Moisture Effects and Viscoelasticity in Polymers Used for Microelectronic Packaging
Type of DegreePhD Dissertation
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In the microelectronics packaging industry, polymer based materials are used widely. Reliable, consistent and comprehensive material property data are required for mechanical design, process optimization, and reliability assessment of electronic packages. In this study, mechanical behavior and mechanical properties of underfill encapsulants and solder mask materials have been investigated. Due to absorption of moisture, softening of the polymer materials can occur as well as new failure modes in electronic assemblies (e.g. popcorn failure). In the first part of this work, the effects of moisture absorption on the stress-strain and creep behavior of an underfill encapsulant were evaluated experimentally. The fabricated and preconditioned uniaxial test specimens were then exposed to moisture for various durations (0, 1, 3, 10, 20, 30, 60 days) in an environmental chamber using three different sets of hygrothermal conditions based on the JEDEC standard. After the moisture exposures, a microscale tension-torsion testing machine was used to measure the room temperature stress-strain behavior of the material. In addition, the viscoelastic mechanical responses of the underfill encapsulant after the moisture exposures were also characterized via creep testing for a large range of applied stress levels. From the recorded results, it was found that the moisture exposures degrade the mechanical properties of the tested underfill including the effective elastic modulus, ultimate tensile stress, and tensile creep rate. Pre-baking was found to increase the initial material properties before moisture exposure, but the degradations due to subsequent moisture exposures occurred in an analogous manner. The moisture absorption into the underfill samples was also characterized by weight gain measurements for up to 60 days of exposure. From the results, it has been shown that maximum water weight gain in the material is about 1%. The results also showed that mechanical properties were only dependent on amount of water that has been absorbed by the material, regardless of how rapidly it was absorbed. In the second part of this study, both frequency dependent dynamic mechanical analysis (DMA) measurements and strain and temperature dependent stress relaxation experiments were performed on a typical underfill material in order to characterize its linear viscoelastic behavior. In both cases, a master curve was determined using the assumption of time-temperature equivalence, and Prony series expansions were utilized to model the material’s relaxation behavior. For both sets of experimental input data, the Prony pairs and shift factors were determined, and the master curves at the reference temperature were shifted to other temperatures for comparing with experimental data. Finally, the developed viscoelastic models were used to both predict the rate dependent stress-strain behavior and the creep behavior of the material. The model predictions were correlated with additional experimental data not used to establish the viscoelastic model, and good correlations were obtained. The developed viscoelastic model for underfill was then used in finite element models of underfilled ball grid array packages (Ultra CSP) subjected to thermal cycling from -40 to 125 oC. Separate simulations were also performed using temperature dependent elastic properties for the underfill material. In both cases, the solder joint fatigue life was estimated, and the effects of using viscoelastic properties for the underfill in solder joint fatigue life simulation were investigated. Using elastic properties of the underfill material overestimates the failure life by calculating lower plastic work. In the third part of this work, the mechanical behavior of a typical UV curable solder mask material has also been explored as a function of ultra violet (UV) curing time, testing temperature, and isothermal aging exposure. In this study, a unique sample preparation technique has been developed to make 80 x 3 mm uniaxial tension test samples with a defined thickness (e.g. 0.30 mm), and both stress-strain and creep tests were performed. The mechanical behavior changes of the solder mask material were recorded for different curing profiles including various durations of UV exposure and subsequent isothermal curing. The results showed that an optimum UV exposure time was critical to provide acceptable mechanical properties. In addition, the stress-strain and creep behavior of the solder mask were recorded for various temperatures from 25 to 125 oC, and the mechanical properties were found to degrade significantly at elevated temperatures. The experimental results showed that variations of thermal curing profile (curing temperature and time) also change the mechanical properties significantly, so that solder masks have a very small optimum processing window. Finally, the effects of isothermal aging at 100 oC on the material behavior were characterized for different aging times. Using the recorded data, the changes in the elastic modulus, strength, and creep rate were characterized as a function of aging time. Significant variations were observed in the elastic modulus and ultimate strength of the aged samples.
- Promod Chowdhury_PhD Dissertation_R8.pdf