When bonds between copper and printed circuit board laminates are subjected to impulsive forces, the need arises to characterize fracture energies corresponding to related, high-speed failure events. Work (or energy) is required to create new surface area—with associated dissipation events—during fracture, and this energy (for a given material system) is dependent on the speed of crack propagation, the locus of failure, and the temperature of the bond when it is broken. Since the 90° peel test has been widely employed in quasi-static fracture testing of film adhesion for printed circuit board applications, this test was first used as a basis to which other test results could be compared. A test fixture was designed and built for quasi-static peel testing that accommodated peeling at different angles and temperatures. A similar test was then desirable for the direct comparison of dynamic fracture events to those quasi-static results. The “loop peel test” was thus developed to mimic the common 90° peel test and to quantify the time- and temperature-dependent fracture energies of peel specimens during low-velocity impact. This test has been successfully used to determine the apparent critical strain energy release rate of copper/epoxy bonds for low-velocity impact conditions (1-10 m/s), for a case of near-interfacial failure. The falling wedge test has also been adapted to estimate the apparent critical strain energy release rate at similar fracture conditions. Four types of printed circuit boards have been analyzed with the above impact test methods as well as with their corresponding quasi-static tests, and the fracture energies measured with the impact tests have been compared to those obtained using quasi-static tests. Fracture energies of the material systems considered were dependent on time (speed of fracture), temperature, and the amount of moisture migration, as determined via humidity conditioning parameters. / Master of Science

The objective of this study was to evaluate and gain a better understanding of the short-term impact performance and the long-term durability of electrically conductive adhesives for electronic interconnection applications. Three model conductive adhesives, designated as ECA1, ECA2 and ECA3, supplied by Emerson & Cuming, were investigated, in conjunction with printed circuit board (PCB) substrates with metallizations of Au/Ni/Cu and Cu, manufactured by Triad Circuit Inc. Effects of environmental aging on the durability of conductive adhesives and their joints were evaluated. All the samples for both mechanical tests and thermal tests were aged at 85%, 100%RH for periods of up to 50 days. Studies of bulk conductive adhesives suggested that both plasticization, which is reversible and further crosslinking and thermal degradation, which are irreversible, might have occurred upon exposure of ECAs to the hot/wet environment. The durability of electrically conductive adhesive joints was then investigated utilizing the double cantilever beam (DCB) test. It was observed that the conductive adhesive joint was significantly weakened following hydrothermal aging, and there was a transition from cohesive failure to interfacial failure as aging continued. A comparative study of the durability of different conductive adhesive and substrate metallization combinations suggested that the resistance of the adhesive joints to moisture attack is related to the adhesive properties, as well as the substrate metallizations. It was noted that the gold/adhesive interface had better resistance to moisture attack than the copper/adhesive interface. A reasonable explanation of this phenomenon was given based upon the concept of surface free energy and interfacial free energy. XPS analysis was performed on the fractured surfaces of DCB samples. For adhesive joints with copper metallization, copper oxide was detected on the failed surfaces upon exposure of the conductive adhesive joints following aging. XPS analysis on the fractured surfaces of adhesive joints with Au metallization suggested that diffusion of Cu to the Au surface might have happened on the Au/Ni/Cu plated PCB substrates during aging. The impact performance of conductive adhesives was quantitatively determined using a falling wedge test. This unique impact resistance testing method could serve as a useful tool to screen conductive adhesives at the materials level for bonding purpose. Moreover, this test could also provide some useful information for conductive adhesive development. This study revealed that the viscoelastic energy, which is a result of the internal friction created by chain motions within the adhesive material, played an important role in the impact fracture behavior of the conductive adhesives. This study also demonstrated that the loss factor, evaluated at the impact environment conditions, is a good indicator of a conductive adhesive's ability to withstand impact loading. / Ph. D.