Modelling of the reliability of flip chip lead-free solder joints at high-temperature excursions

At high-temperature operations of electronic control devices, Tin-Silver-Copper (SnAgCu) alloy solder joints used to assemble the component of the devices are functioning at homologous temperature above 0.8. In such ambient temperatures, solder alloys have limited mechanical strength and will be sensitive to strain rate. The sensitivity of solder properties to creep/visco-plastic deformation increases the rate of accumulation of plastic damage in the alloy and decreases the number of cycles to failure (Nf) of the joints. Most untimely rupture of solder joints in high-temperature electronics (HTE) system usually culminates in colossal loss of resources and lives. Typical incidences are reported in recent automotive and aircraft crashes as well as the collapse of oil-well logging equipment. To increase the mean time to failure (MTTF) of solder joints in HTE, an in-depth understanding and accurate prediction of the response of solder joints to thermally induced plastic strain damage is crucial. This study concerns the prediction of the reliability of lead-free solder joints in a flip chip (FC) model FC48D6.3C457 which is mounted on a substrate and the assembly subjected to high-temperature excursions. The research investigates the effect of the high-temperature operations on reliability of the joints. In addition, the investigation examines the impact of control factors (component stand-off height (CSH), inter-metallic compound (IMC) thickness, number of thermal cycle and solder volume) on Nf of the joints. A model developed in the course of this investigation was employed to create the assembly solder joints architecture. The development of the model and creation of the bump profile involve a combination of both analytical and construction methods. The assembled package on a printed circuit board (PCB) was subjected to accelerated temperature cycle (ATC) employing IEC standard 60749-25 in parts. The cycled temperature range is between -38 oC and 157 oC. Deformation behaviour of SnAgCu alloy solder in the joints is captured using Anand’s visco-plasticity model and the response of other materials in the assembly were simulated with appropriate model. The results demonstrate that the reliability of solder joints operating at elevated temperatures is dependent on CSH, thickness of IMC and solder volume. It also shows that incorporating the IMC layer in the geometric models significantly improves the level of accuracy of fatigue life prediction to ± 22.5% (from the ± 25% which is currently generally accepted). The findings also illustrate that the magnitude of the predicted damage and fatigue life are functions of the number of ATC employed. The extensive set of results from the modelling study has demonstrated the need for incorporating the IMC layer in the geometric model to ensure greater accuracy in the prediction of solder joint service life. The technique developed for incorporating the IMC layer will be of value to R&D engineers and scientists engaged in high-temperature applications in the automotive, aerospace and oil-well logging sectors. The results have been disseminated through peer reviewed journals and also presentations at international conferences

ESTIMATING THE DEMAND FUNCTION OF RICE PRODUCTION IN NIGERIA (1981 – 2018)

The study estimated the demand function for rice in Nigeria from 1981 to 2018. The objective was to estimate the short run and long run demand function (including price elasticities) of rice in Nigeria from 1981 to 2018. Secondary time series data were used for the study. The data were analyzed using inferential statistics of which the Johansen Maximum likelihood method of cointegration was used. The results revealed that the previous years’ demand (0.353) and price of close substitute (0.182) significantly affected demand in the short-run with an ECM (-1) of -0.653 while for long run price of close substitute (0.118), population (1.68) and policy (-0.186) affects demand. The results also showed that the price elasticity of rice demand in the long-run and short-run were -0.033 and -0.093 and were both non-significant at 5 percent level.  The demand for rice is price inelastic.. It was recommended that adequate policy framework aimed at increasing supply of local rice should be pursued as this will reduce the prices of local rice brands and invariably enhance demand for local rice by households as rice was estimated to be own price inelasti

Promoting implementation of sustainable development goals in rural Nigeria: II food security issues and their determinants among cassava-based farming households in Akpabuyo Local Government Area, Cross River State, Nigeria

This study analyzed the food security situation among cassava-based farming households in Akpabuyo local government area of Cross River state, Nigeria. The objectives were to describe their socio – economic characteristics, determine their food security status and its determinants and make recommendation based on the findings of the study. A multi-stage sampling procedure was used in the selection of wards, communities and cassava-based farming households. Data for the study were collected through the use of structured questionnaires administered to one hundred and three (103) households from 53 communities in the Local Government area. Descriptive statistics, food security indices and logistic regression models were used in data analysis and interpretation. The results showed that the percentages of male to female farmers differed considerably, with most households (59%) consisting of married men and women, thirty three percent (33%) of which were aged 46 years and above. Ninety eight percent (98%) of the households were comprised mostly of Christians, thirty-eight percent (38%) of whom had no formal education. Forty eight percent (48%) of the total number of the cassava farming households had less than 4 memberfamily sizes. The results showed also that 63.11% of the farming households were food secure, while 36.89% were food insecure. The food security gap and food surplus index showed that the food-secure households exceeded the food security line by 33%, while 59% of food-insecure household fell below the food security line. Also, the mean per capita expenditure for all households was N12, 684.21 while the mean per capita expenditure for food secure and food insecure households were N9000 and N3684.20 respectively. The estimated regression model gave a pseudo of 0.7304. Food security and Poverty status of the households were negatively correlated. The odd ratio in favour of household food security decreased by 0.03% for every unit increase in poverty (P>0.05%). It is recommended that more of the young and energetic youth population, and more women and the vulnerable be encouraged to engage themselves more in cassava and other crops’ enterprises; that household sizes be controlled to manageable numbers, and sizes of their farm lands optimally expanded. Finally, sources of farmers’ income should be diversified and cassava farmers should be encouraged to form and/or join cooperative societies so as to build and benefit from ensuing “social capital”, as well as be in positions to benefit from possible incentives of government(s).Keywords: Sustainable Development Goals, Food Security, Cassava-based Households, Cross River Stat

Promoting implementation of sustainable development goals in rural Nigeria: I. Poverty issues and its determinants among cassava-based farming households in Akpabuyo Local Government Area, cross river state, Nigeria

Issues of poverty and their major determinants among Cassava-Based Farming Households in Akpabuyo, Cross River state, Nigeria were studied. A multi-stage sampling procedure was used in the selection of wards and cassava-based farming households, while primary data were collected using well-structured questionnaires administered to 103 households and analyzed using standard descriptive statistics, poverty indices and logistic regression models. Socioeconomic statistics of the households showed that the percentages of male to female farmers differed considerably, with 59% of the households consisting of married men and women, 33% were aged 46 years and above. 98% of the households were Christians, 38% had no formal education, and 48% of the total number of the households had less than 4-member family sizes. The poverty line was estimated at N2, 933.25, equivalence of $7. Incidence of poverty among the cassava-based farming households was aggregated at 49% percent, but incidences were observed at thirty four (34%) percent for males and (52%) percent for females. The depth of poverty for cassava-based male farmers was 15% while that of females was 20 percent. The severity of poverty was determined at 9 percent and 11 percents for male- and female-headed households respectively. The strength of association between the dependent and the independent variable was estimated at 0.715, using Nagel Kerke’s coefficient R2 (p>0.05). In order that farming households in Akpabuyo LGA contribute their quota to overall aim of Nigeria to end all forms of poverty by the year 2030, it is recommended that more of the young and energetic youth population be encouraged to engage themselves more in cassava production enterprises and more women, especially the female-headed households and the vulnerable should be empowered to engage in cassava farming. Large household sizes should be discouraged and more incentives should be provided for more household heads. More of the cassava-based farming households should be encouraged to optimally expand the sizes of their cassava farm lands and empowered to diversify their sources of income. Finally, more of the cassava-based farmers in the area should be persuaded to form and/or join cooperative societies so as to build and benefit from ensuing “social capital”

Effective Solder for Improved Thermo-Mechanical Reliability of Solder Joints in a Ball Grid Array (BGA) Soldered on Printed Circuit Board (PCB)

Ball grid array (BGA) packages have increasing applications in mobile phones, disk drives, LC displays and automotive engine controllers. However, the thermo-mechanical reliability of the BGA solder joints challenges the device functionality amidst component and system miniaturisation as well as wider adoption of lead-free solders. This investigation determines the effective BGA solders for improved thermo-mechanical reliability of the devices. It utilised a conducted study on creep response of a lead-based eutectic Sn63Pb37 and four lead-free Tin-Silver-Copper (SnAgCu) [SAC305, SAC387, SAC396 and SAC405] solders subjected to thermal cycling loadings and isothermal ageing. The solders form the joints between the BGAs and printed circuit boards (PCBs). ANSYS R19.0 package is used to simulate isothermal ageing of some of the assemblies at -40℃, 25℃, 75℃ and 150℃ temperatures for 45 days and model the thermal cycling history of the other assemblies from 22℃ ambient temperature for six cycles. The response of the solders is simulated using Garofalo-Arrhenius creep model. Under thermal ageing, SAC396 solder joints demonstrate possession of least strain energy density, deformation and von-Mises stress in comparison to the other solders. Under thermal cycle loading conditions, SAC405 acquired the lowest amount of the damage parameters in comparison. Lead-free SAC405 and SAC387 joints accumulated the lowest and highest energy dissipation per cycle, respectively. It is concluded that SAC405 and SAC396 are the most effective solders for BGA in devices experiencing isothermal ageing and temperature cycling during operation, respectively. They are proposed as the suitable replacement of eutectic Sn63Pb37 solder for the various conditions.University of Derb

Creep-Fatigue Behaviours of Sn-Ag-Cu Solder Joints in Microelectronics Applications

Electronic manufacturing is one of the dynamic industries in the world in terms of leading in technological advancements. At the heart of electronic assembly lies the 'soldering technology' and the 'solder joints' between electronic components and substrate. During the operation of electronic products, solder joints experience harsh environmental conditions in terms of cyclic change of temperature and vibration and exposure to moisture and chemicals. Due to the cyclic application of loads and higher operational temperature, solder joints fail primarily through creep and fatigue failures. This paper presents the creep-fatigue behaviours of solder joints in a ball grid array (BGA), soldered on a printed circuit board (PCB). Using finite element (FE) simulation, the solder joints were subjected to thermal cycling and isothermal ageing. Accelerated thermal cycling (ATC) was carried out using a temperate range from 40℃ to 150℃, and isothermal ageing was done at -40,25,75 and 150℃ temperatures for 45 days (64,800 mins). The solders studied are lead-based eutectic Sn63Pb37 and lead-free SAC305, SAC387, SAC396 and SAC405. The results were analysed using the failure criterion of equivalent stress, strain rate, deformation rate, and the solders' strain energy density. The SAC405 and SAC396 are found to possess the least stress magnitude, strain rate, deformation rate, and strain energy density damage than the lead-based eutectic Sn63Pb37 solder; they have the highest fatigue lives based on the damage mechanisms. This research provides a technique for determining the preventive maintenance time of BGA components in mission-critical systems. Furthermore, it proposes developing a new life prediction model based on a combination of the damage parameters for improved prediction.N/

Generating temperature cycle profile from in-situ climatic condition for accurate prediction of thermo-mechanical degradation of c-Si photovoltaic module

The use of climate specific temperature-cycling profile is critical to precisely quantifying the degradation rate and accurately determining the service fatigue life of crystalline silicon photovoltaic (c-Si PV) module operating in various climates. A reliable in-situ outdoor weathering database is pivotal to generating the required climate specific temperature cycle profile. This research utilizes high-resolution data obtained at five minutes interval from installed c-Si PV modules to generate a temperature cycle profile that is representative of a test site in sub-Saharan Africa climate. The study collected a three-year data from 2012 to 2014 on weathering of c-Si PV module located at College of Engineering, KNUST Ghana. The data site is on latitude 6° 40″ N and longitude 1° 37″ W at an elevation of 250 m above sea level. Analysis of the data on temperature variation and thermally induced stresses demonstrates that the region has a profile with a ramp rate of 8.996 °C/h, a hot dwell time of 228 min, cold dwell time of 369 min. Maximum and minimum module temperatures of 58.9 °C and 23.7 °C, respectively; and a cycle time of 86400 s. Comparison with the IEC 61215 standards for terrestrial PV modules qualification reveals percentage changes of – 91%, 2180%, 3590%, 747% for the ramp rate, dwell (hot and cold) and cycle times, respectively. The generated in-situ temperature cycle profile predicts to qualify accurately, c-Si PV modules operating in the sub-Saharan African test site. The systematic technique employed in this study to generate the in-situ temperature cycle profile would be useful to the thermo-mechanical reliability research community. In addition, photovoltaic design and manufacturing engineers may harness the information to create climate specific robust c-Si PV module. Keywords: Temperature cycling, Ramp rates, Temperature gradient, Accelerated Thermal Cycling (ATC), Co-efficient of Thermal Expansion (CTE), Rainflow counting, In-situ data generatio

Creep response of various solders used in soldering ball grid array (BGA) on printed circuit board (PCB)

In electronics packaging, solder joints play a critical role by providing electrical, thermal and mechanical connections between the package and the printed circuit board (PCB). As the joint is both miniature and brittle, it is the weakest part of the assembly and thus susceptible to untimely damage. This paper presents the creep response of solder joints in a ball grid array (BGA) soldered on a PCB subjected to isothermal ageing in one experiment and temperature cycling in another test. The ageing is simulated in an ANSYS package environment at -40, 25, 75 and 150℃ temperatures applied for 45 days. The thermal cycling profile started from 22℃ and cycled between -40℃ and 150℃ with 15 minutes dwell time at the lowest and highest temperatures. The solders used in the investigation are lead-based eutectic solder alloy and lead-free SAC305, SAC387, and SAC396 solders. The research seeks to qualify these solders against strain and strain energy response for improved reliability in operation. The results show that the lead-free SAC387 accumulated the maximum strain and thus strain energy while the lead-based eutectic solder is found to accrue the least amount of the quantities. Further results show the distribution of damage in the BGA solder bump. Based on the results, it is proposed that lead-free SAC396 is the best replacement of the lead-based eutectic solder in the drive for the achievement of comparable thermo-mechanical reliability of assembled BGA on PCB.The authors thankfully acknowledge the funding contributions of the University of Derb