Pradeep Lall

This study focuses on the feature vector identification and remaining useful life (RUL) estimation of SAC305 solder alloy printed circuit boards (PCBs) of two different configurations during varying conditions of temperature and vibration. The feature vectors are identified using the strain signals acquired from four symmetrical locations of the PCB at regular intervals during vibration. Two different types of experiments are employed to characterize the PCB's dynamic changes with varying temperature and acceleration levels. The first type of analysis emphasizes the vibration characteristic for varying acceleration conditions while holding the temperature levels constant, and the second studies the shift in characteristics for various ambient temperatures by maintaining constant acceleration levels. The above analyses attempt to imitate an electronic board's actual working conditions in an automobile subjected to varying temperature and vibration environments. The strain signals acquired during each of these experiments are compared based on both time and frequency domain characteristics. Different statistical and frequency-based techniques were used to identify the strain signal variations with changes in the environment and loading conditions. The feature vectors in predicting failure at a constant working temperature and load were identified, and as an extension to this work, the effectiveness of the feature vectors during varying conditions of temperature and acceleration levels is investigated. The principal component analysis (PCA) is used as the data reduction and pattern recognition technique to the different operating conditions of vibration environments. The residual of the autocorrelation function (ACF) of frequency and instantaneous frequency (IF) matrices shows behavior that can predict the packages' failure on the PCB irrespective of varying conditions of temperature and acceleration levels. This feature vector's effectiveness is verified on two different configurations of PCB and for the two separate analyses discussed prior in the study. The RUL of the packages was estimated using a deep learning approach based on long short-term memory (LSTM) network. This technique can identify the underlying patterns in multivariate time series data that can predict the packages' life. The ACF's residuals were used as the multivariate time series data in conjunction with the LSTM deep learning technique to forecast the packages' life at different varying temperatures and acceleration levels during vibration.

Lead-free solder joints are the most widely used interconnects in electronic packaging industries. Usually solder joints in most of the electronic devices are exposed to an environment where variation of temperature exists, which indicates cyclic thermal loading to be a very common type of external loading. Moreover, due to difference in the coefficient of thermal expansion (CTE) among dissimilar contact materials, shear stress develops in junctions under thermal loading, which significantly deteriorates the overall reliability. Hence, characterization of lead-free solder materials under thermal loading is essential to predict the performance and deformation behavior of joints in practical applications. A significant portion of the studies in this field are concerned with thermal loading of lead-free solder interconnects, each of which has a very small diameter, in sub-millimeter range. Although the solder balls have very small dimensions, most of the analyses considered them as a bulk material with homogeneous and isotropic properties. However, with the decrease of specimen dimensions, size effects and material directionality play a significant role in deformation mechanisms. Since a very few grains exist in a small specimen, individual grain properties play a vital role on overall material response. Therefore, modeling from the grain structure and orientation point of view could be an effective and more accurate way to predict solder joint deformation behavior under thermal loading. In this study, the effect of grain size and orientation of SAC305 is investigated for predicting anisotropic behavior of solder joints under thermal load. A simplified three-dimensional model of beach-ball configuration solder joint was generated and simulated using ABAQUS finite element (FE) software. Experimentally obtained directional properties such as elastic modulus and CTE were assigned to the computational geometry to create material anisotropy. The effects of material anisotropy were studied for varying grain size specimens, as well as for specimens with varying grain orientation.

Electronic packages are frequently exposed to thermal cycling environments in real life applications. Particularly, the plastic ball grid array (PBGA) is one of the most widely used electronic packages, and consists of various component materials, e.g. solder joint, silicon die, die attachment adhesive, mold compound, solder mask, etc. All of these materials play a significant role on the reliability of the overall PBGA assembly. Hence, it is important to study their mechanical property evolution under the thermal cycling environment. Our prior work has shown that cyclic temperature variations cause the solder joints to change their mechanical behavior. However, the other component materials can also change their mechanical properties during cycling. These changes must be evaluated in order to understand and predict their failure behavior in operation.In our previous study, evolution of mechanical properties of SAC305 solder joints in a PBGA package up to 250 thermal cycles was evaluated using the nanoindentation technique. In this work, the nanoindentation technique was utilized to understand the evolution of mechanical properties (modulus, hardness, and creep behavior) of the die attachment adhesive, silicon die, and solder mask material for various durations of thermal cycling. Test specimens were first prepared by cross sectioning a PBGA package to reveal the different materials, followed by surface polishing to facilitate SEM imaging and nanoindentation testing. After preparation, the package samples were thermally cycled from T = -40 to 125 ^o C in an environmental chamber. At various points in the cycling (e.g. after 0, 50, 100, and 250 cycles), the package was taken out from the chamber, and nanoindentation was performed on above mentioned materials to obtain modulus, hardness, and creep behavior at room temperature (T = 25 ^o C). From the nanoindentation test data, it was found that die attachment and solder mask materials showed increases in modulus and hardness up to 250 cycles. As expected, the silicon die material does not show any significant change in mechanical behavior during same duration of thermal cycling.

Digital printing technologies are rapidly gaining importance for manufacturing electronic devices with their extremely low fabrication cost and be able to print on flexible and conformal substrates such as polyimide rather than conventional FR-4. One of the technologies, Aerosol Jet, works well when it comes to the wide selection of substrates and materials to be able to print. AJP process utilizes aerodynamic focusing to focus a collimated mist of Nano-particles onto a substrate using an inert carrier gas known as sheath gas, such as nitrogen. To account for the wide array of materials, the process further breaks down to two different types of atomization: UA for low viscosity materials (1 to 5cP) which utilizes ultrasonic waves to atomize; and PA for low and high viscosity materials (1 – 100°CP) which utilizes an inert carrier gas to atomize, thereby suspending the Nano-particles into air which travels through a mist tube and gets deposited. Aerosol-jet printing process requires the control of multiple parameters simultaneously for a fine print such as atomizer flow rate, sheath flow rate, nozzle size, print speed, base temperature, UA current, print height (distance between nozzle exit and the substrate), number of passes. Furthermore, for desired pattern, the print process requires a toolpath file to follow which can be generated from a 2D CAD model, implying there is no specification of the line width. The desired line width is obtained by changing the above-mentioned parameters accordingly, which can be time consuming since each material is of a different viscosity and varying metal and solvent composition. In this paper, a detailed approach for making a 2 layer and 5 layer micro-via using metallized silver Nano-particle ink and a dielectric adhesive is shown. Sintering of the nano-particle ink is one of the most important manufacturing step that governs the electrical and mechanical properties of the 3D printed micro-via. Prior data shows that sintering time and temperature have a big effect of the resistivity, shear load to failure and micro structure of the silver nano particles. In this paper, the most appropriate sintering profile for the conductive lines was found by progressively tracking the resistivity for varied candidate sintering profiles. For the best sintering profile obtained, progression of resistivity, shear load to failure and microstructure growth was monitored at each sintering step.

Interest in the field of additively printed electronics has grown owing to added flexibility with design and manufacture of parts in low volumes and lower time between design and prototype. The ability to directly print on a number of substrates, whether be rigid, flexible, or conformable, brings many advantages over traditional techniques used for electronics fabrication. Mainstream digital printing techniques include aerosol-jet, inkjet, dispense-on-demand, gravure offset are becoming more and more well known in the field of printed electronics, each having its own merit. In addition, given the fact that electronic systems are fundamentally multi-material, it is important to understand the material-material interaction including their compatibility, curing conditions, adhesion. Aerosol Jet technology supports a wide range of materials, viscosity ranging from 1cP – 1000cP. It involves use of two atomization processes: Ultrasonic Atomizer (UA) for materials with low range of viscosity (1-5cP); and Pneumatic Atomizer (PA) for higher range (1-1000cP). In multi-material printing, additional problems need to be addressed including degradation in the printed base layer from curing while printing a multilayer substrate. This paper provides an insight gained from investigations for the curing profile for all the layers. The materials chosen for the study are such that both the ultrasonic and pneumatic atomizers can be used simultaneously without having to remove after printing. With that in mind, a Silver Nanoparticle ink as a conducting material is used with low viscosity suitable for UA, and a dielectric as an insulating material with high viscosity suitable for PA. The dielectric will be used in the areas needed only. Process parameters will be developed for a multi-layer substrate, and the consistency of in the printed lines is addressed. In addition, process capability using statistical measures is performed. This results from this study will help gain more understanding into the problems that are not clearly visible while manufacturing a printed multi-layer substrate with vias through Aerosol Jet.

Solder joints in electronic packages often experience fatigue failures due to cyclic mechanical stresses and strains in fluctuating temperature environments. These stresses and strains induced by mismatches in coefficients of thermal expansion lead to damage accumulation and contribute to the crack initiation, crack propagation, and eventually to failure. In this study, we have investigated the cyclic stressstrain behavior of a six element lead free solder alloy SAC-BiNi-Sb (Innolot) optimized for high temperature automotive applications. The main purpose of this work was to investigate the accumulation of damage in this solder material during mechanical cycling. Circular cross-sectioned solder specimens were first reflowed, and these samples were then mechanically cycled for various durations using a Micro-Mechanical tester. Cyclic stress-strain, monotonic stress-strain, and creep tests were then conducted on the prior cycled samples. The cyclic stress-strain curves obtained were studied to observe the degradation of hysteresis loop properties (peak stress, hysteresis loop area, and plastic strain range) with cycling. The monotonic stress-strain and creep test data were plotted, and various mechanical properties were characterized for various levels of cycling. Using the data from the various tests, we have been able to characterize and quantify the cycling induced damage through the observed degradations of several mechanical properties (elastic modulus, yield strength, ultimate strength, and creep strain rate) with the amount of prior cycling. The measured data for the SAC-Bi-Ni-Sb solder were compared to the results from our previous studies conducted on SAC305 and SAC+Bi (SAC_Q) lead free alloys.

Electronics in a number of harsh environments may be subjected to very-low operating temperatures in conjunction with high strain rates. Examples include the automotive underhood environment, in which electronics on-engine and on- transmission may be exposed to low temperatures for sustained period in addition to high strain-rates resulting from engine vibration and road shocks. Further, electronics in downhole applications may be mounted just before the drill tip and be exposed to high strain rate resulting from the deep-well drilling in addition to exposure to extreme cold-temperatures while drilling in cold-climates. Transition of the industry to leadfree solder alloys for second level interconnects has necessitated the fundamental studies for long-term reliability assurance of products in harsh environments. There is scarcity of data for solder alloys at extreme cold temperatures and high-strain rates in the neighborhood of 1-100 per sec. Low strain-rate mechanical properties for SAC allots have shown degradation. In this paper, SAC105 and SAC305 leadfree solder alloys have been tested and studied at several lower surrounding temperature starting from - 65C up to +200C, with high strain rates. The prepared lead-free alloy specimen after reflowing, have been tested at strain rates of 10, 35, 50, and 75 per sec. The calculated experimental data has been fit to the Anand Viscoplasticity model. In order to assess the predictive power of the model, the model predictions from computed parameters have been compared with experimental data.

In temperature changing environments, solder joints often experience fatigue failure due to cyclic mechanical stresses and strains induced by mismatches in the coefficients of thermal expansion. These stresses and strains lead to damage accumulation and contribute to the crack initiation, crack propagation, and eventually to failure. In this study, we have investigated the cyclic stress-strain behavior of SAC305 and SAC_Q reflowed lead free solders that occur at various testing temperatures and with various prior aging conditions. Lead free solder uniaxial test specimens with circular cross-section have been prepared using vacuum suction method and then were aged for 0 to 20 days at 125 °C. The samples were then subjected to cyclic stress-strain loading using a Micro-mechanical tester at different testing temperatures from T = 25 C to T = 100 C. The evolution of hysteresis loops with duration of prior aging was characterized by measuring the strain energy density dissipated per cycle (loop area), peak stress, and plastic strain range. It was observed that aging degrades the mechanical fatigue properties due to microstructural coarsening. At elevated temperatures, a drop in the loop area and peak stress and an increase in the plastic strain range for both lead free reflowed solder materials were obtained. In addition, SAC_Q samples had a higher loop area and peak stress compared to SAC305.

Recent surge in demand for wearable technology products such as activity tracking smartwatches, and for medical devices has necessitated development of flexible secondary lithium ion batteries which also possess high capacity, robustness and thin form factors. Oftentimes, these power sources are only charged up to a partial state of charge (SoC) before use (shallow charge). Their usage continues until the SoC reaches almost zero, after which they are recharged again. Nowadays, the ‘fast-charge ‘feature used to charge the battery at higher C-rates, is a necessity in consumer electronics rather than an amenity. Also, in everyday use, these batteries are exposed to higher-than-ambient temperatures due to perpetual human body contact and also to the high temperatures resulting from poor thermal management in compact devices. This study investigates the compounded influence of partial charge, high temperatures and high C-rates on the capacity degradation of a flexible Li-ion power source subjected to accelerated life testing. The battery current and terminal voltage were logged for multiple charge-discharge cycles and were used to compute the battery capacity and energy efficiency. Finally, a regression model based on several parameters was developed to estimate the battery capacity as a function of the cycle number.

Feature vectors for health monitoring of electronic assemblies under repetitive mechanical shock have been developed for assemblies subject to 3,000g acceleration levels. The resistance and strain measurements of the PCB are acquired during each drop to analyze the changes in the values during the experiment. Analysis on the progression of failure was carried out using frequency-based techniques on the strain signals from different locations of the board and failure of the package was identified from the increase in the resistance values of the package during the drop. Feature vectors selected were based on the time-frequency data as well as the logarithmic decrement of the strain signals during the different drops. Different statistical approaches on identifying the changes in the damping characteristics of the package during drop were also carried out. Statistical analysis on the changes in the resistance values were quantified in accordance with the changes in the strain and correlation of the both were attempted. The dependence on position of the strain gauge on the PCB were also studied by comparing the variation of the feature vectors of the corresponding strain signals. The before and after failure strain signals were compared on the frequency components and as well as the changes in the damping characteristics of the strain signals.