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

Vibration Fatigue of Leaded Solder Joint Interconnects for PCB Electronics

With the increasing prevalence of electronic equipment worldwide, there is also a decrease in the size of the components on their printed circuit boards (PCBs), leading to an increase in the density of these components. A significant amount of failure in electronic equipment is vibration fatigue of solder joints and their attachments. However, the complexity of these PCBs and their components has made finite element modeling (FEM) more complex, adding considerable time to create and analyze a model. This paper aims to provide a literature review for the vibration fatigue of leaded solder components, create a test setup, and validate an analytical solder joint stress model. The literature review provides a walkthrough on modeling PCBs and their components using FEMs and analytical models, fatigue modeling methodology, and fatigue testing data and highlights gaps in the literature. This review was important to compile due to the limited data and the rigor required to find it all when searching. With this literature review collected, testing was to be completed using an analytical model highlighted. Therefore, a setup and procedure have been developed to test the vibration fatigue of leaded solder attachments. The setup combines a test specimen, specimen mounting head, and preliminary model correlation between the test specimen and FEM. Using initial model correlations, an analytical solder joints stress model, and fatigue curves from literature, a vibration fatigue life prediction was made for the test specimen, and tests were run. However, the results were inconclusive and further testing is deemed necessary. Suggestions have been made, such as picking other analytical models to test, modifying the test setup, and increasing the fidelity of local areas in the FEM

The durability of solder joints under thermo-mechanical loading; application to Sn-37Pb and Sn-3.8Ag-0.7Cu lead-free replacement alloy

Solder joints in electronic packages provide mechanical, electrical and thermal connections. Hence, their reliability is also a major concern to the electronic packaging industry. Ball Grid Arrays (BGAs) are a very common type of surface mount technology for electronic packaging. This work primarily addresses the thermo-mechanical durability of BGAs and is applied to the exemplar alloys; traditional leaded solder and a popular lead-free solder. Isothermal mechanical fatigue tests were carried out on 4-ball test specimens of the lead-free (Sn-3.8Ag-0.7Cu) and leaded (Sn-37Pb) solder under load control at room temperature, 35°C and 75°C. As well as this, a set of combined thermal and mechanical cycling tests were carried out, again under load control with the thermal cycles either at a different frequency from the mechanical cycles (not-in-phase) or at the same frequency (both in phase and out-of-phase). The microstructural evaluation of both alloys was investigated by carrying out a series of simulated ageing tests, coupled with detailed metallurgical analysis and hardness testing. The results were treated to produce stress-life, cyclic behaviour and creep curves for each of the test conditions. Careful calibration allowed the effects of substrate and grips to be accounted for and so a set of strain-life curves to be produced. These results were compared with other results from the literature taking into account the observations on microstructure made in the ageing tests. It is generally concluded that the TMF performance is better for the Sn-Ag-Cu alloy than for the Sn-Pb alloy, when expressed as stress-life curves. There is also a significant effect on temperature and phase for each of the alloys, the Sn-Ag-Cu being less susceptible to these effects. When expressed as strain life, the effects of temperature, phase and alloy type are much diminished. Many of these conclusions coincided with only parts of the literature and reasons for the remaining differences are advanced

Through-Life Monitoring of the impact of vibration on the reliability of area array packages using Non- Destructive Testing

In order to keep up with the demands for faster, cheaper and smaller electronics, the packaging industry has evolved tremendously. Area array packages like flip chips and ball grid arrays are therefore widely used in modern day electronics. However, from the reliability standpoint, solder joints in these area array packages are often the weakest link. In case of harsh vibration environments like military and automobile applications, joint failure mainly occurs due to the high stress incurred during extreme environmental conditions that lead to fatigue failures. This thesis aims to study the effects of real time vibration on area array packages (flip chips in particular) using acoustic micro imaging for through life monitoring of the solder joints. Since real time vibration on solder joints have not been studied before, the various steps for successful testing, through life monitoring of the solder joints and data analysis will be investigated and discussed. Based on automobile industry standards, a real time vibration profile was obtained with the help of Delphi experts, who are the industry collaborators of this project. Due to its strong capability to detect discontinuities within materials and interconnections, Acoustic Micro Imaging (AMI) also known as Scanning Acoustic Microscopy (CSAM) has been used to monitor the solder joints. This approach has not previously been used as an effective tool in monitoring solder joints through life performance in vibration testing. The research regime proposed in this thesis was to monitor the health of solder joints through ultrasound images from beginning to failure, and to see how cracks initiate and propagate in them. The effect of the relative position and orientation on the reliability of the solder joints and the flip chips in the PCB was also studied. The data collected was analysed using MATLAB. The results have shown that three types of solder joints- healthy, partially fractured or fractured are formed near the time of complete failure of a flip chip. When about 70- 80% of the flip chips are either partially fractured or fractured a flip chip is expected to fail. The mean pixel intensity and area change in the acoustic image of a partially fractured or fully fractured joint tends to be higher compared to a healthy joint. Crack initiation in a joint occurs at around 35-40% cycling and propagates linearly till 80-85% cycling after which a joint fails. A statistical analysis done on the solder joints showed that the intensity distribution of healthy joints follow a simple Gaussian distribution while that of partially fractured or fractured joint can only be represented by using a mixture of Gaussians. The solder joints near the board edges are the least reliable in a vibration environment. However, solder joints with back to back connections are more reliable than the ones placed in one sided orientation. The most reliable flip chip orientation in a vibration environment is the back to back connection with no offset which was actually found to be the least reliable in the case of thermal cycling. Based on the analysis of the results, a few design guidelines for flip chip layout and orientations in a PCB has also been proposed in this work

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/

Thermal Cycling Life Prediction of Sn-3.0Ag-0.5Cu Solder Joint Using Type-I Censored Data

Because solder joint interconnections are the weaknesses of microelectronic packaging, their reliability has great influence on the reliability of the entire packaging structure. Based on an accelerated life test the reliability assessment and life prediction of lead-free solder joints using Weibull distribution are investigated. The type-I interval censored lifetime data were collected from a thermal cycling test, which was implemented on microelectronic packaging with lead-free ball grid array (BGA) and fine-pitch ball grid array (FBGA) interconnection structures. The number of cycles to failure of lead-free solder joints is predicted by using a modified Engelmaier fatigue life model and a type-I censored data processing method. Then, the Pan model is employed to calculate the acceleration factor of this test. A comparison of life predictions between the proposed method and the ones calculated directly by Matlab and Minitab is conducted to demonstrate the practicability and effectiveness of the proposed method. At last, failure analysis and microstructure evolution of lead-free solders are carried out to provide useful guidance for the regular maintenance, replacement of substructure, and subsequent processing of electronic products

HARMONIC AND RANDOM VIBRATION DURABILITY INVESTIGATION FOR SAC305 (Sn3.0Ag0.5Cu) SOLDER JOINT

ABSTRACT Title of Dissertation: HARMONIC AND RANDOM VIBRATION DURABILITY INVESTIGATION FOR SAC305 (Sn3.0Ag0.5Cu) SOLDER INTERCONNECTS Yuxun Zhou, Doctor of Philosophy, 2008 Dissertation directed by: Professor Abhijit Dasgupta Department of Mechanical Engineering Vibration loading is commonly encountered during the service life of electronic products. However, compared to thermal cycling durability, vibration durability is more complex and has been less investigated. In surface mount technology, solder joints are the primary mechanical, thermal and electrical interconnects between the component and the PWB. So the reliability of solder joints is very crucial for most electronic assemblies. The vibration durability of Pb-free solder joints is the focus of this dissertation. The characteristics of the stress from vibration loading are low amplitude and high frequency, while those from cyclic thermal loading are high amplitude and low frequency. In this study, several exploratory vibration tests were conducted, using both narrow band and broad-band, step-stress excitation at several different isothermal and thermal cycling conditions. The effect of thermal pre-aging on solder joint vibration failures was also investigated. Some of the vibration durability results were analyzed in detail, to obtain quantitative insights into the vibration fatigue behavior of the SAC305 solder material. A time-domain approach was adopted to investigate the durability of solder interconnects under different kinds of vibration and quasi-static mechanical loading. First, the solder interconnects were subjected to narrow-band (harmonic) vibration loading. The test were conducted at the first natural frequency of the test board using constant-amplitude excitation and solder fatigue properties were extracted with the help of a time-domain analysis that is based on quasi-static finite element simulation. Compared to broad-band step-stress vibration durability tests, the advantage of the harmonic constant-amplitude test is less complexity in the model extraction process, hence, less uncertainty in the desired fatigue constants. Generalized strain-based S-N curves have been obtained for both SAC305 and Sn37Pb solder materials. The strain-life model constants show that SAC305 solder material has superior fatigue properties compared to Sn37Pb solder material under low-cycle fatigue loading, while the reverse is true for high-cycle fatigue loading. These results are consistent with test results from other researchers. In actual application, SAC305 assemblies almost always fail before Sn37Pb assemblies under comparable vibration excitation because of (i) higher solder strain at a given excitation level; and (ii) multiple failure modes such as copper trace cracking. Next, durability was investigated under step-stress, broad-band (random) excitation. These test results show that SAC305 interconnects are less durable than Sn37Pb interconnects under the random excitation used in this study, which agrees with the harmonic durability results. The random and harmonic durability results were quantitatively compared with each other in this study. Finite element simulation was used to investigate the stress-strain response in the interconnects. The output of this simulation is the strain transfer function due to the first flexural mode of the PWB. This transfer function is used to obtain the solder strain from the measured board strain. This fatigue assessment method demonstrated that the model constants obtained from the harmonic test overestimate the fatigue life under random excitation by an order of magnitude. The causes for this discrepancy were systematically explored in this study. The effects of cyclic loading and mean stress on the vibration durability were addressed and found to be minimal in this study. The stress-strain curves assumed for the solder material were found to have a very large effect on the durability constants, thus affecting the agreement between harmonic and random durability results. The transient response of the components on the test board under both harmonic and random excitation was also included in the strain transfer function with the help of dynamic implicit simulation, and found to have a much stronger effect on the vibration durability at the high frequencies used in broad-band excitation compared to the low frequency used in narrow-band test. Furthermore, the higher PWB vibration modes may play a strong role and may need to be included in the strain transfer-function. This study clearly reveals that the solder strain analysis for broad-band random excitation cannot be limited to the quasi-static strain transfer-function based on the first PWB flexural mode, that has been used in some earlier studies in the literature. The time-domain approach used in this study provided fundamental and comprehensive insights into the key factors that affect vibration durability under different types of excitation, thus leading to a generalized S-N modeling approach that works for both harmonic and random vibration loading

A STUDY OF THE THERMAL CYCLING PERFORMANCE OF SOLDER JOINTS IN AREA ARRAY PACKAGING

For both the electronics manufacturer and consumer, reliability is an essential characteristic defining the quality of the electronic component and system. Gradual degradation of the electronic components decreases efficiency of the system, and lack of reliability can lead to a significant loss. Efforts at achieving better quality and reliability of electronic components involve the inspection of solder joints in area array packaging. It is of note that solder interconnections are the vulnerable parts of circuit board assemblies (CBA), because they are mainly subjected to various assembly process during electronic manufacturing as well as environmental exposure failures during service. Therefore, the reliability of solder joints is a major concern during the entire life of an area array packaging in order to minimize the electronic failure rate that may lead to large losses. This thesis aims to provide a solution that helps to overcome some of the challenges that can occur during the reliability inspection of solder joints in area array packaging. Firstly, by successfully developing a non-destructive monitoring methodology to study the performance of solder joints under thermal cycling test. The quality of the solder joints in this research work from growth to failure was monitored by using a type of ultrasonic inspection called acoustic micro imaging (AMI). Results indicate that provided a suitable AMI parameters is applied, one can generate a 3D reconstruction of the solder joints images to allow and assess the solder joints’ behaviour in flip chip packages. AMI inspection of solder joints show good agreement with the results obtained that was used to examine how the reliability was affected by the geometry and position of the joints. An automatic segmentation technique was developed that allow to characterize and extract distinctive features of solder joints on different area array packages; such features include mean intensity, structural similarities model and histogram intensity of the region of interest of solder joints. The validation experimental results have been statistically implemented using novel geometrical and time domain features extraction methods like area, form factor and standard deviation. The result from these methods were used to extrapolate the solder joint’s fatigue life at normal operating conditions. Moreover, the analysis of variance (ANOVA) was employed to determine the percentage contribution of solder joints parameters on the acquired images. The results indicated that the thickness of the printed circuit board can affect solder joint reliability

A THERMOMECHANICAL FATIGUE LIFE PREDICTION METHODOLOGY FOR BALL GRID ARRAY COMPONENTS WITH REWORKABLE UNDERFILL

Underfill materials were originally developed to improve the thermo-mechanical reliability of flip-chip devices due to the large coefficient of thermal expansion (CTE) mismatch between the silicon die and substrate. More recently, underfill materials, specifically reworkable underfills, have been used to improve reliability of second level interconnects in ball grid array (BGA) packages in harsh end-use environments such as automotive, military and aerospace. In these environments, electronic components are exposed to mechanical shock, vibration, and large fluctuations in temperatures. Although reworkable underfills improve the reliability of BGA components under mechanical shock and vibration, some reworkable underfills have been shown to reduce reliability during thermal cycling environments. Consequently, this research employs experimental and numerical approaches to investigate the impact of reworkable underfill materials on thermomechanical fatigue life of solder joints in BGA packages. In the first section of the analysis, material characterization of a reworkable underfill is performed to determine appropriate material models for reworkable underfills. In the second analysis section, a variety of underfill materials with different properties are exposed to harsh and benign thermal cycles to determine the stress state responsible for reducing fatigue life of solder joints in BGA packages. In the final analysis section, simulations are performed on the BGAs with reworkable underfill to develop a fatigue life predication methodology that implements a modified mode separation scheme. The model developed in this work provides a working fatigue life approach for BGA packages with reworkable underfills exposed to thermal loading. The results of this study can be utilized by the automotive, military, and aerospace industries to optimize underfill material selection process and provide reliability assessment of BGA components in real world environments

Structural Health Monitoring of Nonlinear Beam under Combined Translational and Rotational Vibration

This study presents a nonlinear dynamic methodology for detecting fatigue damage precursor in an isotropic metallic cantilever beam exposed to harmonic transverse, rotation or combined ¬– transverse and rotation – base excitations. The methodology accounts for important dynamic nonlinearities due to the complex loading generated by uniaxial and multiaxial nonlinear oscillations. These nonlinearities include: 1) structural stiffening due to gyroscopic motion and high-response amplitude at the structure fundamental mode, 2) structural softening due to inertial forces and gyroscopic loads, and localized evolution in the material microstructure due to fatigue damage and 3) cross-axis coupling due to multiaxial loading. The loading intensity and number of vibration cycles intensified these nonlinearities. The damage precursor feature is acquired by quantifying the reduction in the nonlinear stiffness term in the equation of motion due to localized evolution in the material micromechanical properties at high stress concentration regions. Nanoindentation studies near high stress concentration sites confirmed the evolution in the local micromechanical properties, as a function of loading cycles. The nonlinear analytical approach tracks the degradation in the structural stiffness as a function of the nonlinear dynamic response for the uniaxial transverse or rotation base excitation. The change in the dynamic response due to damage precursor is captured experimentally. The nonlinear stiffness terms are found to be sensitive to fatigue damage precursor for translational or rotational excitation. Therefore, the nonlinear stiffness sensitivity to fatigue damage precursor appeared to be a promising metric for structural health monitoring applications. This method is applicable to a cantilever beam only. Additional investigations will be required to extend its applicability to more complex structures. For the combined transverse and rotation base excitation, the experimental and analytic results demonstrated the importance of cross-axis coupling. The Experiments are performed using a unique multiaxial electrodynamic shaker with high controllability of phase and base excitation frequencies. The analytical model captures the modulation in the nonlinear dynamic response behavior seen in the experiments as a function of cross-axis coupling and the phase relation between the axes. Although the model is successful in capturing these general trends, it does not agree with the beam deflection absolute values obtained from the experiments. The discrepancy is due to fatigue damage accumulation during the experiments, which is manifested by a shift in the resonance frequency and an increase in the response amplitude