The viscoplastic steady-state behavior of porous sintered silver is determined at different temperatures using a combination of indentation testing and modeling. The microstructure of the sintered silver consists of agglomerates of silver particles and flakes, separated either by voids or by resin-rich regions. The characteristic length-scale of the microstructural heterogeneity is characterized with electron microscopy and image-processing methods. This study focuses on the dependence of creep behavior on hydrostatic stress and on length-scale, due to the heterogeneous morphology of the sintered material. Testing consists of constant-force indentations using spherical indenters of two different radii at three different temperatures: 25 degrees C,75 degrees C, and 125 degrees C. The indentation results were analyzed using an inverse-fitting approach using detailed global-local computational viscoplastic finite element analysis. The results show that indenters of different radii measure significantly different viscoplastic properties at each stress level and at each temperature. These differences are believed to be due to: (i) the porosity of the sintered material and (ii) the relationship of the indenter tip size to the characteristic length-scale of the heterogeneity of the sintered microstructure. The influence of the first factor is identified by the use of a Drucker-Prager creep model, which incorporates hydrostatic stress (since the inelastic deformation of porous materials is known to be dependent on the pressure). The aim of explicitly accounting for the effect of Factor (i) above is to isolate and identify the remaining difference observed due to length-scale effects (listed in Factor (ii) above), when interrogating the creep behavior with indenters of different radii.

To understand the reliability and repairablityof electronics printed using the AJP process, through experimental and computational modeling, to take full advantage of this technology and realize its industrial potential. This work is sponsored by the members of the CALCE at the University of Maryland, College Park and Laboratory for Physical Science.

Safety critical electronics in automotive, avionic or military applications have to work reliably in a range of harsh conditions, including simultaneous mechanical and thermal loads (Gromala, 2021; Qi et al., 2009 [1, 2]). The durability of electronics, more specifically their board level interconnects, under vibration and thermal loads must be investigated and understood for interconnects assembled with lead-free solder alloys. In this study, isothermal vibration fatigue tests have been carried out at-40 degrees C, 25 degrees C and 125 degrees C. A novel test specimen, specifically designed for combined vibration and temperature cycling experiments, was used for this study. The test PCB contains leadless chip resistor (LCR) components, soldered using the SAC105 solder alloy (Sn98.5Ag1.0Cu0.5). Failed specimens at each temperature were subjected to destructive physical analysis (DPA) for root cause analysis (RCA) of the observed failures. Dynamic finite element analysis (FEA) was carried out and combined with the experimental results in order to generate fatigue strain-life (S-N) curves. Two opposing effects of increasing temperature -viz. decrease of fatigue strength for high-cycle fatigue (HCF) and increase of fatigue ductility for low-cycle fatigue (LCF) -are hypothesized to be responsible for this non-monotonic behavior.

Sintered silver materials (with and without epoxy matrices) are used in microelectronics, as high-temperature interconnect materials, and also as conductor trace materials in printed electronic circuitry. The sintering process results in an interconnected assemblage of discrete agglomerated particles. This results in intrinsic length-scale effects under the action of different stress gradients. In other words, the effective homogenized average continuum-scale material behavior changes with the local magnitude of the stress gradients. Consequently, regions of sharp, localized stress concentrations have to be modeled with different effective continuum material properties, compared with the properties that are relevant for regions that have a uniform stress field. In this study, the focus in on the effective creep behavior, in particular. This length-scale effect is empirically explored in this study using nanoindentation with indenters of different tip radii, causing different stress gradients. Properties estimated by each indenter are compared to demonstrate the dependence of the effective continuum properties on the local length scale effects (generated by the ratio of the tip radius to the characteristic discrete dimension of the sintered particles).

In this work, fatigue damage caused to solder joints in printed wiring assemblies due to the superposition of harmonic vibration and temperature is studied along with different damage superposition approaches. In this work, a non-linear interactive damage superposition method is used, with temperature-dependent vibration damage coefficients that are interpolated from isothermal vibration experiments at selected temperatures. The temperature range -40°C to 125 °C was segmented into multiple sub-segments for this interpolation. By applying these measures, a new procedure for an improved incremental damage superposition approach (IDSA) was developed in this work. This new procedure was then used to superpose the vibration and temperature cycling loads to forecast the fatigue life of SAC105 solder joints for leadless chip resistors under simultaneous vibration and temperature cycling loads. Potential failure sites of the solder joint have been successfully investigated by utilizing this new procedure. Using the developed model, accelerated testing profiles are designed such that the vibration fatigue and temperature cycling fatigue damage are comparable and cause failure in approximately 250 temperature cycles.

Aerosol-Jet Printing (AJP) provides a new method for electronic component manufacturing. Understanding the reliability of electronics printed using the AJP process is essential to take full advantage of this technology and realize its industrial potential. In the current study, we have designed and tested AJP printed samples and conducted failure analysis of those samples that have exhibited early failures. Failures first occurred in the short traces that connect the main traces to the silver pads, due to local stress-raisers caused by local geometric features in the printing geometry. Thermal-Mechanical Finite-Element-Modeling (FEM) has been performed to analyze the cyclic history of thermo-mechanical stress distribution and plastic strain distribution.

Work on establishing an experimental setup for isothermal vibration experiments has been recently introduced. The setup enables tests at temperatures from -40 °C to +150 °C and well-defined load conditions to be applied to specially designed specimens. The specimens can be assembled with LCR or FC components for analysis of their solder joint fatigue behaviour. Here, we present the applied experimental procedure, fatigue results and dominant failure modes observed from tests on lead-free LCR solder joints. The tests not only aim for understanding the solder joint behaviour but also to prepare for tests at combined harmonic vibration and temperature cycling conditions.Harmonic vibration tests were performed on SnAg1.0Cu0.5 solder joints under isothermal conditions at -40 °C and room temperature. Experiments were conducted at various solder strain levels between 800 and 2,000 μstrain at both temperatures. Failure events (>20% resistance change) were detected in real-time during the test. Measures of cycles to failure and FEA calculated strain data have then been used to derive and compare S-N-curves: Solder joint durability is seen to significantly decrease at low temperature conditions. Cross sections were examined to determine failure modes at low and room temperature vibration loading: An increased risk of copper trace cracks was found at low temperature conditions in contrast to a higher probability of solder joint cracks at room temperature.

Electronic assemblies are often utilized in environments in which they are exposed to significant combinations of simultaneous vibration and thermal loads. In this study, vibration durability tests and results under isothermal conditions are presented for SAC105 solder joints of CR0805 chip resistor components. The tests are performed using a specially designed multi-cantilever printed wiring assembly (PWA) containing CR0805 components. Harmonic vibration durability tests were conducted at both room and low temperatures, for multiple deflection levels. The resistance of the resistor assemblies was continuously monitored to detect fatigue degradation of the interconnects during the vibration testing. A detailed 3D transient FE analysis is conducted to examine effective solder joint strains as a result of the applied deflections at various temperatures. The measured cycles to failure and calculated strains were used to derive S-N-plots. A comparison of fatigue behaviour at room and low temperature condition is drawn. Failure analysis results are presented to elucidate the failure modes.

The increasing demand for highly reliable electronic devices, even though they are exposed to harsh use conditions, is one of the main drivers for the development of electronic systems. System development process relies on the selection of materials, technologies and a proper design to meet the mission profile's demands. Among many others, the lead-free solder alloy SnAg1.0Cu0.5 (SAC105) is widely used for many electronic assemblies deployed for various applications. The fatigue behaviour of SAC105 under thermal loads (namely temperature cycling and shock testing) and drop testing has been covered extensively in the literature. Work on damage accumulation under vibration conditions has been accomplished but primarily at room temperature. Therefore, this work aims to expand knowledge of the fatigue behaviour of SAC105 under combined thermal and vibration loading. In this work, vibration durability experiments were conducted at temperatures from -40°C to +125°C and vibration peak-to-peak amplitudes from 0.6 mm to 1.6 mm. Currently, specimens have been subjected to tests with durations of 75×10 6 or 150×10 6 vibration cycles. Cross sections were analysed to relate damage locations and severity to stress conditions (temperature and vibration amplitude). As expected, damage levels were observed to increase with increasing temperatures and vibration amplitudes.

This work focuses on the reliability needs which are caused by the use of recent package solutions for harsh environmental use cases such as assisted or autonomous driving. Simultaneous thermal and mechanical loading of highly integrated packages as Flip-Chip (FC) packages has to be considered, investigated and understood. An earlier introduced test approach used to investigate CR0805 solder joints under combined loading was modified to enable the analysis of FC solder joints. Thus, investigations of solder joint geometries of FC, CSP and BGA packages are now possible. In this work, results on the fatigue behaviour of SnAgCu FC solder joints will be shown. The experiments were conducted under varied harmonic vibration amplitudes at room temperature. A 4.6 x 2.6 mm(2) bare die FC package with a 5 x 5 interconnection grid was tested. Bump size, pad diameter and stand-off are 370 jam, 330 lam and 280 lam, respectively. The damage and fatigue behaviour of the FC solder joints was examined using cross sections. First, test results show damage of solder joints stressed with a peak-to peak deflection of 1.6 mm for up to 75 Million cycles at room temperature. The damage occurred within the solder volume in very close proximity to or at the substrate pad intermetallic interface. Further tests considering varied stress levels are ongoing.

The results on a unique double-cantilever test specimen that has been designed specifically for combined temperature/vibration testing are reported in this study. This specimen consists of two symmetric sets of 4 identical cantilever beams. In prior studies with this double-cantilever test specimen, all the solder joints were stressed to the same level. Thus, a test on one specimen resulted in a single data point on the fatigue curve. In the present study a modification of the double-cantilever test setup is presented which results in a different resonant frequency for each of the two cantilever sets. The asymmetric response produces two different cyclic strain levels in the solder joints on each of the two cantilever sets. Furthermore, a test setup adaption for simultaneous excitation of two test specimens allows four different stress levels in each vibration test. This allows significant time-savings in fatigue testing, compared to sequential testing of four different specimens. Vibration durability results under isothermal conditions are presented, and the potential ability for future experiments with superimposed temperature cycling on vibration is discussed.

Electronic products are often exposed to combined stress loading conditions (both thermal excursions and vibration) during their service life. In order to assess the performance and durability under these conditions, it is important to use temperature dependent fatigue data to assess or more accurately predict vibration damage under more realistic life-cycle conditions. This paper focuses on finding the temperature dependent vibration durability of aged SAC105 solder material. This is accomplished by using a combination of harmonic vibration tests at different temperatures and nonlinear dynamic FEA. The fatigue durability is found to decrease significantly with increasing temperature, in part due to the increase in solder strain, and in part due to degradation of the solder cyclic fatigue properties. Additionally, the room temperature fatigue results were fitted with the Basquin-Coffin-Manson fatigue model to produce the temperature-dependent model constants for the S-N curves.

Nanoindentation has been popular since the 1970s for querying material behavior at extremely small length scales. This paper focuses on using a combination of indentation and FEA methods for characterizing the viscoplastic behavior of composite materials and sintered materials, which pose additional challenges because of the heterogeneous morphology. In particular, this paper focuses on two forms of pressure-less sintered silver interconnect materials: an adhesive-based particulate composite for low temperature applications and a porous sintered version for high-temperature applications. By use of indentation methodology to bound the bulk viscoplastic properties for both of these heterogeneous morphologies, both sintered materials in this study are estimated to be less creep resistant than SAC 305 at operational stress levels, at room temperature.

Moisture can be devastating for electronic components, causing corrosion and electrochemical metal migration, which leads to changes in resistance and short circuits. Although most products (the focus in this study is on solid state lighting products) have a sealed enclosure, the driver electronics are often not as well sealed against the environment. Moisture diffusion through surrounding sealants is a primary pathway of moisture ingress into the electronics enclosures. This study's focus is on quantifying the moisture diffusion rates for various sealant materials and on modeling the moisture ingress rates into sealed containers, using commercial FEA software. This paper also presents a simulation- assisted procedure to extract the moisture diffusion properties for the seal material because most seal structures (gaskets, O-rings, etc.) are not simple onedimensional structures. The resulting geometric correction factor is obtained for toroidal O- rings of circular cross- section and also for elliptic cross- sections caused by compression forces.