Kuo Chuan Liu

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

Because failures in lead-free solder joints occur at locations other than the most highly shear-strained regions, reliability prediction is challenging. To gain physical understanding of this phenomenon, physically based understanding of how elastic and plastic deformation anisotropy affect microstructural evolution during thermomechanical cycling is necessary. Upon solidification, SAC305 (Sn-3.0Ag-0.5Cu) solder joints are usually single or tricrystals. The evolution of microstructures and properties is characterized statistically using optical and orientation imaging microscopy. Inxa0situ synchrotron x-ray measurements during thermal cycling are used to examine how crystal orientation and thermal cycling history change strain history. Extensive characterization of a low-stress plastic ball grid array (PBGA) package design at different stages of cycling history is compared with preliminary experiments using higher-stress package designs. With time and thermal history, microstructural evolution occurs mostly from continuous recrystallization and particle coarsening that is unique to each joint, because of the specific interaction between local thermal and displacement boundary conditions and the strong anisotropic elastic, plastic, expansion, and diffusional properties of Sn crystals. The rate of development of recrystallized microstructures is a strong function of strain and aging. Cracks form at recrystallized (random) boundaries, and then percolate through recrystallized regions. Complications arising from electromigration and corrosion are also considered.

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

The interaction between isothermal aging and long-term reliability of fine-pitch ball grid array (BGA) packages with two different die sizes was investigated. Both 5xa0mmxa0×xa05xa0mm and 10.05xa0mmxa0×xa010.05xa0mm die-attached packages with Sn-3.0Ag-0.5Cu (wt.%) solder balls were used to compare the correlation between the internal strain difference and isothermal aging on microstructural evolution during thermal cycling. To determine their long-term reliability, the samples were isothermally aged and thermally cycled from 0°C to 100°C with 10-min dwell time. Based on Weibull plots for each aging condition, the packages with large dies attached showed shorter characteristic lifetimes, because of the relatively higher stress, but showed less lifetime degradation with isothermal aging compared with the smaller die-attached samples. Microstructure analysis using orientation imaging microscopy (OIM) revealed the evolution of the microstructure during thermal cycling, showing a higher degree of recrystallization inside the bulk solder for joints that were isothermally aged and experienced higher stress. The possible mechanisms giving rise to these observations are discussed.

Impact of Microstructure Evolution and Isothermal Aging on Sn-Ag-Cu Solder Interconnect Board-Level High-G Mechanical Shock Performance and Crack Propagation

The root cause of shock-induced solder joint failures in the range of 800G to 1500G is investigated. Joint stability under various shock and strain level combinations and the impact of isothermal aging on board-level shock performance were analyzed. A test vehicle was developed to obtain various combinations of shock and strain levels in a single board. Using 17xa0mmxa0×xa017xa0mm body-sized ball grid array packages on a shock test board, isothermal aging was applied prior to shock testing to determine the impact of different interface microstructures. Results revealed clear indications of a correlation between shock and strain and a trend of isothermal-aging-induced degradation. A shift in the failure locations was observed based on the interface intermetallic microstructure, and some preliminary evidence for the influence of Sn grain orientation was identified. The interrelated effects of isothermal aging, locally experienced shock and strain levels, and Sn grain orientation on mechanical shock performance are discussed.

In Situ Synchrotron Characterization of Melting, Dissolution, and Resolidification in Lead-Free Solders

Melting and solidification of SAC 305 lead-free solder joints in a wafer-level chip-scale package were examined inxa0situ with synchrotron x-ray diffraction. The chips with balls attached (but not assembled to a circuit board) were reflowed one to three times using a temperature and time history similar to an industrial reflow process. Diffraction patterns from the same joint were collected every 0.5xa0s during the melting and solidification process. The solidification of the Sn phase in the solder joint occurred between 0.5xa0s and 1xa0s. During melting, most of the Sn melted in about 0.5xa0s, but in some cases took 2–5xa0s for the Sn peak to completely disappear. In one instance, the Sn peak persisted for 30xa0s. The Ag3Sn peaks dissolved in about 1–2xa0s, but the Cu6Sn5 peaks from the interface were persistent and did not change throughout the melting and solidification process. Completely different Sn crystal orientations were always developed upon resolidification.

Fracture mechanics of lead-free solder joints under cyclic shear load

This paper reports the experimental and theoretical exploration of the fracture mechanism active in BGA lead-free solder assemblies under high speed shear fatigue test conditions. Our investigation finds that, contrary to common assumption, the crack growth in shear fatigue is not governed by shear stress but more by crack opening stress. Our theoretical analysis indicates that fracture by crack opening mode prevails because non-uniformity in the shear deformation of solder joint creates a body rotation which results in crack opening stress rather than shear. While the crack growth in shear fatigue is found to vary sensitively with variation in the mechanical constraints on the assembly, such as solder shape and elastic modulus of the chip mold, it is also sensitive to variation in solder microstructure. This, the sensitivity to the assembly constraints and solder microstructure, makes it ideal in investigating fatigue properties of solder joints as well as identifying the structural and microstructural features responsible for reliability failure.

The role of elastic and plastic anisotropy of Sn on microstructure and damage evolution in lead-free solder joints

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.

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

Isothermal shear fatigue mechanism of lead free solder joints

This paper presents the results of isothermal shear fatigue tests conducted on Sn-Ag-Cu lead-free and Pb-Sn solder alloys in fully assembled PBGA configurations. Our study, aimed to better understand the fatigue behaviors of solder alloys in the package assembly, reveals that the shear fatigue test is indeed capable of measuring the fatigue properties of the solder itself and may lead to material parameters necessary for the prediction of the solder joints fatigue reliability in general. The cyclic shear fatigue test conducted under various fatigue conditions yields the result suggesting the close linkage between the fatigue in PBGA and the fatigue property of solder alloy. This is a result of the existence of consistency in crack propagation path in solder joint, that is fixed to the chip-side of solder matrix (not interface), irrespective of test conditions that includes temperature, strain range, and frequency. As a result, fatigue failure of Pb-free and Pb-Sn solder can be fitted to the Coffin-Manson fatigue model with a reasonable consistency. The resulting numerical constants, namely ductility coefficient and ductility exponent, are consistent with what is expected from general consideration of metallic fatigue system. However, their temperature dependence shows a significant deviation from expectation, probably due to an addition of thermal strain to the shear strain and its variation with temperature.