Microstructural Evolution of SAC305 BGA Joints during Extreme High Temperature Aging

Solder joints provide mechanical support, and electrical and thermal interconnection between packaging levels in microelectronics assemblies. Proper functioning of these interconnections and the reliability of the electronic packages depend largely on the mechanical properties of the solder joints. Lead free solders typically provide excellent thermo-mechanical properties and are commonly used as interconnections in electronic packages. However, the exposure of lead free solder joints to complicated thermal histories can degrade their mechanical behavior. For example, aging of lead free solders at high temperature leads to microstructural evolution resulting in reduced mechanical properties and increased creep deformations. Previous investigations on mechanical characterization of lead free solders have mainly emphasized mechanical testing at temperatures up to 125 °C. However, many electronic products are deployed in harsh environment applications including well drilling, geothermal energy, automotive power electronics, and aerospace engines, where solders are exposed to very high temperatures from 125-200 °C. Mechanical properties of lead free solders at elevated temperatures are limited.In this work, we have explored Mechanical behavior and aging effects in SAC305 (96.5Sn-3.0Ag-0.5Cu) solder joints at several extreme high testing temperatures (T = 125, 150, 175, and 200 °C) using the method of nanoindentation. A special high temperature stage and test protocols were used within the nanoindentation system to carefully control the testing temperature, and to make the measurements insensitive to thermal drift problems. Solder joints were extracted from 14 x 14 mm PBGA assemblies (0.8 mm ball pitch, 0.46 mm ball diameter) that were built as part of the iNEMI Characterization of Pb-Free Alloy Alternatives Project. Since the properties of SAC solder joints are highly dependent on crystal orientation, polarized light microscopy was utilized to determine the orientation of the tested joints. For all the experiments, only single grain solder joints were used to avoid introducing any unintentional variation from changes in the crystal orientation across the joint cross-section.To study aging effects, solder joints were preconditioned for 0, 1, 5, 10, and 30 days at T = 125 °C in a box oven. Nanoindentation testing was then performed on the aged specimens at four different test temperatures (T = 125, 150, 175, and 200 °C) to extract the elastic modulus, hardness, and creep performance of the aged material. Throughout this study, a constant force of 10 mN was applied for 900 seconds to monitor the creep displacements and measure the creep strain rate as a function of both temperature and prior aging conditions.As expected, our results have shown that mechanical properties and creep strain rate of solder joints are highly dependent on the test temperature and degrade significantly as the temperature increases. In particular, the aging induced degradation rates for high temperature testing (175-200 °C) were about 10000X than those observed at 125 °C. In addition, the effects of aging on solder joints properties become much more significant as the aging time and test temperature increases.

Solder joints provide mechanical support, electrical and thermal interconnection between packaging levels in microelectronics assembly systems. Proper functioning of these interconnections and the reliability of the electronic packages depend largely on the mechanical properties of the solder joints. Lead free solders provide excellent thermomechanical properties and commonly used as interconnections in electronic packages. However, environmental conditions, such as, operating temperature, aging temperature, and aging time significantly affect these properties due to the microstructural evolution of the solder that occurs during aging. The most well-known and widely observed changes are coarsening of the Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn and Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> intermetallic compounds (IMCs)present in the eutectic regions between beta-Sn dendrites. In this work, we have investigated the variations in mechanical behavior of several SAC and SAC+X lead free solder alloys including SAC305 (96.5Sn-3.0Ag-0.5Cu)and SAC_Q subjected to high temperature aging. Specimens have been preconditioned at several extreme temperature aging conditions including T = 125 and 200°C. At each extreme temperature condition, several durations of aging were considered including 0, 1, 5 and 20 days. With each set of aging conditions, stress-strain tests were performed on the aged specimens. The evolution of the stress-strain behavior with aging temperature and time was determined. In addition, microstructural evolution of solder alloys during extreme high temperature aging has been explored, and aging induced coarsening of IMCs has been explored using Scanning Electron Microscopy (SEM)to validate our experimental results. Our experimental results show substantial degradations of the mechanical properties including initial modulus, yield stress, and ultimate tensile strength of lead-free solders with extreme temperature aging. The results suggested that there are large challenges to obtaining solder joint reliability when the joints are exposed to long-term high temperature aging. The addition of dopants (e.g. Bi)in the traditional SAC alloys significantly mitigated the high temperature aging induced degradations in both the microstructure and mechanical properties.

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 the observed material behavior variations of Sn-Ag-Cu (SAC) lead free solders during room temperature aging (25°C) and elevated temperature aging (125°C) were unexpectedly large and universally detrimental to reliability. Such 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. However, there has been little work in the literature, and the work that has been done has concentrated on the degradation of solder ball shear strength (e.g. Dage Shear Tester). Current finite element models for solder joint reliability during thermal cycling accelerated life testing are based on traditional solder constitutive and failure models that do not evolve with material aging. Thus, there will be significant errors in the calculations with the new lead free SAC alloys that illustrate dramatic aging phenomena. In the current work, we have extended our previous studies to include a full test matrix of aging temperatures and solder alloys. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on four different SAC alloys (SAC105, SAC205, SAC305, SAC405) that were aged for various durations (0–6 months) at room temperature (25°C), and several elevated temperatures (50, 75, 100, and 125°C). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. 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. In this paper, we report on the creep results. The chosen selection of SAC alloys 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). In order to reduce the aging induced degradation of the material behavior of the SAC alloys, we are testing several doped SAC alloys in our ongoing work. These materials include SAC0307-X, SAC105-X, and SAC305-X; where the standard SAC alloys 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, etc.) has become widespread to enhance shock/drop reliability, and we have extended this approach to examine the ability of dopants to reduce the effects of aging and extend thermal cycling reliability.

FCBGAs are finding applications in automotive underhood environments where they may be subjected to sustained temperatures of 125-200°C for sustained periods during operation. While, FCGBAs have been previously used in consumer applications where operating temperatures typically range in 55-85°C, relatively little is known on methods to design damage-tolerant packages in automotive underhood environments. There is insufficient information on plastic encapsulated electronic components capable of surviving high temperatures for long periods (>100,000 hours). In this paper, the aging effects of a number of epoxy molding compounds subjected to sustained high temperature long term aging have been studied. Two popular molding compounds, EMC-1 and EMC-2 are studied under three aging temperatures: 100°C, below glass transition temperature, 125°C, around glass transition temperature and 150°C, above glass transition temperature. The dynamic mechanical analyzer (DMA) and thermomechanical Analysis (TMA) are conducted to study the evolution of viscoelasticity, glass transition region and coefficient of thermal expansion as the function of aging time and temperature, including pristine, 40 days, 80 days, 120 days, 240 days and 360 days. TGA tests are conducted to study the weight loss change as the function of aging temperature and aging time. SEM and optical microscope are applied to study the microstructure evolution of EMCs.

The effects of isothermal aging on the tensile properties of Cu/Sn–58Bi/Cu soldered joints were investigated. Experimental results show that the scallop-shaped Cu6Sn5 and planar Cu3Sn formed at the interface between solder and Cu substrate during reflowing and aging. The thickness of the intermetallic compounds (IMCs) increased almost linearly with the square root of aging time, and aging at 120 °C yielded a much faster growth of the IMCs layer than that of samples aged at 100 °C. The IMCs growth rate constants were 6.02 × 10−18 and 1.85 × 10−18 m2 s−1 for solder joints aged at 120 and 100 °C, respectively. The tensile strength of the Sn–58Bi/Cu soldered joints decreased slightly with the increasing aging time and temperature. The failure was dominated by the mixed fracturing in both the solder and the Cu6Sn5 grains irrespective of their thermal aging conditions. However, the fracture pattern tended to transform from ductile to brittle with increasing aging time and temperature. The Bi segregation and voids were observed around the Cu/Cu3Sn interface as the long term aging at high aging temperature was carried out, which resulted in reduction of tensile strength of solder joints.

High voltage direct current (HVDC) cables develop rapidly due to the increased demand for renewable sources and long distance power transmission, recently. Cross-linked polyethylene (XLPE) insulated direct current (DC) cable is one of main HVDC cables. However, XLPE insulation materials incline to ageing and degradation in the long-term high temperature environment, which leads to the reduction of the electric, physicochemical and mechanical properties. In this paper, the effects of a single thermal ageing on the DC electrical conductivity of HVDC cable XLPE insulation materials were investigated in 130, 140 and 150 oC by accelerated ageing experiments in nitrogen atmosphere to simulate the pressure environment of cable insulation. The change trend and mechanism of DC electrical conductivity at different ageing temperature and time were analyzed through Fourier transform infrared (FTIR) spectra. The experiments and analysis results indicated that the electrical conductivity of XLPE insulation after thermal ageing firstly reduced and were below that of unaged XLPE, and then gradually increased during the whole ageing temperature and time range by the combined effect of thermal decomposition, post-crosslinking and the diffusion of the low molecular weight substance. The higher the thermal ageing temperature was, the greater the changes of the electrical conductivity of XLPE insulation were. Under the same ageing time of 700 h, the DC electrical conductivity of XLPE insulation at different ageing temperature followed well the Arrhenius equation. Based on data processing and analyzing, the electrical conductivity activation energies of XLPE before and after thermal ageing were obtained and it was found that the activation energies for aged XLPE insulation were superior to unaged XLPE. The thermal ageing increased the electrical conductivity activation energy. In the FTIR spectra, it can be seen that the intensity of carbonyl absorption peak at about 1722 cm−1 increased after thermal ageing. The carbonyl index has been calculated by the rate of peak near 1722 and 2020 cm−1 to evaluate the ageing degree of XLPE insulation materials. It was found that the carbonyl index of unaged XLPE was the minimum and increased as the ageing time under different thermal ageing temperature. At high thermal ageing temperature, the carbonyl index increased rapidly. The high temperature contributed to the thermal decomposition of XLPE insulation materials, which caused oxygen groups increasing in molecular chain and the increasing rate of the carbonyl was accelerated.

Thermal aging effects on the fatigue life of Sn-3.0Ag-0.5Cu (wt%, SAC305) solder joint are investigated. Two series of experiments were conducted: 1) at different temperatures after aging for 72 hours; 2) with different aging time at the cryogenic temperature. The fatigue life of SAC305 solder joint were analyzed statistically by using a modified Weibull distribution model, which takes the thermal aging effect into account and the results agree well with the experimental data. After aging for 72 hours, the longest fatigue life was obtained by aging at cryogenic temperature. With the aging temperature raises, fatigue life decreases. The fatigue life after aging at cryogenic temperature for different aging time was found to decrease with the increasing of aging time. An empirical formula that describe the relation between fatigue life and aging time under the cryogenic temperature was proposed. The fatigue life of solder joints can be increased by aging at cryogenic temperature and the aging time should be controlled to be around 72 hours.

Performance of asphalt concrete pavement significantly degrades during the aging of asphalt binders. The master curves of complex shear modulus and phase angle are recognised as useful tools to characterise the rheological properties of aging asphalt binders. The main shortcoming of existing master curves is that the effects of aging cannot be captured and predicted effectively. This study developed a new method to construct such aging master curves that can be extended to predict the complex shear modulus and phase angle at a wide range of aging time and temperature. The DSR tests were conducted on three different types of asphalt binders with three aging temperatures and four aging durations. The master curves of complex shear modulus and phase angle were constructed for aging asphalt binders through time-temperature-aging shift by performing both rotation and horizontal aging shift. Rotation and horizontal aging shift were determined as the function of aging time and aging temperature. Therefore, it becomes possible and convenient to construct single aging master curves at a wide range of aging temperature and aging time.

Lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging. In particular, their microstructures evolve significantly causing dramatic changes in their mechanical properties, as well as reductions in reliability of lead free electronic assemblies subjected to aging. To better understand aging induced degradations, we have been utilizing Scanning Electron Microscopy (SEM) and Scanning Probe Microscopy (SPM) to monitor aging induced microstructural changes occurring within fixed regions in lead free solder joints. Such "fixed region" approaches allow for quantitative analysis of the microstructural evolution, removing the limitations of many prior studies where aged and non-aged microstructures were taken from two different samples and could only be qualitatively compared. In addition, time-lapse imagery of the microstructure evolution in a particular region of a solder joint can be recorded as a function of the aging time. In our recent paper presented at ECTC 2017, we illustrated the SEM and SPM approaches for small fixed regions in a SAC305 solder joint sample, and provided initial aging data for up to 10 hours of aging. In the current study, we have extended this work in several ways. In particular, microstructural evolution has been studied for several different regions from several different SAC305 joints, and both short term (up to 50 hours) and long term (up to 1500 hours) aging of the joints have been performed. For both short term and long term aging experiments, the microstructure evolutions were observed in solder joint samples subjected to aging at T = 125 °C. The microstructures in several fixed regions of interest were recorded after predetermined time intervals of aging, which were 1 hour (up to 50 hours) and 250 hours (up to 1500 hours) for the short term and long term aging samples, respectively. The size metrics for each IMC particle were tracked during the aging processes, and the results correlated to observed mechanical behavior changes. Quantitative analysis of the microstructures has shown that the number of IMC particles decreases during aging, while the average diameter of the particles increases significantly (more than 200% for short term aging, and more than 300% for long term aging). The changes in particle size were rapid during the first 30 hours of aging, and then became significantly slower. This matches the aging induced degradations previously observed in the mechanical behavior of lead free solders. Conventional models for IMC particle coarsening could not fit the combined short term and long term aging data found in this study. Histogram charts were also used to illustrate distribution of the particle sizes present at each aging time, and to show that that changes occurred during aging. In particular, the number of particles of any given size degreased with aging, while the width of the histogram increased, indicating a larger distribution of different particle sizes as aging progressed.

Exposure of lead free solder joints to high temperature isothermal aging conditions leads to microstructure evolution, which mainly includes coarsening of the intermetallic (IMC) phases. In our previous work, it was found that the coarsening of IMCs led to degradation of the overall mechanical properties of the SAC solder composite consisting of β-Sn matrix and IMC particles. However, it is not known whether the isothermal aging changes properties of the individual β-Sn and IMC phases, which could also be affecting to the overall degradation of properties. In this study, the aging induced variations of the mechanical properties of the β-Sn phase, and of Sn-Cu IMC particles in SAC solder joints have been explored using nanoindentation. SAC solder joints extracted from SuperBGA (SBGA) packages were aged for different time intervals (0, 1, 5, 10 days) at T = 125 °C. Nanoindentation test samples were prepared by cross sectioning the solder joints, and then molding them in epoxy and polishing them to prepare the joint surfaces for nanoindentation. Multiple β-Sn grains were identified in joints using optical polarized microscopy and IMCs were also observed. Individual β-Sn grains and IMC particles were then indented at room temperature to measure their mechanical properties (elastic modulus and hardness) and time dependent creep deformations. Properties measured at different aging time were then compared to explore aging induced degradations of the individual phases. The properties of the individual phases did not show significant degradation. Thus, IMC coarsening is the primary reason for the degradation of bulk solder joint properties, and changes of the properties of the individual phases making up the lead free solder material are negligible.

Impact of thermal aging on the intermetallic compound particle size and mechanical properties of lead free solder for green electronics

Solid/solid state interfacial reactions in the lead-free solders/Cu- 2.3%wt. Fe alloy (C194) couples

Moisture susceptibility is one of the key issues of warm mix asphalt (WMA). In this research, the moisture susceptibility of asphalt mixtures and binders containing Sasobit warm mix additive was investigated in comparison to that of hot mixture asphalt (HMA) through laboratory aging experiments. The WMA asphalt mixtures were aged in the laboratory at three aging temperatures and times. The moisture susceptibility of the asphalt mixtures was measured through the laboratory immersed Marshall test and freeze-thaw splitting test. The surface free energy (SFE) of asphalt binders extracted from WMA asphalt mixtures was determined by the sessile drop method. The results show that the aging time and temperature have a significant effect on the improvement in moisture susceptibility in terms of both the freeze-thaw splitting strength ratios and the residual Marshall stability of asphalt mixtures containing Sasobit warm mix additive. The SFE of asphalt binders extracted from Sasobit warm mix increased with the aging time and temperature. The moisture susceptibility of the asphalt mixtures and binders containing Sasobit warm mixes was similar to or even greater than that of HMA aging under certain conditions.

Lead-free solder has been used to replace toxic lead-containing solder. Sn 42 Bi 58 solder has attracted a lot of attention for its low-cost, low melting temperature. Nanoparticles, such as metallic, nonmetallic and oxidant, have been doped into solders to improve their performance. In this study, we investigated the effect of ZrO 2 nanoparticles on the mechanical properties of Sn 42 Bi 58 solder joints. Firstly, solder balls were made from solder paste by stencil printing and reflow. Next, solder balls were reflowed to form solder joints on Cu pad metalized with Au/Ni layers. Then, solder joints were put into an isothermal aging oven for different aging time. Shear tests were performed to investigate the shear strength of both solder joints subjected to aging time. Scanning electron microscope (SEM) was employed to study microstructure evolution in solder matrix. The results showed that the shear strength of Sn 42 Bi 58 solder joints was reinforced with the incorporation of ZrO 2 nanoparticles. The growth of intermetallic compounds (IMCs) in ZrO 2 doped solder joints was observed to be impeded.

The kinetics of carbide precipitation in a fully processed 2.3 wt Pct silicon, 0.66 wt Pct aluminum electrical steel with carbon contents of 0.005 to 0.016 wt Pct were investigated over the temperature range from 150 to 760 °C and times from 30 seconds to 240 hours. The size, morphology, and distribution of the carbide phases, as functions of aging time and temperature, were determined by optical and transmission electron microscopy. The 1.5T core loss was also evaluated and correlated with the changes in precipitation. Distinct C curves were observed for the formation of grain-boundary cementite at temperatures above 350 °C and a transition carbide ({100} α habit plane) at temperatures below 350 °C. Grain-boundary cementite had a relatively small effect on core loss. The large increases in core loss that accompanied transition carbide precipitation peaked at specific aging temperatures depending on the carbon content of the steel. Once a transition carbide dispersion was initially established at a given aging temperature, particle coarsening and core loss changes were generally insensitive to aging time. The influence of a combined addition of silicon and aluminum on the solubility of cementite and the transition carbide in iron was estimated and discussed.