Evolution of Mechanical Properties and Microstructure in SAC Bulk Solder and Solder Joints During Thermal Cycling Exposures

Electronic packages are frequently exposed to thermal cycling during their service life between low to high temperature extreme. Similar phenomena can be observed in solder joints during the characterization of thermal-mechanical fatigue behavior. This variation in temperature causes the evolution of mechanical and microstructural behavior of solder joints. Also, dwelling at high temperature extreme causes the mechanical properties reduction of solder joints due to thermal aging phenomena which eventually leads to the change in microstructure. In literature, the effect of thermal aging on the mechanical behavior evolution has been reported by several research groups, but the evolution of mechanical and microstructural properties under different thermal cycling exposure is limited. In our prior study, reduction of mechanical properties of SAC305 lead-free solder material under different thermal cycling exposures have been reported for up to 5 days of thermal cycling. It was found that thermal cycling with long ramp period and dwell time has severe effect on mechanical properties reduction. In our present study, previous study has been extended up to 100 days along with the mechanical behavior evolution of solder joints under stress free condition at different thermal cyclic loading. Particularly, the evolutions of mechanical behavior in both bulk SAC305 miniature solder bar samples and small SAC305 solder balls under stress free condition have been investigated for several thermal cycling profiles, and then the results were compared. Reflow solidification technique with a controlled temperature profile has been used to prepare bulk solder specimens for uniaxial tensile testing. Optical microscopy has been used to figure out the single grain BGA solder balls after grounding and polishing to avoid grain orientation effect during nanoindentation technique. Then, both bulk solder bars and solder balls were thermally cycled between −40 C to +125 °C under a stress-free condition (no load) in a thermal chamber. Several thermal loading were adopted such as (1) 150 minutes cycles with 45 minutes ramps and 30 minutes dwells, (2) air-to-air thermal shock exposures with 30 minutes dwells and near instantaneous ramps, (3) 90 minute cycles with 45 minutes ramps and 0 minutes dwells (thermal ramp only), and (4) Isothermal aging at high temperature extreme (no cycle). After each thermal cycling exposure, mechanical properties evolution of both solder bars and solder balls were recorded in terms of effective elastic modulus (E), hardness (H), yield strength (YS), and ultimate tensile strength (UTS). For the BGA solder balls, the evolution of mechanical properties was measured using nanoindentation. Moreover, mechanical properties evolution of both specimens was compared in terms of normalized properties with respect to elapsed time under different thermal cycling exposures. Finally, the microstructural evolution of bulk solder bars was observed under slow thermal cycling exposures with elapsed time.

Spatiotemporal Trends of Temperature Extremes in Bangladesh Under Changing Climate Using Multi-statistical Techniques

The rise in the frequency and magnitude of extreme temperature phenomena across the globe has led to the recurrent incidence of global climate hazards, which have had severe effects on socioeconomic development. The daily maximum and minimum temperature datasets of 27 sites in Bangladesh were used to detect spatiotemporal trends of temperature extreme over Bangladesh during 1980-2017 based on ten temperature extreme indices using multi-statistical modeling namely linear regression, Pearson correlation coefficient and factor analyses. Besides, mutation analyses based on the Mann-Kendall test, Sen’s slope estimator and Pettit test were employed to show the changing trend in extreme temperature. Results show that except for warmest days, the warm indices showed an increasing trend, mainly since the 2000s, while the growth rate was faster, and the response to global climate warming was sensitive. The cold indices demonstrated a reverse trend since the 2010s. Diurnal temperature range (DTR) and summer days (SU) increased faster, implying that the rising speed of daily max temperature was higher than of daily min-temperature in Bangladesh. The detrended fluctuation analysis (DFA) revealed a continuous increase in temperature extreme in the future except for cold days. The probability distribution functions (PDF) analysis revealed an evident variation of the curves in recent decades compared to the past three decades. Besides the warm night, DTR and SU primarily control the general warming trend of temperature extremes over Bangladesh during the study period. The mutation of the warm indices occurred before the cold index, indicating that the warm indices were more sensitive to global climate warming. The temperature extremes recognized in our research suggest that elevated warm temperature extremes due to global climate warming may have huge implications on the sustainable development of Bangladesh in the forthcoming period.

Trend in temperature extreme indices over Mongolia and associated large-scale circulations

Employing the percentile-based indices, TN10p, TX10p, TN90p, and TX90p during 1961–2018, we evaluate temporal and spatial trends in extreme temperature at 54 stations over Mongolia. Statically significant changes in temperature extremes in the warm (TN90p and TX90p) and cool indices (TN10p and TX10p) are found. The rate of increase in the number of warm nights and days are respectively 1.5 and 1.9 days decade− 1, while the cool nights and days show a declining trend of -0.8 and − 1.5 days decade− 1, respectively. Despite the fact that the trends are most vigorous during June-July-August, seasonal variations can be seen. Also, spatial distributions of the trends reveal weakest magnitudes in Gobi Desert, while strongest in the west and north of Mongolia. The large-scale atmospheric circulations account for changes in the temperature extreme indices. The East Atlantic, East Atlantic/western Russian, and Scandinavian patterns, and the Arctic Oscillation is found to contribute the most to the interannual variation in the temperature extremes.

Evaluation of the Creep Response of Lead Free Solder Materials Subjected to Thermal Cycling

In the electronic assembly arena, lead is being targeted due to the concern regarding environmental pollutants. So, the lead-free solder and its reliability are getting highlighted. During qualification testing or actual use lead-free solders in electronic assemblies, they are often subjected to thermal cycling. In the lead-free solder material, microstructural evolution and material property degradation occurs due to the thermal aging phenomena during dwell periods at the high temperature extreme of thermal cycling. In addition, during ramping between low and high temperature extreme, lead-free solders can experience additional aging phenomena. In our prior work, we have compared material properties (stiffness and strength) degradation of lead-free solder materials in isothermal aging and five different thermal cycling exposures. Changes in material properties were higher for all the thermal cycling exposures compared to the aging. In addition, microstructural evolution and material property degradation were exacerbated as the ramp rate decreased in the thermal cycling. In this study, the creep behavior evolutions occurring in SAC305 lead free solder subjected to isothermal aging and slow thermal cycling exposures have been investigated. Uniaxial test specimens were prepared by reflowing solder in rectangular cross-section glass tubes with a controlled temperature profile. Afterwards, the reflowed samples were exposed to either isothermal aging at 125 °C, or to thermal cycling from −40 to +125 °C, under a stress-free condition (no load) for various durations in an environmental chamber. A slow thermal cycling profile, e.g. 150 minutes cycle with 45 minutes ramps and 30 minutes dwells, was chosen for this study as it was found in our previous work that it caused the most detrimental effects on the mechanical behavior evolution. The thermally exposed samples were isothermally aged for 0, 1, 2, and 5 days; or were thermally cycled for 0, 48, 96, and 240 slow thermal cycles, which had the same aging times at the high temperature extreme of T = 125 °C. After aging or cycling, creep testing was performed at room temperature on the thermally exposed samples at three different stress levels (10, 12, 15 MPa). The evolutions of the secondary creep strain rate were obtained as a function of the stress level, as well as the net aging time at the high temperature extreme, and then compared. Results showed that secondary creep strain rate increased dramatically with equivalent aging time, and that the degradation effects were larger for slow thermal cycling than for pure aging. For example, the creep rate increased by 3.0–3.4X for 5 days of pure aging at T = 125 °C; while they increased by 10.9–13.1X for 240 thermal cycles, when there had been 5 days of equivalent aging at the T = 125 °C high temperature extreme.