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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.

International audience ; 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 ...

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.

International audience ; 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 ...

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.

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.

International audience ; 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 ...