Yishu Wang

Effects of Sn grain c- axis on electromigration in Cu reinforced composite solder joints

Sn grain orientation in solder matrix has recently been considered as one of the principal failure contributions in lead-free solder joints. Since β-Sn exhibits high anisotropy, the differences of electromigration behavior of solder joints with various grain orientations are dramatic. With different angle (θ) between the c-axis of Sn grain and the direction of electron flow, the solder joints often demonstrate varying degrees of electromigration-induced damage. The present investigation illustrated significant differences in microstructural features on the surface of Cu reinforced composite solder joints, even with similar angle θ. Two Cu particles reinforced Sn3.5Ag composite solder joints with the similar angle θ of 35° and 40° were selected to investigate the effects of Sn grain c-axis on electromigration under high current stressing for 528xa0h. A large number of Cu6Sn5 compounds were observed and occupied nearly the whole surface with clear polarization effect in one sample. In contrast, very few Cu6Sn5 were found on the surface of the solder matrix in the other sample after current stressing, and the Cu6Sn5 which was formed after reflow disappeared with subsequent current stressing. Even though polarization effect was not obvious, and the cathode interfacial dissolution was observed on such sample. Systematic study revealed that the angle θ alone was not a sole factor to determine the electromigration damage. It has to be considered along with the coordinate system containing the orientation of the c-axis of the Sn grain, a more dominative factor for the diffusing species.

IMC growth behavior along c-axis of Sn grain under current stressing

Due to the asymmetric structure of Sn unit cell, the electromigration behaviors of Sn based solder joints were significantly depended on the crystal orientation of Sn grain. The present investigation illustrated the effect of Sn grain c-axis on the intermetallic compounds (IMCs) growth under current stressing. The Cu particles reinforced Sn3.5Ag solder joint was selected to obtain various observed surfaces of the sample after 46 days current stressing. The front side, back side, right side and anode side of the sample represented different phenomena, which reflected and proved the importance of c-axis orientation on Cu atomic migration in three-dimension space. Besides, another composite solder joint was polished severe times on the observed surface which was full of IMCs. It showed that the IMCs were covered on the Sn-based solder, and there was crack at the cathode underneath the front surface after 38 days current stressing. The IMC growth behavior would be well understood by systematic study in this work.

Effect of the Angle Between Sn Grain c-Axis and Electron Flow Direction on Cu-Reinforced Composite Solder Joints Under Current Stressing

With a body-centered tetragonal crystal structure, Sn grains were demonstrated to have highly anisotropic behaviors in various properties. The electromigration behavior of lead-free solder was impacted by the grain orientations. In this paper, the angle between the c-axis and the electron flow direction in composite solder joints (angle θ) was proven to be an important factor during electromigration. The effects of angle θ on the electromigration of composite solder joints were investigated in this paper. Cu particle-reinforced Sn3.5Ag solder joints were stressed under a current density of 104xa0A/cm2 at room temperature. After 336xa0h current stressing time, different electromigration phenomena occurred at the two sides of the grain boundary in the composite solder joint which contained two Sn grains with different angle θ. The Sn grains with the larger angle θ had a smaller growth rate of Cu6Sn5. In addition, a composite solder joint with a single Sn grain was set as the contrast and its angle θ was smaller than that of the composite solder joint with two Sn grains. The growth rate of Cu6Sn5 in the composite solder joint with a single grain was faster than that of the composite solder joint with two Sn grains.

Effect of Thermal Mechanical Behaviors of Cu on Stress Distribution in Cu-Filled Through-Silicon Vias Under Heat Treatment

Through-silicon vias (TSV) are facing unexpected thermo-mechanical reliability problems due to the coefficient of thermal expansion (CTE) mismatch between various materials in TSVs. During applications, thermal stresses induced by CTE mismatch will have a negative impact on other devices connecting with TSVs, even leading to failure. Therefore, it is essential to investigate the stress distribution evolution in the TSV structure under thermal loads. In this report, TSVs were heated to 450°C at different heating rates, then cooled down to room temperature after a 30-min dwelling. After heating treatment, TSV samples exhibited different Cu deformation behaviors, including Cu intrusion and protrusion. Based on the different Cu deformation behaviors, stress in Si around Cu vias of these samples was measured and analyzed. Results analyzed by Raman spectrums showed that the stress distribution changes were associated with Cu deformation behaviors. In the area near the Cu via, Cu protrusion behavior might aggravate the stress in Si obtained from the Raman measurement, while Cu intrusion might alleviate the stress. The possible reason was that in this area, the compressive stress

Effect of annealling process on microstructure of Cu-TSV

sigma_{theta }

Microstructure and intermetallic compounds in Sn-3Ag-3Bi-3In solder joints on Cu matrix

σθ induced by thermal loads might be the dominant stress. In the area far from the Cu via, thermal loads tended to result in a tensile stress state in Si.

Study of the growth behavior of prism-type Cu 6 Sn 5 in the liquid solder

With the urgent demand for the reduction of harmful element in electronic product, lead-free solder is considered to be potential brazing alloy due to the toxicity of lead. The major effect of long time exposured under solder Sn-Pb base solder are proven to increase nervous system disorder risk. Sn3Ag0.5Cu is a normal lead-free solder product with the content of Sn beyond 90%. It can lead to the formation of β-Sn after remelting. Crystal c-axis of β-Sn is 0.3181nm, crystal a-axis and b is 0.5832nm. The crystal orientation will close to each other after reflow soldering. Sn crystals showed strong anisotropy because of the body-centered tetragonal structure and Recrystallization begins due to the thermal mechanical fatigue. Recrystallization caused by thermal mechanical fatigue could deteriorate solder joint mechanical property. Because it is unclear about mechanism of recrystallization, it is necessary to investigate the thermal and mechanical force factors respectively in order to study stress affection to recrystallization performance. In this paper, inhomogeneous evolution of the microscopic structure and crystal orientation with different quantity of deformation were studied. All the samples were machining from lead-free welding rod by wire cut electrical discharge machining. Experiments were conducted with tensile specimens made by SAC305 at a speed of 0.2um/s per second on tensile machine. Before and after specimens approach tensile strength, different samples with different load times 10 seconds, 50 seconds, 150 seconds and 250 seconds were analyzed by scanning electron microscope, and electron back-scattered diffraction, and so as the fracture region. The surface grain characteristics and the fracture mechanisms were discussed. Inhomogeneous evolution of different deformation were investigated. The experimental results show that crystal misorientation in adjacent grains were getting larger with increasing deformation. Located different area of tensile specimens, the distribution of low angle grain boundaries and high angle grain boundaries showed significant difference. Its believed that with high angle grain boundaries are transformation of low angle grain boundaries caused by the activation process of the slip systems and high density dislocation movement. Central area of specimens shows that no obvious recrystallization means the number of movable dislocations are not sufficient to transform low angle grain boundaries to high angle grain boundaries.

Effects of POSS on the interfacial reactions between Sn-3.5Ag solders and Cu substrates during soldering

Three-dimensional (3D) package technology is currently considered as one of the best solution to beyond Moores Law and achieve miniaturization, high density and multi-functional of the device. Copper filled in through silicon via (Cu-TSV) technology with good electrical performance and high reliability, is a new type of 3D packaging technology. Cu-TSV has been widely used in integrated circuit technology due to its excellent interconnection performance. Residual stress could be generated in Cu-TSV during the manufacturing process which could be relaxed by annealing treatment. However, the Cu protrusion out of the TSV could also occurred during annealing process due to the large temperature variation and the severe mismatch of thermal expansion coefficient between copper (17.6×10−6/K) and silicon (2.8×10−6/K). This could be a potential threat to reliability of the back-end interconnect structure, since it might lead to cracking or even failure of the device. In this paper, the TSV samples were annealed at different temperatures and times to study the protrusion and microstructure of the copper. The heating temperatures were 250°C, 350 °C and 450°Cwith 30min, 60min and 90min, respectively. The protrusion behaviors and microstructure of different samples were further characterized and compared. The protrusion behaviors were characterized by Focus Ion Beam (FIB) and Confocal Laser Scanning Microscope (CLSM), which reveal that the annealing temperature and heating time have a significant effect on the protrusion. With the annealing temperature and time increase, the protrusion behavior becomes more and more obvious, however, the annealing time has little effect than annealing temperature on the protrusion. The cross section microstructure of TSV was also studied by Electron Backscatter Diffraction (EBSD). The results showed that the Cu grain size of all specimen increases but the grain orientation no significant changes after annealing. The grain size is positive correlation to the annealing temperature. The residual stress in Cu and Si around the TSV was measured using Raman spectrometer. This work could help to find out the optimized annealing condition which could further improve the reliability of TSV structure.