deformation mechanism for the solder interconnects, i.e., grain boundary sliding, which would have been impossible prior to recrystallization. The grain orientation has a strong effect on damage generation and the subsequent failure mode; initiation and propagation of cracks could be facilitated by the intrinsic anisotropic thermomechanical responses of the differently oriented grains, leading to a change in the crack propagation path and corresponding failure mode.

“…[10][11][12][13] However, fatigue cracks are frequently observed in the solder bulk instead of in the interfacial IMCs in Pb-free solder interconnects under thermal cycling tests. 14,15 Therefore, it becomes increasing important to study the orientation, microstructure, and deformation behavior of the Pb-free solder alloy itself to obtain better understanding of the failure modes of solder interconnects. Especially, for finite-element modeling, a solder interconnect should not be assumed to be a homogeneous and isotropic structure, because this cannot represent the actual stress and strain distribution in the solder interconnect without the details of the b-Sn grain shape, number, and corresponding crystallographic orientations.…”

Section: Introductionmentioning

confidence: 99%

Grain Orientation Evolution and Deformation Behaviors in Pb-Free Solder Interconnects Under Mechanical Stresses

Chen

Wang

Han

et al. 2011

In this study, the effect of mechanical stresses on the recrystallization behavior of Sn-3.0Ag-0.5Cu Pb-free solder interconnects was studied by fourpoint cyclic bending, shear tests, and tensile tests. Scanning electronic microscopy with electron backscattered diffraction was used to characterize the microstructure and crystallographic orientation of the solder interconnects. The results show that recrystallization occurs under the different mechanical stresses in these tests at room temperature, and that recrystallized grains evolve from subgrains by rotation. Microhardness measurements after shear tests show that the hardness of the recrystallized microstructure was decreased by 15% and 41%, respectively, compared with that of the nonrecrystallized and as-solidified microstructures. Furthermore, the finegrained microstructure produced after recrystallization facilitated grain boundary sliding. Therefore, the deformation and cracking behaviors were localized in the degraded recrystallized microstructures once recrystallization occurred, accelerating failure of the solder interconnects.