Bite Zhou

Slip, Crystal Orientation, and Damage Evolution During Thermal Cycling in High-Strain Wafer-Level Chip-Scale Packages

Wafer-level chip-scale package samples with pre-cross-sectioned edge rows were thermally cycled to study microstructure evolution and damage development. Electron backscattered diffraction (EBSD) and high-energy x-ray diffraction were used to obtain Sn grain orientations and the average coefficient of thermal expansion normal to the board in every joint of the package for samples in the as-fabricated and thermally cycled conditions. The results indicated a near-random distribution of joint orientation. Optical, scanning electron microscopy, and EBSD methods were used to characterize microstructure changes in pre-cross-sectioned samples due to thermal cycling. Slip trace analysis and Orientation Imaging Microscopy™ (OIM) show that slip systems with high Schmid factors (estimated global shear stress based on the package neutral point) are responsible for the observed microstructure evolution during thermal cycling, which provides information about slip systems that are more easily activated. Two joints were analyzed in detail to evaluate slip activity at different stages of their thermal history. The first case showed that a solidification twin grain boundary misorientation deviated from the twin relationship due to slip activity during thermal cycling, which can influence damage development and the path of crack propagation. The second case showed a new grain orientation developing due to gradual lattice rotation about the Sn [110] axis by a continuous recrystallization mechanism. This rotation was correlated with the operation of slip system

Multiscale Modeling of the Effect of Micro-alloying Mn and Sb on the Viscoplastic Response of SAC105 Solder

\{ 110 )\langle \left. {001} \right]

The interaction between grain orientation evolution and thermal cycling as a function of position in ball grid arrays using orientation image microscopy

{110)⟨001. Small tin whiskers emerged from the initially polished chip interface and grew with increasing thermal cycles until a crack developed in the solder that relieved the stress. As the local stresses are not known experimentally, this analysis provides observations that can be compared with a crystal plasticity model simulation.

Mechanistic modeling of the anisotropic steady state creep response of snagcu single crystal

The interaction between isothermal aging and the long-term reliability of wafer-level chip-scale packages with Sn-3.0Ag-0.5Cu (wt%) solder ball interconnects is investigated. On isothermally aging at 100 and 150°C for 500 h and then thermally cycling from 0 to 100°C with 10 min of dwell time, the lifetime of the package is reduced by approximately 29%, depending on the aging condition. The microstructural evolution is observed during thermal aging and thermal cycling using orientation image microscopy. A Sn grain orientation structure transformation is observed. Different mechanisms after aging at various conditions are identified, and their impacts on the fatigue life of solder joints discussed.

The Role of Elastic and Plastic Anisotropy of Sn in Recrystallization and Damage Evolution During Thermal Cycling in SAC305 Solder Joints

This study investigates the time-dependent viscoplastic response of two relatively new SAC105-X solders—SAC105-05Mn (Sn1.0Ag0.5Cu (SAC105) doped with 0.05 wt.% Mn), and SAC105-55Sb (SAC105 doped with 0.55 wt.% Sb). The results showed that the addition of Mn or Sb increases the creep resistance of SAC105 solder by one to two orders of magnitude at the tested stress levels of 2–20 MPa. The addition of Mn as a fourth alloying element promotes homogeneous distribution of micron-scale Cu6Sn5 intermetallic compounds (IMCs), thereby reducing their interparticle spacing as compared to that of SAC105. On the other hand, addition of Sb does not change the spacing of the Cu6Sn5 particle, but promotes the formation of uniformly sized Sn dendritic lobes, homogeneously distributed in the whole solder joint. Moreover, Sb also forms a solid solution with Sn and strengthens the Sn matrix in SAC105-55Sb itself. The effects of these microstructural changes (obtained using image processing) on the secondary creep constitutive response of SAC105 solder interconnects were then modeled using a mechanistic multiscale creep model. The mechanistic model was able to accurately capture the trends in the secondary creep constitutive response of the alloys and to explain the improvement in creep resistance of SAC105 due to the addition of Mn and Sb.

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

The elastic, thermal expansion, and plastic anisotropy of Sn is examined to assess how anisotropy affects the microstructural evolution and damage nucleation processes in SAC305 solder joints. Examination of all joints in a package indicates that upon solidification, crystal orientations are nearly randomly distributed. Initial studies of cracked joints after thermal cycling showed that orientations with the c-axis parallel to the joint interface (red orientations) are more likely to crack arising from tensile stresses during the hot part of the cycle. Subsequent studies show that package design has a large influence on how the microstructure evolves; higher strain designs stimulate recrystallization at earlier times. Recrystallization appears to be strongly correlated with crack nucleation and propagation processes, as red orientations often develop and lead to crack nucleation and propagation. The details of the recrystallization process depend strongly on the plastic slip and recovery processes arising from the specific crystal orientation / temperature / strain history that makes microstructural evolution of each joint unique. The unique history for each joint implies that worst case scenarios need to be identified and models developed that can predict microstructural evolution that leads to worst case scenarios.

Sn-Ag-Cu Solder Joint Microstructure and Orientation Evolution as a Function of Position and Thermal Cycles in Ball Grid Arrays Using Orientation Imaging Microscopy

Thermally cycled PBGA packages with a full array of 196 solder joints after various pre-conditions are examined to observe the microstructure evolution of Sn-Ag-Cu solder joints during aging and thermal cycling, focusing on Sn grain orientation. Each PBGA package was polished to obtain plan view cross sections of every solder joint, and characterized using both Polarized Optical microscopy and Orientation Imaging Microscopy (OIM). Based on observations using OIM images, we obtained a distribution map based on the Sn crystal c-axis orientation. Each precondition show its own signature distribution related to the thermal aging and thermal cycling history. Further analysis, combining the dye and pry and plan view observation, revealed the correlation between the c-axis orientations and the fatigue cracks caused by thermal cycling. A strong relationship between evolving crystal orienations where the c-axis becomes aligned with the plane of the package and cracking is identified. A combined study and observation of Polarized light images and OIM provides further understanding about deformation and microstructure evolution processes that occur during thermal cycling. A continuous recrystallization mechanism may account for the changes in grain orientation that lead to susceptibility to cracking.

Impact of Isothermal Aging on Long-Term Reliability of Fine-Pitch Ball Grid Array Packages with Sn-Ag-Cu Solder Interconnects: Die Size Effects

A multiscale modeling framework is proposed in this study to capture the influence of the inherent elastic anisotropy of single crystal Sn and the inherent heterogeneous microstructure of a single crystal SnAgCu (SAC) solder grain on the secondary creep response of the grain. The modeling framework treats the SAC microstructure as having several distinct length scales. The smallest length scale (Tier 0) consists of the Sn BCT lattice. The eutectic Sn-Ag micro-constituent, consisting of nanoscale Ag3Sn IMC particles embedded in the single crystal BCT Sn matrix, is termed Tier 1. The single-crystal SAC microstructure, consisting of Sn dendrites and surrounding eutectic Sn-Ag phase, is termed Tier 2. Dislocation recovery mechanisms, such as Orowan climb and detachment from nanoscale Ag3Sn particles, are found to be the rate controlling mechanisms for creep deformation in the eutectic Sn-Ag phase (Tier 1) of a SAC single crystal. The anisotropic secondary creep rate of eutectic Sn-Ag phase (Tier 1), is then modeled using the above inputs and the saturated dislocation density calculated for dominant glide systems during secondary stage of creep. Saturated dislocation density is estimated as the equilibrium saturation between three competing processes: (1) dislocation generation; (2) dislocation impediment caused by back stress from pinning of dislocations at IMCs; and (3) dislocation recovery due to climb/detachment from IMCs. Secondary creep strain rate of eutectic Sn-Ag phase in three most facile slip systems is calculated and compared against the isotropic prediction. At low stress level secondary steady state creep rate along (110)[001] system is predicted to be ten times the creep rate along (100)[0-11] system. However, at high stress level, secondary steady state creep rate along (110)[001] system is predicted to be ten thousand times the creep rate along (100)[0-11] system. The above predictions are in strong agreement with (1–4) orders of magnitude of anisotropy observed in steady state secondary creep response in SAC305 solder joints tested under identical loading conditions in experiments conducted by several authors. The above model is then combined with Eigen-strain methods and average matrix stress concepts to homogenize the load sharing between the Sn dendrites and the surrounding eutectic Ag-Sn matrix. The resulting steady state creep rates are predicted for a few discrete single crystal SAC305 specimens. Very good agreement is observed between the predicted steady state creep rate and the measured creep rates for two SAC305 single crystal specimens.Copyright