The size and crystal orientation of Sn grains in Pb-free, near eutectic Sn-Ag-Cu solder joints were examined. A clear dependence of the thermomechanical fatigue response of these solder joints on Sn grain orientation was observed (Sn has a body centered tetragonal crystal structure). Fabricated joints tend to have three orientations in a cyclic twin relationship, but among the population of solder balls, this orientation triplet appears to be randomly oriented. In thermally cycled joints, solder balls with dominant Sn grains having the particular orientation with the c-axis nearly parallel to the plane of the substrate were observed to fail before neighboring balls with different orientations. This results from the fact that the coefficient of thermal expansion of Sn in the basal plane (along the alpha-axis) is half the value along the c-axis; joints observed to be damaged had the maximum coefficient of thermal expansion mismatch between solder and substrate at the joint interface, as well as a tensile stress modes during the hot part of the thermal cycle. Localized recrystallization was observed in regions of maximum strain caused by differential expansion conditions, and its connection with crack nucleation is discussed.

Influence of Sn Grain Size and Orientation on the Thermomechanical Response and Reliability of Pb-free Solder Joints

The effects of aging on mechanical behavior of lead free solders have been examined by performing creep tests on four different SAC alloys (SAC105, SAC205, SAC305, SAC405) that were aged for various durations (0-4 months) at room temperature (25degC), and several elevated temperatures (75, 100, and 125 degC). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. Variations of the creep properties were observed and modeled as a function of aging time and aging temperature. In addition, the chosen selection of SAC alloys has allowed us to explore the effects of silver content on aging behavior.

The effects of aging temperature on SAC solder joint material behavior and reliability

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that large degradations occur in the material properties (stiffness and strength) and creep behavior of Sn-Ag-Cu (SAC) lead free solders during aging. These effects are universally detrimental to reliability and are exacerbated as the aging temperature and aging time increases. Conversely, changes due to aging have been shown to be relatively small in conventional Sn-Pb solders. Aging effects for lead free solder materials are especially important for the harsh applications environments present in high performance computing and in automotive, aerospace, and defense applications. In the current investigation, we have extended our previous studies to include a full test matrix of aging temperatures and SAC lead free solder alloys. In an attempt to reduce the aging induced degradation of the material behavior of SAC solders, we are also exploring various doped SAC-X alloys. These materials are SAC solders that have been modified by the addition of small percentages of one or more additional elements (X). Using dopants (e.g. Bi, In, Ni, La, Mg, Mn, Ce, Co, Ti, Zn, etc.) has become widespread to enhance shock/drop reliability, wetting, and other properties; and we have extended this approach to examine the ability of dopants to reduce the effects of aging and extend thermal cycling reliability. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on solder samples that were aged for various durations (0–6 months) at room temperature (25 °C), and several elevated temperatures (50, 75, 100, and 125 °C). Four “standard” SAC alloys have been examined in this work including SAC105, SAC205, SAC305, and SAC405. This selection has allowed us to explore the effects of silver content on aging behavior (we have examined SACN05 with N= 1%, 2%, 3%, and 4% silver; with all alloys containing 0.5% copper). The doped SAC solder materials being considered in our ongoing studies include SAC0307-X, SAC105-X, and SAC305-X. In this work, we will concentrate on presenting the results for SAC0307-X (SAC-X), where X is 0.1%Bi. This alloy has been proposed as a lower cost SAC variation suitable for enhancing drop reliability. For all of the solders, variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time and aging temperature. Our findings show that the doped SAC-X alloy illustrates reduced degradations with aging for all of the aging temperatures considered. The stress-strain and creep mechanical properties of SAC-X are better than those of SAC105 after short durations of aging, and approach those of SAC205 with longer aging times. After long term aging, the SAC-X alloy was found to have more stable behavior than all of the standard SACN05 alloys. Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes.

Reduction of lead free solder aging effects using doped SAC alloys

The deformation behavior of lead free solder joints was characterized under shear loading. The test vehicle was a 0.5mm pitch wafer level chip scale package (WLCSP). Five alloys were compared: Sn/sub 63/Pb/sub 37/, Sn/sub 95.5/Ag/sub 4.0/Cu/sub 0.5/, Sn/sub 96.5/Ag/sub 3.0/Cu/sub 0.5/, Sn/sub 96.5/Ag/sub 3.5/, and Sn/sub 99.3/Cu/sub 0.7/. The strain rate range was between 1.0/sec and 10/sup -7//sec, and test temperature was 25C. Both un-aged and thermally aged samples were tested. The aging condition was 24hrs at 125C. At strain rates of 10/sup -2//sec and above, all of the alloys had comparable strength, except for Sn/sub 99.3/Cu/sub 0.7/, which had 20% to 30% lower strength. Sn/sub 63/Pb/sub 37/ was the most strain rate sensitive, and hence had the lowest strength below 10/sup -4//sec strain rate. All alloys showed a 10% to 30% reduction in strength with aging. The solder joints exhibited very high ductility. In some cases strains of 100% were achieved before the joints started to fail. The ductility of Sn/sub 63/Pb/sub 37/ increased with aging, but the ductility of the lead free alloys all decreased with aging. Sn/sub 96.5/Ag/sub 3.0/Cu/sub 0.5/ showed the largest decrease in ductility with aging. The steady state creep behavior observed in the current study is fairly consistent with other published work. However, the absolute value of the joint strength is a little higher than data on bulk samples. All of the alloys exhibited significant primary creep before steady state conditions were achieved. This behavior is not always reported in the literature, and hence it deserves more investigation.

Shear deformation of lead free 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. In situ 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.

The role of elastic and plastic anisotropy of Sn in recrystallization and damage evolution during thermal cycling in SAC305 solder joints

The creep response of solder joints in a microelectronic package, which are subjected to aggressive thermo-mechanical cycling (TMC) during service, often limits the reliability of the entire package. Furthermore, during TMC, the microstructures of the new lead-free solders (Sn–Ag and Sn–Ag–Cu) can undergo significant in situ strain-enhanced coarsening, resulting in in-service evolution of the creep behavior. In this paper, the coarsening kinetics of Ag 3 Sn particles in SnAg-based solder are studied, and the results are correlated with impression creep data from individual microelectronic solder balls subjected to thermal aging treatments. Coarsening influences creep behavior in two ways. At low stresses, the creep rate increases proportionately with precipitate size. At high stresses, precipitate coarsening influences creep response by altering the threshold stress for particle-limited creep. Based on these observations, a microstructurally adaptive creep model for solder interconnects undergoing in situ coarsening is presented.

Effect of thermo-mechanically induced microstructural coarsening on the evolution of creep response of SnAg-based microelectronic solders

The creep behavior of solder joints plays a key role in their reliability in microelectronic applications, necessitating the development of accurate creep testing methodologies and constitutive creep models for the various emerging Pb-free solder alloys. Because of the significant microstructural/compositional differences between bulk solder alloys and tiny microelectronic joints, bulk creep properties are usually not suitable for reliability assessment of flip-chip and ball-grid-array (BGA) solder joints. In this paper, we present results on impression creep of Sn-Ag solder, which allows testing of very small material volumes, and report on the miniaturization of the apparatus to allow direct testing of individual microelectronic solder balls attached to a BGA substrate within relatively short times and with minimal sample preparation. A microstructurally adaptive creep model, which accounts for the effects of microstructural aging on the creep response of solder joints, and is capable of adjusting itself as solder joint microstructures evolve during service, is presented, along with experimental determination of the relevant coarsening kinetics parameters.

S.G. Jadhav

Papers

Effect of thermo-mechanically induced microstructural coarsening on the evolution of creep response of SnAg-based microelectronic solders

Impression creep testing and constitutive modeling of Sn-based solder interconnects