Characterising Solder Materials from Random Vibration Response of their Interconnects in BGA Packaging

Solder interconnection in electronic packaging is the weakest link, thus driving the reliability of electronic modules and systems. Improving interconnection integrity in safety-critical applications is vital in enhancing application reliability. This investigation qualifies the random vibration response of five essential solder compositions in ball grid array (BGA) solder joints used in safety-critical applications. The solder compositions are eutectic Sn63Pb37 and SnAgCu (SAC) 305, 387, 396, and 405. Computer-aided engineering (CAE) employing ANSYS FEA and SolidWorks software is implemented in this investigation. The solder Sn63Pb37 deformed least at 0.43 µm, followed by SAC396 at 0.58 µm, while SAC405 deformed highest at 0.88 µm. Further analysis demonstrates that possession of higher elastic modulus and mass density culminates in lower solder joint deformation. Stress is concentrated at the periphery of the solder joints in contact with the printed circuit board (PCB). The SAC396 solder accumulates the lowest stress of 14.1 MPa, followed by SAC405 at 17.9 MPa, while eutectic Sn63Pb37 accrues the highest at 34.6 MPa. Similarly, strain concentration is found at the interface between the solder joint and copper pad on PCB. SAC405 acquires the lowest elastic strain magnitude of 0.0011 mm/mm, while SAC305 records the highest strain of 0.002 mm/mm. These results demonstrate that SAC405 solder has maximum and SAC387 solder has minimum fatigue lives

MODELING RATE DEPENDENT DURABILITY OF LOW-Ag SAC INTERCONNECTS FOR AREA ARRAY PACKAGES UNDER TORSION LOADS

The thesis discusses modeling rate-dependent durability of solder interconnects under mechanical torsion loading for surface mount area array components. The study discusses an approach to incorporate strain-rate dependency in durability estimation for solder interconnects. The components under study are two configurations of BGAs (ball grid array) assembled with select lead-free solders. A torsion test setup is used to apply displacement controlled loads on the test board. Accelerated test load profile is experimentally determined. Torsion test is carried out for all the components under investigation to failure. Strain-rate dependent (Johnson-Cook model) and strain-rate independent, elastic-plastic properties are used to model the solders in finite element simulation. Damage model from literature is used to estimate the durability for SAC305 solder to validate the approach. Test data is used to extract damage model constants for SAC105 solder and extract mechanical fatigue durability curve

Rinnakkainen yhdistelmätestausmenetelmä elektronisten kokoonpanojen kokonaisvaltaisempaan ja tehokkaampaan luotettavuustestaukseen

Perinteisesti elektroniikan kokoonpanojen luotettavuutta on testattu kiihdytetyillä elinaikatesteillä yhtä rasitusparametriä hyväksikäyttäen. Tässä diplomityössä pyrittiin kehittämään uusi rinnakkainen yhdistelmätestausmenetelmä, joka simuloisi tuotteiden todellisia käyttöolosuhteita todenmukaisemmin. Rasituksien yhdistämisen uskotaan myös nopeuttavan vauriomekanismejä ja täten lyhentävän testaukseen käytettyä aikaa. Työ keskittyi termomekaanisten ja mekaanisten rasitusten yhdistämiseen. Työn kirjallisuusosa käsitteli kiihdytettyjä elinaikatestejä. Oleelliset yhden rasituksen testimenetelmät käytiin läpi ennen syventymistä useita rasitusparametrejä hyödyntäviin testeihin kirjallisuusselvityksen avulla. Työn kokeellisessa osassa yhdistettiin tehosyklaus ja tärinätestaus yhdeksi testimenetelmäksi. Ennen rasitusten yhdistämistä suoritettiin yhden rasitusparametrin tehosykli- ja tärinätestit varsinaisten testiparametrien määrittämiseksi ja rinnakkaistestauksen vikaantumismekanismien selvittämiseksi. Tulokset olivat yhtäpitäviä aikaisempien asiasta suoritettujen tutkimusten kanssa, elinaikojen yhdistelmätestauksessa ollessa huomattavasti lyhyempiä kuin oli oletettu erillisten yhden rasitusparametrin testien tulosten perusteella. Yhdistelmätestauksessa havaitut vikaantumismoodit olivat hyvin samankaltaisia tärinätesteissä havaittujen moodien kanssa ja vaurioitumisnopeuden kasvun oletettiin johtuvan juoteliitoksien lämpötilasta aiheutuvien mekaanisten ominaisuuksien muutoksista. Tulokset osoittivat, että testiparametrien huolellisella valinnalla voidaan uudella menetelmällä saavuttaa todellisia käyttöympäristöjä todenmukaisemmin edustava rasitusympäristö sekä lyhentää testiaikoja merkittävästi. On myös huomioitava, että yksittäin lähes merkityksettömillä rasituksilla saattaa olla merkittäviä yhteisvaikutuksia luotettavuuteen, joita ei voida huomioida perinteisillä yhden rasituksen testeillä.Traditionally the reliability and lifetime predictions of electronic assemblies have been conducted by employing single load accelerated life tests. This thesis aimed to develop a new concurrent reliability testing method that would offer a more realistic representation of actual use environments. Additionally, the combination of several loadings is expected to accelerate the damage accumulation, thus decreasing the associated testing times and costs. The focus was on the combination of thermomechanical and mechanical loads. Accelerated life tests were analyzed in the literature part. The relevant conventional accelerated life tests utilizing a single loading parameter were discussed before addressing multiple loading tests by giving a literature review on the current status of the subject. The experimental part combined power cycling and vibration loading into a single concurrent test method. To determine the actual test parameters and to clarify the failure modes and mechanisms in concurrent loading, single loading tests were conducted before the loads were combined. The experimental results were consistent with the previous results reported in literature with observed lifetimes considerably shorter than expected based on the single load test results. Observed failure modes in concurrent testing closely resembled those observed in pure mechanical loading, but the accelerated damage accumulation rate was attributed to the temperature related change of material properties. It was concluded that when the loading parameters are carefully set, significant improvements in both the lifelikeness of the loading conditions and in effectiveness can be achieved with the new test method. The results indicated that even under relatively small magnitude single loads the interactions of the various loadings can have significant effects on reliability that cannot be accounted for with single load tests

Development of convective reflow-projection moire warpage measurement system and prediction of solder bump reliability on board assemblies affected by warpage

Out-of-plane displacement (warpage) is one of the major thermomechanical reliability concerns for board-level electronic packaging. Printed wiring board (PWB) and component warpage results from CTE mismatch among the materials that make up the PWB assembly (PWBA). Warpage occurring during surface-mount assembly reflow processes and normal operations may cause serious reliability problems. In this research, a convective reflow and projection moire warpage measurement system was developed. The system is the first real-time, non-contact, and full-field measurement system capable of measuring PWB/PWBA/chip package warpage with the projection moire technique during different thermal reflow processes. In order to accurately simulate the reflow process and to achieve the ideal heating rate, a convective heating system was designed and integrated with the projection moire system. An advanced feedback controller was implemented to obtain the optimum heating responses. The developed system has the advantages of simulating different types of reflow processes, and reducing the temperature gradients through the PWBA thickness to ensure that the projection moire system can provide more accurate measurements. Automatic package detection and segmentation algorithms were developed for the projection moire system. The algorithms are used for automatic segmentation of the PWB and assembled packages so that the warpage of the PWB and chip packages can be determined individually. The effect of initial PWB warpage on the fatigue reliability of solder bumps on board assemblies was investigated using finite element modeling (FEM) and the projection moire system. The 3-D models of PWBAs with varying board warpage were used to estimate the solder bump fatigue life for different chip packages mounted on PWBs. The simulation results were validated and correlated with the experimental results obtained using the projection moire system and accelerated thermal cycling tests. Design of experiments and an advanced prediction model were generated to predict solder bump fatigue life based on the initial PWB warpage, package dimensions and locations, and solder bump materials. This study led to a better understanding of the correlation between PWB warpage and solder bump thermomechanical reliability on board assemblies.Ph.D.Committee Chair: Dr. Ume, I. Charles; Committee Member: Dr. Book, Wayne; Committee Member: Dr. Kim, Yeong; Committee Member: Dr. Pan, Jiahui; Committee Member: Dr. Sitaraman, Suresh; Committee Member: Dr. Wu, C. F. Jef

ADVANCED STATISTICAL ANALYSIS FOR TAIL-END PROBABILITY PREDICTION AND PERFORMANCE RESPONSE CALCULATION OF SEMICONDUCTOR PACKAGING PRODUCTS WITH A LARGE NUMBER OF INPUT VARIABLES

Stochastic reliability modeling capabilities are developed and implemented for semiconductor packaging problems with a very large number of input variables (> 10 input variables). The capabilities are aimed at three critical areas in the semiconductor packaging product development: (1) prediction of tail-end probability (i.e., assembly yield loss) by advanced uncertainty propagation (UP) analyses, (2) determination of the statistical distributions of unknown design and/or manufacturing parameters by advanced statistical model calibrations, and (3) determination of the performance response of high-dimensional problems by developing an advanced metamodeling scheme. In the first part, a comprehensive stochastic model is proposed and implemented to predict package-on-package (PoP) stacking yield loss based on non-contact open. The model takes into account all pad locations at the stacking interface while considering the statistical variations of warpages as well as solder ball and joint heights. The goal is achieved by employing (1) advanced approximate integration-based approach, called eigenvector dimension reduction (EDR) method, for the UP analysis; (2) the stress-strength interference (SSI), and (3) the union of events. The proposed approach is capable of handling the number of input variables much larger than that has been conceived as the practical limit of the UP analysis. The model can be used effectively to control the input uncertainties, and thus to achieve a yield goal for a given set of PoP designs. In the second part, the unknown statistical distributions of two effective elastic properties of Sn-3.0Ag-0.5Cu solder joint of leadless chip resistors (LCRs), induced by an assembly condition, are determined by the advanced statistical model calibration. The UP analysis also utilizes the EDR method, which allows to take into account the statistical variations of six additional known input variables, including die thickness, solder joint height, termination length, and thickness and elastic moduli of a printed circuit board. The cyclic bending test results of LCR assemblies are used in conjunction with the maximum likelihood metric to obtain the statistical distributions of the effective properties. The cycles-to-failure distribution of the identical LCR assemblies subjected to a different loading level is predicted accurately by the calibrated model, which corroborates the validity of the proposed approach. In the third part, an advanced metamodeling scheme, called partitioned bivariate Cut-high dimensional model representation (PB Cut-HDMR), is developed to consider the statistical correlation among input variables and to further reduce the computational burden encountered for high-dimensional problems without compromising accuracy. The statistical correlation is handled by eigen-decomposition of a covariance matrix. The latter is achieved by the HDMR-factorial design (HDMR-FD) hybrid method. The validity of the proposed scheme is verified by comparing the performance of the proposed scheme with the full bivariate Cut-HDMR. The proposed scheme is implemented successfully to construct an accurate metamodel for a problem with 12 input variables among which 2 pairs are correlated

Reliability evaluation of stacked die BGA assemblies under mechanical bending loads

This thesis presents a reliability evaluation of stacked die ball grid array (BGA) assemblies under mechanical bending loads. The test specimens used in this investigation were four die stacked BGAs assembled on printed circuit boards (PCBs) with eutectic tin-lead solder and gold over nickel finishes, both as-reflowed and after aging. The failure envelopes of both types of specimen were quantified in terms of PCB flexural strain and strain rate. The experimental data from cyclic bending tests at three strain amplitudes with a constant strain rate have been used to determine the effect of strain amplitudes on cycles to failure. The experimental data from cyclic bending tests were combined with the data from impact tests to determine the effect of strain rate to cycles to failure. The failure sites associated with each test condition were identified, and failure site transition phenomena are reported and discussed

Thermo-mechanical reliability studies of lead-free solder interconnects

N/ASolder interconnections, also known as solder joints, are the weakest link in electronics packaging. Reliability of these miniature joints is of utmost interest - especially in safety-critical applications in the automotive, medical, aerospace, power grid and oil and drilling sectors. Studies have shown that these joints' critical thermal and mechanical loading culminate in accelerated creep, fatigue, and a combination of these joints' induced failures. The ball grid array (BGA) components being an integral part of many electronic modules functioning in mission-critical systems. This study investigates the response of solder joints in BGA to crucial reliability influencing parameters derived from creep, visco-plastic and fatigue damage of the joints. These are the plastic strain, shear strain, plastic shear strain, creep energy density, strain energy density, deformation, equivalent (Von-Mises) stress etc. The parameters' obtained magnitudes are inputted into established life prediction models – Coffin-Manson, Engelmaier, Solomon (Low cycle fatigue) and Syed (Accumulated creep energy density) – to determine several BGA assemblies' fatigue lives. The joints are subjected to thermal, mechanical and random vibration loadings. The finite element analysis (FEA) is employed in a commercial software package to model and simulate the responses of the solder joints of the representative assemblies' finite element models. As the magnitude and rate of degradation of solder joints in the BGA significantly depend on the composition of the solder alloys used to assembly the BGA on the printed circuit board, this research studies the response of various mainstream lead-free Sn-Ag-Cu (SAC) solders (SAC305, SAC387, SAC396 and SAC405) and benchmarked those with lead-based eutectic solder (Sn63Pb37). In the creep response study, the effects of thermal ageing and temperature cycling on these solder alloys' behaviours are explored. The results show superior creep properties for SAC405 and SAC396 lead-free solder alloys. The lead-free SAC405 solder joint is the most effective solder under thermal cycling condition, and the SAC396 solder joint is the most effective solder under isothermal ageing operation. The finding shows that SAC405 and SAC396 solders accumulated the minimum magnitudes of stress, strain rate, deformation rate and strain energy density than any other solder considered in this study. The hysteresis loops show that lead-free SAC405 has the lowest dissipated energy per cycle. Thus the highest fatigue life, followed by eutectic lead-based Sn63Pb37 solder. The solder with the highest dissipated energy per cycle was lead-free SAC305, SAC387 and SAC396 solder alloys. In the thermal fatigue life prediction research, four different lead-free (SAC305, SAC387, SAC396 and SAC405) and one eutectic lead-based (Sn63Pb37) solder alloys are defined against their thermal fatigue lives (TFLs) to predict their mean-time-to-failure for preventive maintenance advice. Five finite elements (FE) models of the assemblies of the BGAs with the different solder alloy compositions and properties are created with SolidWorks. The models are subjected to standard IEC 60749-25 temperature cycling in ANSYS 19.0 mechanical package environment. SAC405 joints have the highest predicted TFL of circa 13.2 years, while SAC387 joints have the least life of circa 1.4 years. The predicted lives are inversely proportional to the magnitude of the areas of stress-strain hysteresis loops of the solder joints. The prediction models are significantly consistent in predicted magnitudes across the solder joints irrespective of the damage parameters used. Several failure modes drive solder joints and damage mechanics from the research and understand an essential variation in the models' predicted values. This investigation presents a method of managing preventive maintenance time of BGA electronic components in mission-critical systems. It recommends developing a novel life prediction model based on a combination of the damage parameters for enhanced prediction. The FEA random vibration simulation test results showed that different solder alloys have a comparable performance during random vibration testing. The fatigue life result shows that SAC405 and SAC396 have the highest fatigue lives before being prone to failure. As a result of the FEA simulation outcomes with the application of Coffin-Manson's empirical formula, the author can predict the fatigue life of solder joint alloys to a higher degree of accuracy of average ~93% in an actual service environment such as the one experienced under-the-hood of an automobile and aerospace. Therefore, it is concluded that the combination of FEA simulation and empirical formulas employed in this study could be used in the computation and prediction of the fatigue life of solder joint alloys when subjected to random vibration. Based on the thermal and mechanical responses of lead-free SAC405 and SAC396 solder alloys, they are recommended as a suitable replacement of lead-based eutectic Sn63Pb37 solder alloy for improved device thermo-mechanical operations when subjected to random vibration (non-deterministic vibration). The FEA simulation studies' outcomes are validated using experimental and analytical-based reviews in published and peer-reviewed literature.N/

Brayton heat exchanger unit development program (alternate design)

A Brayton Heat Exchanger Unit Alternate Design (BHXU-Alternate) consisting of a recuperator, a heat sink heat exchanger, and a gas ducting system, was designed and fabricated. The design was formulated to provide a high performance unit suitable for use in a long-life Brayton-cycle powerplant. Emphasis was on double containment against external leakage and leakage of the organic coolant into the gas stream. A parametric analysis and design study was performed to establish the optimum component configurations to achieve low weight and size and high reliability, while meeting the requirements of high effectiveness and low pressure drop. Layout studies and detailed mechanical and structural design were performed to obtain a flight-type packaging arrangement, including the close-coupled integration of the BHXU-Alternate with the Brayton Rotating Unit (BRU)

Solder Interconnect Life Prediction under Complex Temperature Cycling with Varying Mean and Amplitude

Electronic devices are under concurrent loading of the power cycling of the devices and the temperature cycling from the surrounding environment. Temperature histories resultant from these concurrent loading would be a complex temperature cycling with varying cyclic temperature mean and amplitude, as well as spatial thermal gradient. This study developed modeling approaches and quantified accuracies for predicting solder interconnect life under complex temperature cycling. Three modeling approaches were presented in this study: 1) modeling the strain energy under the resultant complex temperature cycling and employing the energy based fatigue life models; 2) segmenting the resultant complex temperature cycle into multiple simple temperature cycles with a single temperature range for each first, then assessing the life expectancy of the solder interconnect under the segmented simple temperature cycles and at last applying Miner's rule to superpose the damage; 3) estimating solder damage under the resultant complex temperature cycling by a standard temperature cycling with a single temperature range. Two case studies were included in this thesis: 1) chamber controlled complex temperature cycling with mini cycles occurring at the upper excursion on ceramic leadless chip carriers assembled by Sn36Pb62Ag2 and SnAg3.0Cu0.5 solder (without spatial thermal gradient); 2) combined temperature and power cycling on plastic ball grid array assembled by Sn63Pb37 and SnAg3.0Cu0.5 solder (with spatial thermal gradient). Physical tests were also conducted to quantify the developed modeling approaches