Experimental strain energy density dissipated in SAC305 solder joints during different thermal cycling conditions using strain gages measurements

International audienceDespite being widely investigated for the last two decades, solder joints thermomechanical durability assessment remains a major concern for industries wishing to switch from lead-based (SnPb) to lead-free electronics. Amongst the variety of lead-free solder compositions, 96.5Sn-3.0Ag-0.5Cu (SAC305) solder alloy has become the preferred substitute to classic SnPb solders. However, unlike SnPb assemblies, the return on experience is limited and the microstructure is very different for SAC305 solder joints. The use of SAC305 solder paste requires to understand the mechanical and fatigue behaviors of the soldered interconnects. This paper presents the experimentation based on strain gages measurements, allowing the determination of the shear stress-strain response of SAC305 solder joints subjected to different thermal cycling conditions. The area of the experimental shear strain-stress hysteresis loops gives the values of the strain energy density corresponding to each thermomechanical loading. The finite element modeling of the test assembly showed a good correlation between experimental and numerical strain energy densities. The experimental shear strain-stress curves also provide the necessary data to derive SAC305 solder joints constitutive laws

Influence of P+ layer parameters on 4H-SiC UV PiN photodetector characteristics

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Anharmonicity of the antiferrodistortive soft mode in barium zirconate BaZrO3_3

Barium zirconate (BaZrO$_3$) is one of the very few perovskites that is claimed to retain an average cubic structure down to \SI{0}{\K}, while being energetically very close to an antiferrodistortive phase obtained by condensation of a soft phonon mode at the R point of the Brillouin zone boundary. In this work, we report a combined experimental and theoretical study of the temperature dependence of this soft phonon mode. Inelastic neutron and x-ray scattering measurements on single crystals show that it softens substantially from \SI{9.4}{\meV} at room temperature to \SI{5.6}{\meV} at \SI{2}{\K}. In contrast, the acoustic mode at the same R point is nearly temperature independent. The effect of the anharmonicity on the lattice dynamics is investigated non-perturbatively using direct dynamic simulations as well as a first-principles based self-consistent phonon theory, including quantum fluctuations of the atomic motion. By adding cubic and quartic anharmonic force constants, quantitative agreement with the neutron data for the temperature dependence of the antiferrodistortive mode is obtained. The quantum fluctuations of the atomic motion are found to be important to obtain the proper temperature dependence at low temperatures. The mean squared displacements of the different atoms are determined as function of temperature and are shown to be consistent with available experimental data. Adding anharmonicity to the computed fluctuations of the Ba-O distances also improves the comparison with available EXAFS data at \SI{300}{\K}

Experimental strain energy density dissipated in SAC305 solder joints during different thermal cycling conditions using strain gages measurements

Despite being widely investigated for the last two decades, solder joints thermomechanical durability assessment remains a major concern for industries wishing to switch from lead-based (SnPb) to lead-free electronics. Amongst the variety of lead-free solder compositions, 96.5Sn-3.0Ag-0.5Cu (SAC305) solder alloy has become the preferred substitute to classic SnPb solders. However, unlike SnPb assemblies, the return on experience is limited and the microstructure is very different for SAC305 solder joints. The use of SAC305 solder paste requires to understand the mechanical and fatigue behaviors of the soldered interconnects. This paper presents the experimentation based on strain gages measurements, allowing the determination of the shear stress-strain response of SAC305 solder joints subjected to different thermal cycling conditions. The area of the experimental shear strain-stress hysteresis loops gives the values of the strain energy density corresponding to each thermomechanical loading. The finite element modeling of the test assembly showed a good correlation between experimental and numerical strain energy densities. The experimental shear strain-stress curves also provide the necessary data to derive SAC305 solder joints constitutive laws

Experimental SAC305 shear stress-strain hysteresis loop construction using Hall's one-dimensional model based on strain gages measurements

Temperature-induced solder joint fatigue is a main reliability concern for aerospace and military industries whose electronic equipment used in the field is required to remain functional under harsh loadings. Due to the RoHS directive which eventually will prevent lead from being utilized in electronic systems, there is a need for a better understanding of lead-free thermomechanical behavior when subjected to temperature variations. Characterizing solder joints properties remains a challenge as viscoplastic behavior during thermal cycling is complex, and their small dimensions prevent direct measures from being performed. This paper reports the experimentation based on strain gage measurements, allowing the construction of the shear stress-strain hysteresis loop corresponding to Sn3.0Ag0.5Cu (SAC305) solder joints behavior during thermomechanical loading. This methodology, initially developed in 1984 by P. Hall for Sn60Pb40 interconnects, allows the measurement of the strain energy density dissipated during temperature cycles. The approach developed in this study has two objectives: 1) conduct thermal cycles until failure in order to get the number of cycles to failure (N1%), 2) instrument a specific assembly with strain gages to plot the corresponding shear stress-strain hysteresis loop and allow experimental characterization of SAC305 solder joints. Custom daisy-chained 76 I/O Ceramic Ball Grid Array (CBGA76) components were designed and assembled on flame retardant (FR-4) multi-layered (8 copper ground planes) Printed Circuit Boards (PCB). The component and the PCB were optically characterized to measure their corresponding Coefficient of Thermal Expansion (CTE). Four strain gages were specifically placed at the center of the assembly on top and bottom faces of both PCB and CBGA76 component. As-reflowed solder joints were also investigated to ensure that the resulting microstructure and β-Sn grain morphology are representative of those usually observed in SAC305 solder joints after reflow. A slow rate thermal cycle profile was then considered to allow viscoplastic phenomena to occur and the corresponding SAC305 solder joints shear stress-strain hysteresis loop was plotted. Failure analysis revealed that Ag3Sn coarsening and recrystallization occurred which ensure the representativeness of the microstructural changes usually observed on failed SAC305 interconnections during thermomechanical damage. The correlation between the measured strain energy density and measured lifetime corresponds to one point of the energy based fatigue curve for SAC305 solder joints

Mechanical fatigue assessment of SAC305 solder joints under harmonic vibrations

Vibration-induced solder joint fatigue is a main reliability concern for aerospace and military industries whose electronic equipment used in the field is required to remain functional under such loading. Due to the RoHS directive which eventually will prevent lead from being utilized in electronic systems, there is a need for a better understanding of lead-free mechanical behavior under vibration conditions. This study reports the durability of Sn3.0Ag0.5Cu (SAC305) solder joints subjected to harmonic solicitations at three specific temperatures (-55°C, 20°C and 105°C). A test assembly is designed and consists in a single daisy-chained 1152 I/O ball grid array (FBGA1152) package assembled on a flame retardant (FR-4) printed circuit board (PCB). The vibration levels are imposed by a controlled deflection at the center of the board at its natural frequency. The electric continuity is monitored to determine the number of cycles to failure of each sample. Mode shape measurements with a scanning vibrometer are also conducted and correlated with Finite Element Analysis (FEA) to ensure accurate calculation of stress within the critical solder balls at the corners of the component. The failed specimens are then cross-sectioned in order to determine failure modes. A comparison of SAC305 durability with SnPb36Ag2 solder is given, along with a set of lifetime measurements for two complementary assemblies: 68 I/O Leadless Chip Carrier (LCC68) and 324 I/O Plastic Ball Grid Array (PBGA324). It turns out that SAC305 outperforms SnPb36Ag2 and the effect of temperature on the mechanical durability of SAC305 appears to be minor. Failure analysis points out different failure modes such as ductile and brittle cracks at the interface between the solder bulk and the component, along with pad cratering-induced copper trace failures. FEA calculations provide data to estimate the high cycle fatigue (HCF) behavior of SAC305 solder under harmonic vibrations

Microstructural evolutions of Sn-3.0Ag-0.5Cu solder joints during thermal cycling

Temperature-induced solder joint fatigue is a main reliability concern for aerospace and military industries whose electronic equipment used in the field is required to remain functional under harsh loadings. Due to the RoHS directive which eventually will prevent lead from being utilized in electronic systems, there is a need for a better understanding of lead-free thermomechanical behavior when subjected to temperature variations. As solder joints mechanical properties are dependent of their microstructural characteristics, developing accurate solder joint fatigue models means to correctly capture the microstructural changes that undergo the solder alloy during thermal cycling. This study reports the Sn3.0Ag0.5Cu (SAC305) solder joints microstructural evolution during damaging temperature cycles. Electron BackScatter Diffraction (EBSD) analysis was conducted to assess the SAC305 microstructure corresponding to a specific damage level. Investigated microstructural features included the β-Sn grain size and crystallographic orientation, as well as the grain boundary misorientation and Ag3Sn intermetallic compound (IMC) size. As-reflowed and damaged components were also mechanically characterized using nanoindentation technique. The microstructural analysis of SAC305 solder joints prior to thermal cycling showed a highly textured microstructure characteristic of hexa-cyclic twinning with two β-Sn morphologies consisting of preferentially orientated macrograins known as Kara's beach ball, along with smaller interlaced grains. The main observation is that recrystallization systematically occurred in SAC305 solder joints during thermal cycling, creating a high population of misoriented grain boundaries leading to intergranular crack initiation and propagation in the high strain regions. The recrystallization process is accompanied with a progressive loss of crystallographic texture and twinning structure. Ag3Sn IMCs coalescence is another strong indicator of SAC305 solder damage since the bigger and more spaced the IMCs are the less dislocation pinning can prevent recrystallization from occurring

Mechanical fatigue assessment of SAC305 solder joints under harmonic and random vibrations

Vibration-induced solder joint fatigue is a main reliability concern for aerospace and military industries whose electronic equipment used in the field is required to remain functional under harsh loadings. Due to the RoHS directive which eventually will prevent lead from being utilized in electronic systems, there is a need for a better understanding of lead-free mechanical behavior under vibration conditions. This study reports the durability of Sn3.0Ag0.5Cu (SAC305) solder joints subjected to harmonic solicitations at three specific temperatures (-55°C, 20°C and 105°C) and random vibrations at ambient temperature (20°C). A test assembly was designed and consisted in a single daisy-chained 1152 I/O ball grid array (FBGA1152) package assembled on a flame retardant (FR-4) printed circuit board (PCB). The vibration levels were imposed by a controlled deflection at the center of the board at its natural frequency. The electric continuity was monitored to determine the number of cycles to failure of each sample. Mode shape measurements with a scanning vibrometer were also conducted and correlated with finite element analysis (FEA) to ensure accurate calculation of strain within the critical solder balls at the corners of the component. The failed specimens were then cross-sectioned in order to determine failure modes. A comparison of SAC305 durability with SnPb36Ag2 solder is given, along with a set of lifetime measurements for two complementary assemblies: 68 I/O Leadless Chip Carrier (LCC68) and 324 I/O Plastic Ball Grid Array (PBGA324). For the tested harmonic vibration levels, SAC305 outperforms SnPb36Ag2. Furthermore, the effect of temperature on the mechanical durability of SAC305 appears to be minor. Failure analysis pointed out different failure modes on PCB and component side, along with pad cratering and copper trace failures. FEA calculations allows the determination of the SAC305 fatigue curve to estimate the high cycle fatigue (HCF) behavior of SAC305 solder under harmonic vibrations. The random vibrations durability of SAC305 solder was assessed using the same test assembly (FBGA1152) which was subjected to three different levels of Power Spectral Density (PSD) at 20°C. The random vibrations tests were conducted within a frequency band ranging from 500 Hz to 900 Hz around the natural frequency. The chosen PSD levels applied were 0.04, 0.10 and 0.20 g2/Hz. Using power-law fitting, the results give a first estimation of the durability of SAC305 solder joints subjected to random vibrations

Enhance quality care performance: Determination of the variables for establishing a common database in French paediatric critical care units

Abstract Selected variables for the French Paediatric Intensive Care registry. Rationale, aims, and objectives Providing quality care requires follow-up in regard to clinical and economic activities. Over the past decade, medical databases and patient registries have expanded considerably, particularly in paediatric critical care medicine (eg, the Paediatric Intensive Care Audit Network (PICANet) in the UK, the Australian and New Zealand Paediatric Intensive Care (ANZPIC) Registry in Australia and New Zealand, and the Virtual Paediatric Intensive Care Unit Performance System (VPS) in the USA). Such a registry is not yet available in France. The aim of this study was to determine variables that ought to be included in a French paediatric critical care registry. Methods Variables, items, and subitems from 3 foreign registries and 2 French local databases were used. Items described each variable, and subitems described items. The Delphi method was used to evaluate and rate 65 variables, 90 items, and 17 subitems taking into account importance or relevance based on input from 28 French physicians affiliated with the French Paediatric Critical Care Group. Two ratings were used between January and May 2013. Results Fifteen files from 10 paediatric intensive care units were included. Out of 65 potential variables, 48 (74%) were considered to be indispensable, 16 (25%) were considered to be optional, and 1 (2%) was considered to be irrelevant. Out of 90 potential items, 62 (69%) were considered to be relevant, 23 (26%) were considered to be of little relevance, and 5 (6%) were considered to be irrelevant. Out of 17 potential subitems, 9 (53%) were considered to be relevant, 6 (35%) were considered to be of little relevance, and 2 (12%) were considered to be irrelevant. Conclusions The necessary variables that ought to be included in a French paediatric critical care registry were identified. The challenge now is to develop the French registry for paediatric intensive care units

The GenTree Dendroecological Collection, tree-ring and wood density data from seven tree species across Europe

The dataset presented here was collected by the GenTree project (EU-Horizon 2020), which aims to improve the use of forest genetic resources across Europe by better understanding how trees adapt to their local environment. This dataset of individual tree-core characteristics including ring-width series and whole-core wood density was collected for seven ecologically and economically important European tree species: silver birch (Betula pendula), European beech (Fagus sylvatica), Norway spruce (Picea abies), European black poplar (Populus nigra), maritime pine (Pinus pinaster), Scots pine (Pinus sylvestris), and sessile oak (Quercus petraea). Tree-ring width measurements were obtained from 3600 trees in 142 populations and whole-core wood density was measured for 3098 trees in 125 populations. This dataset covers most of the geographical and climatic range occupied by the selected species. The potential use of it will be highly valuable for assessing ecological and evolutionary responses to environmental conditions as well as for model development and parameterization, to predict adaptability under climate change scenarios