David Locker

High strain rate constitutive behavior of SAC105 and SAC305 leadfree solder during operation at high temperature

Industry migration to leadfree solders has resulted in a proliferation of a wide variety of solder alloy compositions. The most popular amongst these are the Sn-Ag-Cu family of alloys like SAC105 and SAC305. Electronics subjected to shock and vibration may experience strain rates of 1-100 per sec. Electronic product may often be exposed to high temperature during storage, operation and handling in addition to high strain rate transient dynamic loads during drop-impact, shock and vibration. Properties of leadfree solder alloys at high strain rates at low and high temperatures experienced by the solder joint during typical mechanical shock events are scarce. Previous studies have showed the effect of high strain rates and thermal aging on the mechanical properties of leadfree alloys including elastic modulus and the ultimate tensile strength. The ANAND viscoplastic constitutive model has been widely used to describe the inelastic deformation behavior of solders in electronic components. In this study, SAC105 and SAC305 leadfree alloys have been tested at strain rates of 10, 35, 50 and 75 per sec at various operating temperatures of 50°C, 75°C, 100°C and 125°C. Full-field strain in the specimen have been measured using high speed imaging at frame rates up to 75,000 fps in combination with digital image correlation. The cross-head velocity has been measured prior-to, during, and after deformation to ensure the constancy of cross-head velocity. Stress-Strain curves have been plotted over a wide range of strain rates and temperatures. Experimental data for the pristine specimen has been fit to the ANANDs viscoplastic model.

Effect of Temperature on the High Strain Rate Properties of SAC Leadfree Alloys at Temperatures up to 200°C

Electronics in downhole drilling, automotive and aerospace applications is expected to sustain prolonged periods of operation at high temperature while being subjected to shock and vibration. High temperature shock and vibration loads may be generally in the strain rate range of 1-100 per sec. Material data on properties at elevated temperatures and high strain rates is scarce. There is lack of material models that capture the mechanical deformation under transient dynamic loads at elevated temperatures. Anand Viscoplasticity models which have been widely used for modeling of material behavior of solder alloys require nine-constants to describe material constitutive behavior. Prior published data on Anand Constants for SAC leadfree solder alloys is limited to either low strain rates or lower temperatures in the neighborhood of 125°C. In this paper, mechanical properties and the non-linear constitutive behavior has been reported for two of the commonly used lead free alloys including Sn1Ag0.5Cu (SAC105) and Sn3Ag0.5Cu (SAC305) for strain rates 1-100 per sec at elevated temperatures up to 200°C. The stress-strain behavior has been measured using uniaxial tension specimen fabricated using reflow profile representative of production assemblies. Data has been reported at discrete constant strain rates including 10, 35, 50 and 75 per sec at various operating temperatures of 50°C, 75°C, 100°C, 125°C, 150°C, 175°C, and 200°C. The measured experimental data has been fit to the Anand Viscoplasticity model for both the alloys and the model constants verified by predicting the measured stress-strain behavior for the complete range of temperatures and strain rates and comparing the predictions with the prior experimentally measured values.

A Study on the Evolution of the High Strain Rate Mechanical Properties of SAC105 Leadfree Alloy at High Operating Temperatures

Electronics products may often be exposed to high temperature during storage, operation and handling in addition to high strain rate transient dynamic loads during drop-impact. Electronics subjected to drop-impact, shock and vibration may experience strain rates of 1–100 per sec. There are no material properties available in published literature at high strain rate at elevated temperature. High temperature and vibrations can contribute to the failures of electronic system. The reliability of electronic products can be improved through a thorough understanding of the weakest link in the electronic systems which is the solder interconnects. The solder interconnects accrue damage much faster when subjected to Shock and vibration at elevated temperatures. There is lack of fundamental understanding of reliability of electronic systems subjected to thermal loads. Previous studies have showed the effect of high strain rates and thermal aging on the mechanical properties of leadfree alloys including elastic modulus and the ultimate tensile strength. Extended period of thermal aging has been shown to affect the mechanical properties of lead free alloys including elastic modulus and the ultimate tensile strength at low strain rates representative of thermal fatigue [Lee 2012, Motalab 2012]. Previously, the microstructure, mechanical response and failure behavior of leadfree solder alloys when subjected to elevated isothermal aging and/or thermal cycling [Darveaux 2005, Ding 2007, Pang 2004] have been measured. Pang [1998] has showed that young’s modulus and yield stress of Sn-Pb are highly depending on strain rate and temperature. The ANAND viscoplastic constitutive model has been widely used to describe the inelastic deformation behavior of solders in electronic components. Previously, Mechanical properties of lead-free alloys, at different high strain rates (10, 35, 50, 75 /sec) and elevated temperature (25 C-125 C) for pristine samples have been studied [Lall 2012 and Lall 2014]. Previous researchers [Suh 2007 and Jenq 2009] have determined the mechanical properties of SAC105 at very high strain rate (Above 1000 per sec) using compression testing. But there is no data available in published literature at high strain rate and at elevated temperature for aged conditions. In this study, mechanical properties of lead free SAC105 has been determined for high strain rate at elevated temperature for aged samples. Effect of aging on mechanical properties of SAC105 alloy a high strain rates has been studied. Stress-Strain curves have been plotted over a wide range of strain rates and temperatures for aged specimen. Experimental data for the aged specimen has been fit to the ANAND’s viscoplastic model. SAC105 leadfree alloys have been tested at strain rates of 10, 35, 50 and 75 per sec at various operating temperatures of 50°C, 75°C, 100°C and 125°C. The test samples were exposed to isothermal aging conditions at 50°C for different aging time (30, 60, and 120 Days) before testing. Full-field strain in the specimen have been measured using high speed imaging at frame rates up to 75,000 fps in combination with digital image correlation. The cross-head velocity has been measured prior-to, during, and after deformation to ensure the constancy of cross-head velocity.Copyright

High strain rate stress-strain measurement of SAC105 leadfree alloy at temperatures up to 200°C

Material data on properties at elevated temperatures and high strain rates is scarce. There is lack of material models that capture the mechanical deformation under transient dynamic loads at elevated temperatures. Electronics in downhole drilling, automotive and aerospace applications may often be exposed to high temperature during storage, operation and handling in addition to high strain rate transient dynamic loads. Strain rates in the neighborhood of 1-100 per sec may be encountered during operation. Anand Viscoplasiticity models which have been widely used for modeling of material behavior of solder alloys require nine-constants to describe material constitutive behavior. Prior published data on Anand Constants for SAC leadfree solder alloys is limited to either low strain rates or lower temperatures in the neighborhood of 125°C. In this study, mechanical properties of lead free SAC105 has been measured for strain rates 1-100 per sec at elevated temperatures higher than 125°C. Stress-Strain curves have been plotted over a wide range of strain rates and temperatures for pristine specimen which have been fabricated using reflow profiles representative of solder-joint production assemblies. The fabricated SAC105 leadfree alloys specimen have been tested at strain rates of 10, 35, 50 and 75 per sec at various operating temperatures including 150°C, 175°C, and 200°C. Experimental data for the specimen has been fit to the Anands viscoplastic model. Furthermore, the Anand Model has been used to model the uniaxial tensile test and predict the stress-strain behavior of the solder alloys at elevated temperature. The comparison of the model predictions with the experimental data indicates that the model constants capture the non-linear material behavior for the SAC105 solder alloy accurately.

Mechanical deformation behavior of SAC305 at high strain rates

Electronic products are subjected to high G-levels during mechanical shock and vibration. Failure-modes include solder-joint failures, pad cratering, chip-cracking, copper trace fracture, and underfill fillet failures. The second-level interconnects may be experience high-strain rates and accrue damage during repetitive exposure to mechanical shock. Industry migration to lead free solders has resulted in proliferation of a wide variety of solder alloy compositions. One of the popular tin-silver-copper alloys is Sn3Ag0.5Cu. The high strain rate properties of lead free solder alloys are scarce. Typical material tests systems are not well suited for measurement of high strain rates typical of mechanical shock. Previously, high strain rates techniques such as the Split Hopkinson Pressure Bar (SHPB) can be used for strain rates of 1000 per sec. However, measurement of materials at strain rates of 1-100 per sec which are typical of mechanical shock is difficult to address. In this paper, a new test-technique developed by the authors has been presented for measurement of material constitutive behavior. The instrument enables attaining strain rates in the neighborhood of 1 to 100 per sec. High speed cameras operating at 300,000 fps have been used in conjunction with digital image correlation for the measurement of full-field strain during the test. Constancy of cross-head velocity has been demonstrated during the test from the unloaded state to the specimen failure. Solder alloy constitutive behavior has been measured for SAC305 solder. Constitutive model has been fit to the material data. Samples have been tested at various time under thermal aging at 25°C and 125°C. The constitutive model has been embedded into an explicit finite element framework for the purpose of life-prediction of lead free interconnects. Test assemblies has been fabricated and tested under JEDEC JESD22-B111 specified condition for mechanical shock. Model predictions have been correlated with experimental data.

Evolution of high strain rate and high temperature mechanical properties of SAC305 with long term storage up to 1-year

The effect of aging on mechanical properties of SAC 305 at low strain rate has been investigated. For high strain rate constitutive mechanical behavior, a number of researchers relied on Split Hopkinson Pressure bar and the strain rate range is from 500/s to 3000/s. However, for typical drop and shock, the strain rate range is from 1/s to 100/s. There is a general scarcity of data for solder materials in this strain rate range. Therefore knowing the mechanical properties of lead free solder at this high strain rate range is very important for design and optimization of package reliability. It is possible that failure may happen at initial shock incident or may result from cumulative damage from sequential shock and vibration events. In addition, isothermal aging and thermal cycling may cause significant changes of mechanical properties of solder alloys due to evolving of microstructure. These changes are large especially in harsh environment such as high temperature and long-term aging. Consequently, a complete understanding of high strain rate and high temperature behaviors of solder alloy after long period of aging is necessary to perform a better design and optimization in electronics. A viscoplastic model was proposed by Anand [1982, 1989] to describe materials that depend on both operating temperature and strain rate. Recently, it has been broadly used to characterize viscoplastic deformation of lead-free solder materials. However, the Anand constants of SAC305 for high strain rate and high temperature condition at long-term aging are not available. In order to compute the constants for this model, uniaxial tensile tests have been done at a wide range of high strain rate and high temperature conditions within different aging period. In this study, different weighted impact hammers were introduced which enable attaining different high strain rates around 1 to 100 /s. A load cell is on the top of the specimen-grip, which is used to calculate tensile load dynamically. Additionally, a small thermal chamber is used to control the operating temperatures. High-speed data acquisition system was built to capture the stress-strain curves of specimen. Tensile stress-strain curves have been plotted over a wide range of strain rates (8 =10, 35, 50, 75 /s) and temperatures (T = 25, 50, 75, 100, 125, 150, 175, 200°C) at different aging periods (Pristine, 60, 120, 180, 240, 300, 360 days). Totally, seven groups of Anand constants have been computed based non-linear least square curve fitting procedures. In addition, the correctness of the predicted model has been verified by comparing with experimental data.

Effect of storage on high strain rate mechanical properties of SAC105 at operating temperature up to 200°C

Electronic devices used in higher temperatures environments such as well logging in the oil and natural gas industries, under hood automotive applications, military applications may be exposed to very high temperatures around 200°C and high strain rates. Previously, lead based solder alloys have been used in such applications but due to health concern, use of lead-free solder alloys (SAC) has been increased. High temperature shock and vibration loads may be generally in the strain rate range of 1–100 per sec. Temperatures in drilling wells can exceed 200°C. Lead free solder material properties may keep evolving when they exposed to thermal aging or high temperatures. There are no published data for effect of aging of SAC105 leadfree solder alloy at high strain rate at very high operating temperature (150°C-200°C). For better design, reliability and process optimization, we need more reliable, consistent solder constitutive equations and material properties. Previously, Materials properties for SAC solder alloys at different strain rates and temperatures have been reported by many researchers. Mechanical properties for SAC solder alloys at operating temperatures up to 125°C and high strain rates (1075 per sec) have been reported by Lall for aged and unaged solder [1-9]. Many constitutive models have been used to represent the material behavior for SAC solder alloys. Anand constitutive model is widely used to describe the deformation behavior for SAC solder alloys as well as Sn-Pb based solder. Anand parameters have been computed for SAC105 and SAC305 for high strain rate and elevated temperature [4-9]. However, there is no prior reported measurement of anand parameters for SAC105 solders during operation at 200°C, at high strain rate. In this study, mechanical properties and constitutive behavior of lead free SAC105 has been measured for high strain rate (10–75 per sec) at elevated temperature (25°C–200°C) for thermal aged samples.

Effect of simultaneous high temperature and vibration on MEMS based vibratory gyroscope

Combined effects of high temperature and vibration can significantly attenuate the life of electronics used in automotive, military and navigation applications. No prior studies exist which examine the simultaneous effects of high temperature plus vibration and analyze failure modes and failure criterion for MEMS based gyros and accelerometers. Accelerometers and gyroscopes make a great complement to one another, as the latter is not affected by gravity. Both are used alongside each other in motion-capture, vehicle navigation, and missile-control and flight guidance applications. In order to have a reliable harsh environmental reliability data set these MEMS devices need to be monitored under conditions mimicking their areas of applications. In this paper a test vehicle with a, MEMS gyroscope, LPY510AL, has been tested under: high temperature exposure at 55°C combined with vibration profile(s) set at 14G. The test boards with gyroscopes were later subjected to rotations between 07s and 1007s. Scale Factor(s) were computed for both the pristine and vibration conditions. This paper also highlights the effect of thermal cycling on zero bias stability of LPY510AL. The test boards were subjected to a standard thermal cycling profile, −40°C to 85°C for 250 loops. Shift in output parameters for the gyro has been examined incrementally until failure. The survivability of LPY510AL has been demonstrated as a function of change in the output parameters.

Effect of Prolonged Storage up to 1-Year on the High Strain Rate Properties of SAC Leadfree Alloys at Operating Temperatures up to 200 °C

Transient dynamic loads in addition to prolonged periods of high temperature exposure are a part of number of high profile applications with high reliability needs. Examples include - electronics in automotive applications may be mounted under the hood or in the trunk of the car resulting in prolonged periods of high temperature exposure followed by operation under vibration while at environmental temperature extremes, and electronics in downhole drilling applications may be mounted close to the drill tip resulting in exposure to transient dynamic loads. High strain rates encountered subsequent to prolonged periods of non-operational high temperature storage encountered in downhole drilling, and automotive underhood applications require the development of computational tools and techniques for prediction of material deformation behavior and reliability. In this study, mechanical properties of lead free SAC105 and SAC305 has been measured for strain rates 1-100 per sec at elevated temperature 200°C after prolonged storage for periods up to 1-year. Stress-Strain curves have been plotted over a wide range of strain rates and temperatures for pristine specimen which have been fabricated using reflow profiles representative of solder-joint production assemblies. The fabricated SAC105 and SAC305 leadfree alloys specimen have been tested at strain rates of 10, 35, 50 and 75 per sec at various operating temperatures of 50°C, 75°C, 100°C, 125°C, 150°C, 175°C, and 200°C. Experimental data for the aged specimen has been fit to the ANANDs viscoplastic model.

Effect of Mean Temperature on the Evolution of Strain-Amplitude in SAC Ball-Grid Arrays during Operation under Thermal Aging and Temperature Excursions

Electronics in automotive applications may be used for a number of safety critical systems including lane-departure warning, collision avoidance, drive assist systems, and adaptive cruise control. Furthermore, electronics in fully-electric vehicles may be used for power generation and management. Automotive electronics may be mounted on engine or on transmission or in the base of the automobile and may be subjected to operational temperature excursions in addition to environmental temperature extremes. Further, automotive electronics systems may be subjected to prolonged periods of storage at ambient environmental low or high temperatures. There is need for tools and techniques for proactive assessment of consumed life, remaining useful-life, and spot assessment of thermo-mechanical reliability of electronics to assure reliable operation for the automotive benchmark of 10-years, 100,000 miles. In this study, the effect of thermal aging on thermal cycling reliability and the evolution of strain has been studied using digital image correlation. Leadfree assemblies which have been subjected to prolonged periods of aging have been subsequently subjected to thermal cycling and the strain amplitude experienced in the solder joints has been measured using digital image correlation. These strain state results then were correlated with microstructural damage rate (obtained from a separate study) to develop a damage mapping model. Finally, a new approach of life model along with Remaining Useful Life (RUL) estimation technique has been presented based upon microstructural damage rate.