CALCE EPSC Graduate Student Theses and Dissertation Abstracts (2010)

Challa, Vidyu (Ph.D. Mechanical Engineering)
Effect of Joule heating on the reliability of stamped metal land grid array sockets
Performance requirements in high end microprocessors have increased tremendously in the last several years, leading to higher I/O counts and interconnect densities. As greater currents pass through the microprocessor interconnect, higher temperatures driven by Joule heating are expected to pose reliability risks to high pin count microprocessor sockets. In this study Joule heating and its effect on the reliability of stamped metal land grid array (LGA) sockets was investigated using a combination of experimental and numerical methods. A methodology to determine socket temperature environments under electrical loading was developed. Knowledge of socket operating temperatures can allow original equipment manufacturers (OEMs) and socket manufacturers to test for and mitigate failure mechanisms under thermal aging. The factors that influence Joule heating and contribute to premature socket failure were examined. Processor temperature, contact alloy and contact pitch were all found to significantly influence socket temperatures driven by Joule heating, with the contact alloy and processor temperature having the most significant effects. The resulting temperatures at higher currents were found to significantly influence the mechanical properties of the polymer housing and adversely affect socket stress relaxation behavior. The properties of the polymer housing were found to be sensitive to temperature owing to its visco-elastic nature. Polymer housing relaxation was therefore identified as a principle contributor to failure in stamped metal sockets under high temperature environments. In the latter part of the study, numerical modeling was used to develop a methodology for assessing socket life expectancy under temperature and deformation loads. A full visco-elastic characterization of the polymer housing was conducted and the measured properties were subsequently used to model socket stress relaxation time to failure. The results of this study indicate that socket temperatures under electrical loading can be significantly higher than those called for by EIA test specifications for LGA sockets. Passing tests that are not stringent enough to account for worst case scenarios can pave the way for field failures. The methodology outlined in this dissertation may be used to determine socket temperature environments and their effect on socket life expectancy.

Cuddalorepatta, Gayatri (Ph.D. Mechanical Engineering)
Evolution of the Microstructure and Viscoplastic Behavior of Miroscale SAC305 Solder Joints as a Function of Mechanical Fatigue Damage
The effect of mechanical cycling fatigue damage and isothermal aging histories on the evolution of the constitutive and fatigue responses, and microstructure of microscale SAC305 solder joints is investigated. In particular, the study examines if joint dependent behavior should be expected from as-fabricated and cycled microscale SAC305 joints that exhibit an initial non-homogenous coarse-grained Sn microstructure. In addition, the ability of traditionally used macroscale constitutive models based on continuum mechanics to represent the viscoplastic constitutive behavior of the non-homogenous as-fabricated microscale SAC305 specimens is explored. Insights into the effect of key microstructural features and dominant creep mechanisms influencing the measured viscoplastic behavior of SAC305 are provided using a multi-scale mechanistic modeling framework.

Modified lap-shear microscale SAC305 specimens are characterized using the thermomechanical microscale test setup (TMM). Microscale SAC305 solder specimens show significant piece-to-piece variability in the viscoplastic constitutive properties under identical loading histories in the as-fabricated state. The mechanical response is strongly influenced by the grain microstructure across the entire joint, which is non-repeatable and comprises of very few highly anisotropic Sn grains. The statistical non-homogeneity in the microstructure and the associated variability in the mechanical properties in the microscale SAC305 test specimen are far more significant than in similar Sn37Pb specimens, and are consistent with those reported for functional microelectronics solder interconnects. In spite of the scatter, as-fabricated SAC305 specimens exhibit superior creep-resistance (and lower stress relaxation) than Sn37Pb.

Macroscale creep model constants represent the non-homogeneous behavior of microscale joints in an average sense. Macroscale modeling results show that the range of scatter measured from macroscale creep model constants is within the range of scatter obtained from the stress relaxation predictions. Stress relaxation predictions are strongly sensitive to the inclusion or exclusion of primary creep models. The proposed multiscale framework effectively captures the dominant creep deformation mechanisms and the influence of key microstructural features on the measured secondary creep response of microscale as-fabricated SAC305 solder specimens. The multiscale model predictions for the effect of alloy composition on SAC solders provide good agreement with test measurements. The multiscale model can be extended to understand the effects of other parameters such as aging and manufacturing profiles, thereby aiding in the effective design and optimization of the viscoplastic behavior of SAC alloys.

Accumulated fatigue damage and isothermal aging are found to degrade the constitutive and mechanical fatigue properties of the solder. The scatter gradually decreases with an increasing state of solder damage. Compared to the elastic-plastic and creep measurements, the variability in the fatigue life of these non-homogenous solder joints under mechanical fatigue tests is negligible. Recrystallization is evident under creep and mechanical fatigue loads. Gradual homogenization of the Sn grain microstructure with damage is a possible reason for the observed evolution of scatter in the isothermal mechanical fatigue curves. The yield stress measurements suggest that SAC305 obeys a hardening rule different from that of isotropic or kinematic hardening. The measured degradation in elastic, plastic and yield properties is captured reasonably well with a continuum damage mechanics model from the literature.

Douglas, Stuart (M.S. Mechanical Engineering)
High accelerations Produced Through Secondary Impact and Its Effect on Reliability of Printed Wiring Assemblies
The focus of this thesis is the investigation of extremely high accelerations through secondary impact and its effect on reliability of printed wiring assemblies. The test equipment consists of a commercially available drop system and a commercially available attachment termed a Dual Mass Shock Amplifier (DMSA), which extends the impact acceleration range to as much as 30,000 Gs by utilizing secondary impact dynamics. Further secondary impacts between the test vehicle and fixture are intentionally generated in simulation and tested experimentally to imitate board 'slap' phenomena in product assemblies, and to generate even further amplification of the acceleration at various locations on the test specimen. In this thesis a detailed description of the test equipment and modeling techniques are provided. Model complexity ranges from simple analytic closed-form rigid-body mechanics to detailed nonlinear dynamic finite element analysis. The effects of different equipment design parameters (table mass, spring stiffness, table clearance) are investigated through parametric modeling. The effects of contact parameters (constraint enforcement algorithms, stiffness, damping) on model accuracy are explored. Test fixtures for high shock accelerations are discussed and used for board level reliability testing of printed wire assemblies containing WLCSP49s and MEMS microphones.

Farley, Daniel (Ph.D. Mechanical Engineering)
Development of fatigue models for copper traces on printed wiring assemblies under quasi-static cyclic mechanical bending
This dissertation investigates the fatigue durability of copper (Cu) traces on printed wiring assemblies (PWAs) under quasi-static cyclic mechanical flexure, using experimental results from a set of three-point bending fatigue tests, finite element (FE) modeling of the stresses generated during the cyclic bending tests, and response surfaces (RS) to facilitate iterative assessment of the model constants.

Cyclic three-point bend tests were conducted on land grid array (LGA) components during this investigation. Failure analysis revealed the fatigue failure sites to be in the Cu traces, at the outer edge of the foot-print of the solder joint. A three-dimensional, elastic-plastic FE model simulating the event (based on a global and local modeling strategy) was used to determine the stresses and strains occurring at the failure site during the cyclic loading. Parametric studies were conducted to examine the influence of elastic-plastic constitutive behavior on the stress and strain states at the failure site. Results of the parametric studies were captured in compact meta-models, using polynomial response surfaces.  The durability data was collected from the experiment and used in conjunction with these models, to develop a set of compatible constitutive and fatigue model constants that best fit the behavior observed.

Since the loading was not fully reversed, a mean stress correction factor was needed. Existing correction methods, such as the modified Morrow model, were found to be deficient for tensile means stresses, due to high mean stresses predicted by classical constitutive models.  A new correction model was proposed, based on a “tanh” term, which forced a saturation of the mean stress effect at higher stress levels for tensile means stresses. This saturation effect was also considered for compressive loading, termed the BCS model (“B” for “bounded” effect of the mean stresses), and compared with the standard unbounded model, termed the UCS model.

A detailed iterative methodology was developed to iterate the Cu elastic-plastic constitutive model constants as well as the cyclic fatigue model constants needed to satisfy the observed durability behavior. This iterative model was based on the average strain values in cross section of the trace, at the failure site.  The resulting fatigue model constants were termed the “averaged fatigue constants (AFCs).

To further improve on the fatigue constants, the fatigue damage initiation and propagation behavior were considered separately, using a continuum damage mechanics method termed the successive initiation method.  In this phase of the study, the constitutive model constants were those determined from the AFC model.  This method uses an incremental damage growth concept rather than a classical fracture propagation concept, since there is distributed damage observed in the experiment. The resulting fatigue constants were termed the incremental fatigue constants (IFCs).

Finally, the validity of the modeling approach and the developed AFC and IFC model constants are explored, using results from a published case study of four-point cyclic bend tests of leadless chip resistors (LCRs). The model appears to predict the results reasonably well.

George, Elviz (M.S. Mechanical Engineering)
Reliability of Lead-Free Solders under High Temperature Thermal Cycling
Eutectic tin-lead solder has a low melting point and is not suited for certain high temperature applications. Hence, for these applications, the industry had to rely on specialized solders with higher material and processing costs. Widely available lead-free solders have higher melting points and has become the new industry standard. The objective of the research is to study the reliability of these common lead free solders under high temperature application conditions. The effects of temperature cycling with a peak temperature of 185°C on PBGAs (144 I/O and 256 I/O), LQFPs, and surface mount resistors assembled with SAC305 and Sn3.5Ag solder pastes on ENIG and Sn-based proprietary board finishes were studied. The durability data of the components with different solder pastes and board finishes were compared using Weibull++ and MINITAB. Detailed failure analysis including X-ray, optical microscopy, and cross-sectional analysis using Environmental Scanning Electron Microscopy (ESEM) and Energy-Dispersive X-ray Spectroscopy (EDS) were performed. Micro-structural analysis including comparison of IMC composition and thickness would be performed. Finally recommendations are provided based on the results. Other projects: calcePWA is a simulation software that determines the effects of various loading conditions on printed wiring boards to estimate the cycles to failure. The virtual qualification of Printed Circuit Assemblies using the in-house simulation software has been performed for various companies. Life cycle analysis and qualification of PCAs under various loading conditions like unpowered temperature cycling, powered temperature cycling, and vibration, were also carried out. Various regional/national/international reliability and qualification standards were compared and recommendations were provided based on the analysis. In addition, the best practices followed by the computer industry in the qualification of printed wiring assemblies were studied and evaluated.

Goswami, Arindam (Ph.D. Mechanical Engineering)
Quantitative Hermeticity Assessment Of Packages With Micro To Nano-Liter Cavities
Hermeticity is a measure of the “leak-proof ness” of packages with internal cavities and is critical for ensuring proper operation of the devices/circuits enclosed in them.  The most widely used hermeticity detection technique in the industry is the helium fine leak test.  The exiting conduction based governing equation is examined to investigate the volume dependant limits of the test when applied to metal sealed MEMS packages.  The results clearly indicate that the test has limited applicability for small internal volumes (10-6 cc – 10-3 cc).  The limited applicability of the guidelines specified in Method 1014.11 of the MIL-STD-883F document for hermeticity characterization is also characterized. 

To cope with these limitations, a regression analysis based procedure is developed and implemented to extract the true leak rate from the apparent leak data.  While the apparent leak rate obtained directly from the He mass spectrometer changes with the test parameters, the true leak rate remains constant and this can be used as a metric to evaluate a package seal.

The hermeticity of polymer sealed MEMS packages is also studied.  Unlike metal sealed packages, gas transport in polymer sealed packages occurs via diffusion.  A gas diffusion based model is proposed to study the hermetic behavior of these packages.  An effective numerical scheme is developed to implement this model and simulate the change in cavity pressure as gas flows into or out of the cavity through the polymeric seal.  An optical interferometry based leak test is developed to experimentally measure this change in cavity pressure.  The experimental data is used to verify the validity of the proposed numerical scheme and the assumption of adiabatic boundary conditions made in the numerical model. An inverse method is presented to determine the two diffusion properties, diffusivity and solubility, of the polymeric seal by using the experimental data iteratively with the numerical data. The proposed method offers unique advantages over the routinely practiced/existing gas diffusion property measurement techniques.

Gregory, Patrice (Ph.D. Mechanical Engineering)
Comparison of interconnect failures of electronic components mounted on Fr-4 boards with Sn37Pb and Sn3.0Ag0.5Cu solders under rapid loading conditions
Electronic circuit boards can experience rapid loading through shock or vibration events during their lives; these events can happen in transportation, manufacture, or in field conditions. Due to the lead-free migration, it is necessary to evaluate how this rapid loading affects the durability of a leading lead free solder alternative (Sn3.0Ag0.5Cu) assemblies as compared with traditional eutectic lead based solder Sn37Pb assemblies. A literature review showed that there is little agreement on the fatigue behavior of Sn37Pb solder assemblies and Sn3.0Ag0.5Cu solder assemblies subjected to rapid loading. To evaluate the failure behavior of Sn37Pb and Sn3.0Ag0.5Cu solder assemblies under rapid loading conditions, leadless chip resistors (LCR), ball grid arrays (BGA), small outline integrated circuits (SOIC), and small outline transistors (SOT) were subjected to four point bend tests via a servo-hydraulic testing machine at printed wiring board (PWB) strain rates greater than 0.1/s. The PWB strain was the metric used to evaluate the failures. The PBGAs and LCRs were examined with both Sn37Pb and Sn3.0Ag0.5Cu solders. There was no significant difference found in the resulting test data for the behavior of the two solder assembly types in the high cycle fatigue regime. PBGA assemblies with both solders were also evaluated at a higher strain rate, approximately 1/s, using drop testing. There was no discernable difference found between the assemblies as well as no difference in the failure rate of the PBGAs at this higher strain rate. The PWB strain was converted to an equivalent solder stress index using finite element analysis. This equivalent stress index value was used to compare the results from the LCR and BGA testing for Sn37Pb and Sn3.0Ag0.5Cu. Independently generated BGA data that differed with respect to many testing variables was adjusted and incorporated to this comparison. The resulting plot did not show any significant differences between the behaviors of the two solder assemblies under rapid loading outside of the ultra low cycle fatigue regime, where the assemblies with Sn37Pb solder outperformed the assemblies with SnAgCu solder.

Jaai, Rubyca (M.S. Mechanical Engineering)
Fusion Prognostics Methodology for Prognostics and Systems Health Management
Prognostics and systems health management technology is an enabling discipline of technologies and methods with the potential of solving reliability problems that have been manifested due to complexities in design, manufacturing, environmental and operational conditions, and mainenance.Over the past decade, research has been conducted in PHM to provide benefits such as advance warning of failures, enable forecasted maintenance, improve system qualification, extend system life, and diagnose intermittent failures that can lead to field failure returns exhibiting no-fault-found symptoms. While there are various methods to perform prognsotics, including model-based and data-driven methods, these methods have some key disadvantages. This thesis presents a fusion prognostics approach, which combines or "fuses together" the model based and data-driven approaches, to enable increasingly better estimates of remaining useful life. A case study using an electronics system to illustrate a step by step implementation of the fusion approach is also presented. the various benefits of the fusion approach and suggestions for future work are included.

Kwon, Daeil (Ph.D. Mechanical Engineering)
Detection of Interconnect Failure Precursors Using RF Impedance Analysis
Many failures in electronics result from the loss of electrical continuity of common board-level interconnects such as solder joints. measurement methods based on DC resistance such as event detectors and data-loggers have long been used by the electronics industry to monitor the reliability of interconnects during reliability testing. DC resistance is well-suited for characterizing electrical continuity, such as identifying an open circuit, but it is not useful for detecting a partially degraded interconnect. Degradation of interconnects, such as cracking of solder joints due to fatigue or shock loading, usually initiates at an exterior surface and propogates towards the interior. A partially degraded interconnect can cause the RF impedance to increase due to skin effect, a phenomenon wherein signal propogation at frequencies above several hundred MHz is concentrated at the surface of a conductor. therefore, RF impedance exhibits greater sensitivty compared to DC resistance in detecting early stages of interconnect degradation and provides a means to prevent and predict an important cause of electronics failures.

This research identifies the applicability of RF impedance as a means of a failure precursor that allows for prognostics on interconnect degradation based on electrical measurement. it also compares the ability of RF impedance with that of DC resistance to detect early stages of interconnect degradation, and to predict the remaining life of an interconnect. To this end, RF impedance and DC resistance of a test circuit were simultaneously monitored during interconnect stress testing. the test vehicle included an impedance-controlled circuit board on which a surface mount component was soldered using two solder joints at the end terminations. During stress testing, the RF impedance exhibited a gradual non-linear increase in response to the early stages of solder joint cracking while the DC resistance remained constant. The gradual increase in RF impedance was trended using prognostic algorithims in order to predict the time to failure of solder joints. this prognostic approach successfully predicted solder joint remaining life with a prediction error of less than 3%. Furthermore, it was demonstrated both theoretically and experimentally that the RF impedance analysis was able to distinguish between two competing interconnect failure mechanisms: solder joint cracking and pad cratering. These results indicate that RF impedance provides reliable interconnect failure precursors that can be used to predict interconnect failures. Since the performance of high speed devices is adversely affected by early stages of interconnect degradation, RF impedance analysis has the potential to provide improved reliability assessmentfor these devices, as well as accurate failure prediction for current and future electronics.

Paquette, Beth (M.S. Mechanical Engineering)
Harmonic Vibration Testing of Electronic Components Attached to Printed Wiring Boards with SAC305 and Eutectic SnPb Solder
Ball grid arrays attached to printed wiring boards with conventional tin-lead solder (63/37) and one of the leading lead-free tin-silver-copper solders (SAC305) were tested at high and low load levels of harmonic vibration. Leadless chip resistors attached to printed wiring boards with conventional tin-lead solder and lead-free solders (SAC105 and SAC305, and tin-nickel-copper, SN100C) were tested at low levels of harmonic vibration. The tests were conducted near the natural frequency of the assemblies to accelerate testing and to generate high cycle fatigue failures in a reasonable amount of time. The results showed that there are nearly negligible differences in the high cycle fatigue life between the SnPb and SAC305 solders. SN100C and SAC105 were less durable. A master durability plot was generated for SAC305 and SnPb to confirm the negligible-difference between the solders. A safe area was defined be used as a design goal for survivablity for circuit board design.

Patil, Nishad (Ph.D. Mechanical Engineering)
Prognostics of Insulated Gate Bipolar Transistors
Insulated gate bipolar transistors (IGBTs) are the devices of choice for medium and high power, low frequency applications. IGBTs have been reported to fail under excessive electrical and thermal stresses in variable speed drives and are considered as reliability problems in wind turbines, inverters in hybrid electric vehicles and railway traction motors. There is a need to develop methods to detect anomalous behavior and predict the remaining useful life (RUL) of IGBTs to prevent system downtime and costly failures. In this study, a framework for prognostics of IGBTs was developed to provide early warnings of failure and predict remaining useful life. The prognostic framework was implemented on non punch through (NPT) IGBTs. Power cycling of IGBTs was performed and the gate-emitter voltage, collector-emitter voltage, collector-emitter current and case temperature was monitored in-situ during aging. The on-state collector-emitter current (ICE(ON)) and collector-emitter voltage (VCE(ON)) were identified as precursors to IGBT failure. Electrical characterization and X-ray analysis was performed before and after aging to map degradation in the devices to observed trends in the precursor parameters. A Mahalanobis distance based approach was used for anomaly detection. The initial ICE(ON) and VCE(ON) parameters were used to compute the healthy MD distance. This healthy MD distance was transformed and the mean and standard deviation of the transformed MD data was obtained. The µ+3σ upper bound obtained from the transformed healthy MD was then used as a threshold for anomaly detection. This approach was able to detect anomalous IGBT behavior before failure. Upon anomaly detection, a particle filter approach was used for predicting the remaining useful life of the IGBTs. A system model was developed using the degradation trend of the VCE(ON) parameter. This model was obtained by a least squares regression of the IGBT degradation curve. The tracking and prediction performance of the model with the particle filter was demonstrated.

Srinivas, Vikram (M.S. Mechanical Engineering)
Mechanical Testing of Printed Wiring Assemblies with Experience in Torsional Load; Bend Load and Vibration Load (Random, Sinusoidal and Classical Shock) Testing to Evaluate Second-Level Interconnects Reliability.
Torsion loading is experienced by printed wiring assemblies (PWA) present in portable electronics such as cell phones, PDAs, laptops and mp3 players. This leads to shear stresses in the second level solder interconnect used to mechanically and electrically connect surface mount packages on PWA. Reliability assessment was performed using torsional loading of PWAs to compare durability of select lead free solders (SAC305, SAC105 and SN100C) with SnPb as the baseline for comparison. It was found that SAC305 solder shows comparable durability to SnPb solder paste where as SN100C and SAC105 show significantly lower durability. Impact of placement on the PWA, package geometry (resistor2512s and ball grid arrays (BGAs)) and loading conditions (PWA strain range/strain rate) on the durability was determined to generate models and determine acceleration factor. This included exploratory stress test to determine load conditions to precipitate fatigue failures. Solder strain was determined using a transfer function from board strain using FEA. As an extension to the work, package on package (PoP) technology was also evaluated. Package on Package consists of two BGAs stacked one on top of the other to increase miniaturization. PoP can be assembled by stacking the BGAs initially followed by a single reflow or assembled through successive reflows. This detailed study included durability testing of bottom BGA, top BGA, prestacked PoP and PoP to isolate the critical component from the reliability perspective. This work required extensive failure analysis techniques such as X-Ray analysis (non-destructive tests), E-SEM analysis (invasive tests).

Other Research Projects · Rework process is used to increase yield in the electronics industry. However, the impact on performance of such processes on lead-free solders has to be studied. The impact on durability to mechanical loading of rework process on ball grid arrays, thin small outline packages(TSOPs) and resistor 2512 packages for lead based(SnPb) and lead free (SAC305) solder pastes was evaluated. The impact of multiple reworks and thermal stresses on adjacent components is also addressed in this study. This evaluation was performed using four point bend testing. · Diminished part supply of SnPb parts has led companies with RoHS exemptions to procure SAC components and reball them with SnPb solder paste. Reballing is the attachment of new solder after removal of previous solder ball usually to address defects. The impact on durability to mechanical loading of reballing process on BGA packages for packages with same dimensions but different IO count (256IO and 676IO) was performed. Also different methods of solder removal (Low Temperature Wave Solder and Solder Wick) and attachment (Ball drop and Preform) were compared to determine process reliability. This evaluation was performed using four point bend testing. · Solder dipping may be used to replace the original finish with SnPb solder for tin whisker risk mitigation. However, refinishing leads to thermo mechanical damage and impact on mechanical durability as a function of original part lead, finish, solder paste (lead based and lead free solders) and reflow profile on TSOP and resistor2512 packages was assessed in this study. This evaluation was performed using four point bend testing. · Installation of the Shaker Facility for vibration testing and design of fixture. Finite Element Analysis model calibration using Classical Shock experiments and Random Vibration tests. · Extensive experience with Failure Analysis procedures such as cross-sectioning, X-ray, dyes and pry and optical microscopy. · Experience with setting up data acquisition systems to perform in-situ monitoring of strain, resistance and temperature.

Wang, Yong (Ph.D. Mechanical Engineering)
Integrated Measurement Technique To Measure Curing Process-dependent Mechanical And Thermal Properties Of Polymeric Materials Using Fiber Bragg Grating Sensors
An innovative technique based on a fiber Bragg grating (FBG) sensor is proposed to measure the critical mechanical properties of polymeric materials.  The properties include (1) chemical shrinkage evolution during curing, (2) modulus evolution during curing, (3) glass transition temperature (4) coefficient of thermal expansion (CTE), and (5) visco-elastic properties.  Optimum specimen configurations are proposed from the theoretical analysis.  Then an efficient numerical procedure is established to determine the material properties from the measured Bragg wavelength (BW) shift.  The technique is implemented with various polymeric materials.  The measured quantities are verified through a self-consistency test as well as the exiting testing methods such as a warpage measurement of a bi-material strip, and a TMA measurement.  The evolution properties obtained at a curing temperature are extended further by combining them with the conventional isothermal DSC experiments.  Based on the existing theories, the evolution properties can be predicted at any temperatures.

The proposed technique greatly enhances the capability to characterize the mechanical properties and behavior of polymeric materials.  Since the specimen preparation is very straightforward, the proposed method can be routinely practiced and the measurement can be completely automated.  It will provide a much-needed tool for rapid but accurate assessment of polymer properties, which, in turn, will enhance the accuracy of predictive modeling for design optimization of a microelectronics product at the conceptual stage of product development.

Woodworth, Laura (M.S. Mechanical Engineering)
Characterization of Time and Temperature Dependent Mechanical Properties of Advanced Polymers Using Bragg Grating Sensors
The use of polymers in electronic packaging is continuously increasing, due to their relative ease of manufacturability and low cost.  Since polymers exhibit time and temperature dependent behavior, their visco-elastic properties must be characterized in order to predict the behavior of package assemblies during manufacturing and operation.  The testing methods for visco-elastic properties have been developed for many decades and some of them are routinely practiced using commercially available equipment.  However, some of the methods are too time-consuming or complex to be implemented routinely by non-experts; the specimen preparation and the testing conditions are very critical to reliable and repeatable measurements.  A novel method is proposed to characterize the visco-elastic behavior rapidly but accurately.  The method utilizes a polymer cured around a fiber Bragg grating (FBG) to form a complete specimen.  An instantaneous mechanical load is applied to the specimen while equilibrated at a temperature within an environmental chamber, and the Bragg wavelength (BW) shift is documented as a function of time.  The load applies an instantaneous, constant stress to the polymer substrate, which in turn applies a strain to the fiber.  The relationship between the BW shift and the creep compliance can be derived directly from the theoretical behavior of the FBG.  The creep compliance can then be obtained from the BW shift data at each temperature.  By undergoing a de-convolution process the creep compliance can be converted into the time dependent relaxation modulus.  This process can then be repeated for a range of temperatures, which results in relaxation modulus data as well as the initial modulus for each temperature. 

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