Abstract:The goal of a durable exterior clear coating has eluded generations of coatings technologists, despite long-standing desire amongst the public for such a coating. The journey towards this goal initially focused on modifications to coating formulation, but took a completely different direction when it was found that a UV-transparent silicone clear coating on wood modified with chromic acid met consumer expectations of coating durability. This finding sparked world-wide interest in wood pre-treatments as a way of enhancing the durability of clear coatings. This interest initially focused on transition metal compounds, but has now shifted in the direction of organic and inorganic photostabilizers or even more drastic pre-treatments. Pre-treatments that dimensionally stabilize wood, protect it from microbial degradation and photostabilize lignin, when combined with flexible, photostable, coatings provide the next way-stop on the journey towards achieving the goal of durable exterior clear coatings for wood. This paper reviews this journey, drawing upon our research and that of other groups who have focused on this elusive goal. OPEN ACCESSCoatings 2015, 5 831

We hypothesize that plasma treatments that increase the adhesion and penetration of transparent water, and solvent-borne polyurethane coatings into black spruce wood will improve the performance of coated wood exposed to accelerated weathering. We tested this hypothesis by modifying wood samples with plasma for 30, 180, 600, and 1200 s, and measuring coating penetration and adhesion using light microscopy and a mechanical pull-off test, respectively. Plasma treatment did not improve coating adhesion, but the solvent-borne coating showed deeper penetration into plasma-modified wood, and its resistance to accelerated weathering was better on plasma-modified wood than on untreated controls. Plasma treatments enhanced the penetration of water-borne polyurethane into wood, but the treatments did not improve weather-resistance of the coating. Plasma treatment increased the wettability of wood surfaces, and prolonged plasma treatment etched cell walls, increasing their porosity. These effects may explain the positive effect of plasma treatment on coating penetration, and the increased weather-resistance of the solvent-borne polyurethane on plasma-modified wood. In conclusion, our results indicate that the ability of plasma treatment to improve coating performance on black spruce depends on the coating type, and the effects of the treatment on the surface microstructure of wood.

This is Part I of a two-part paper on adhesively bonded flip-chip-on-flex (FCOF) microelectronic assemblies. This paper examines the use of anisotropically conductive adhesives (ACAs), and the companion paper (Part II) addresses the use of non-conductive adhesives (NCAs). Two types of FCOF dies, bonded with ACAs, were subjected to temperature cycling and repeated temperature shock durability tests. The specimens have Au-plated bumps and the ACA contains Au-plated polymer particles. Specimens were fabricated with different bonding pressures. Scanning Electron Microscopy (SEM) investigations were performed to characterize the distribution and shape of the conducting particles.Contact resistance measurements, conducted throughout the temperature cycling durability test for 1000 temperature cycles between 20 • C and 115 • C, showed that resistance varied cyclically with each temperature cycle, but there was no increase in the average resistance over the duration of the test. In contrast, cyclic thermal shock tests between −50 • C and 115 • C produced failures within 200-2500 cycles, depending on the specimen configuration.Studies were conducted to examine the effect of specimen configuration and temperature on: (i) interconnect resistance, based on electro-thermal modeling; and (ii) mechanical contact stresses, based on thermomechanical modeling. Modeling techniques used detailed finite element analysis (FEA) as well as simpler rapid-assessment models based on 2D variational methods (for example, the Raleigh-Ritz method) and 1D electrical resistance networks. These models were used to parametrically investigate the influence of interconnect design, bonding pressure, temperature and interconnect degradation mechanisms, on the contact resistance and contact stresses.Results suggested that the thermal expansion of the ACA during temperature cycling did not cause any changes in the contact resistance at the contact interface, thus suggesting the possibility of bonding at AuAu interfaces. During thermal shock, the expansion mismatch stress between the die and Printed Wiring Board (PWB) at the low temperature (−50 • C) was probably sufficient to fracture this bond. The issue of gold-to-gold bonding will be addressed in a future paper.