To increase the energy conversion efficiency and power production, the size of wind turbine blades has continued to grow larger. As such, the need to identify defects and monitor the structural health becomes more critical to maintain reliability and prevent costly catastrophic failures. Larger blades are subject to an increased likelihood of operational damage and manufacturing defects. Usually, glass-reinforced spar caps are inspected with the assistance of strong flashlights, while nondestructive testing (NDT) inspections are performed only on carbon-reinforced blades which are not translucent. These operations provide qualitative information only. Therefore, there is a growing need for inspection techniques that can provide quantitative information for assessing the structural health of the blades. In recent years, different NDT techniques capable of accurately delivering surface and internal data have been developed. Nondestructive testing systems should be able to quickly and efficiently scan large areas, to be economically beneficial to the wind turbine manufacturing industry. Within this paper, 3 different NDT techniques for the inspection of manufactured wind turbine blades' spar caps are compared: terahertz inverse synthetic aperture radar, infrared thermography, and X-ray imaging. Each is investigated to evaluate their ability to detect the presence of defects of concern that are created during the blade manufacturing process. Through a set of experimental tests, the advantages, challenges, and drawbacks of each used technique are evaluated and compared in the context of the needs of the wind turbine industry. This research provides the foundation of empirical comparisons that can lead to the development of more accurate NDT resulting in the construction of more reliable and less-expensive wind turbine blades. KEYWORDS infrared thermography, nondestructive testing, terahertz inverse synthetic aperture radar, wind turbine blades, X-ray imaging

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JEDEC identifies in J-STD-020C, that all devices should be baked for 24 hours minimum at 125°C +5°C/ -0°C to remove moisture from the package so that it will be "dry" [1]. This bake is required prior to other Reliability stresses [2 ], again to "dry" the plastic encapsulated devices. When this preparation technique was first established, most of the mold compounds used were comprised of OCN / ECN novolac resins with glass transition temperatures (Tg) significantly greater than this 125°C bake. The mold compounds that are used today have a variety of chemistries and often have Tg regions near or below the 125°C bake temperature. This change in mold compound chemistry has resulted in two unpredicted responses to the 125°C bake. Some mold compounds will undergo additional cross linking during this conditioning (drying) bake and will result in a more fully cured matrix. Additional mold compounds that have their Tg regions at or near this 125°C bake temperature will undergo a change in their inherent equilibrium state, known as physical aging. This will result in a polymer network with a reduced free volume space between polymer chains that would ultimately result in performance behavioral changes when the material is exposed to the moisture soak conditioning, and reflow preconditioning required prior to reliability testing. This paper will describe the analytical methods for characterizing this change in equilibrium state, and explore the effect of this change in free volume space on the behavior of three mold compound systems.An alternative drying procedure utilizing vacuum desiccation will be used for comparative purposes during subsequent characterization experiments. ANSYS modeling will be used to demonstrate the moisture diffusion rate through the mold compound of the package. BackgroundSeveral failure mechanisms within plastic encapsulated microcircuits(PEMs) associated to moisture ingress and ionic diffusion through the mold compound had been identified and characterized [4,5]. In order to characterize the reliability of the PEM, JEDEC (the semiconductor engineering standardization body of the Electronic Industries Alliance) identifies several reliability stress tests that are designed to bring out failure mechanisms associated to moisture exposure. As a "drying" step prior to accelerated environmental exposure, several of the JEDEC specifications [1,2,3] require a 24 hour minimum bake at 125°C +5°C/-0°C to remove the resident moisture from the package. After this bake the devices under test (DUT) are exposed to various elevated temperature and relative humidity environments. Failure mechanisms such as delamination, package cracking, corrosion, and intermetallic