Debonding Under Fatigue Loading at Polymer/Inorganic Interfaces
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P3.13.1
Debonding Under Fatigue Loading at Polymer/Inorganic Interfaces Bree M. Sharratt1 and Reinhold H. Dauskardt2 Department of Aeronautics and Astronautics, Stanford University 2 Department of Materials Science and Engineering, Stanford University Stanford, CA 94305-2205 1
ABSTRACT The mechanisms associated with cycle-by-cycle damage accumulation resulting in fatigue crack propagation between a highly constrained polymer layer and an adjacent elastic substrate are explored. Specifically, cyclic fatigue-induced crack growth between a bisphenol F model epoxy system and a passivated silicon substrate under Mode I loading is reported. Preliminary findings regarding the effects of fatigue load ratio on interfacial crack growth rates are presented. While intermediate crack growth rates were significantly accelerated under cyclic loading, the near-threshold crack growth behavior under cyclic and monotonic loading was surprisingly similar. INTRODUCTION The effect of fatigue loading on the subcritical interfacial crack growth between a polymer layer and an adjacent substrate is of significant interest for the integrity of a variety of components. In the case of high-density flip-chip packaging, underfill epoxies are used to support the thermo-mechanical stresses that develop in the closely-packed solder ball grid array. By filling in the standoff between the chip and the board with an epoxy, the reliability of the solder connections is improved [1,2]. However, pervasive challenges persist associated with interfacial crack growth driven by significant residual stresses from thermal expansion mismatch between the epoxy and the chip. In addition, accelerated crack growth under alternating stresses due to thermomechanical cycling may occur and is the subject of the present study. The interface selected for study was formed between an unfilled bisphenol F epoxy bonded to a silicon substrate with a SiO2 or Si3N4 passivation. Previous work examined the subcritical crack growth behavior of this interface under monotonic loading [3]. While the interface was only marginally susceptible to stress corrosion, an unexpected plateau region was observed at growth rates on the order of 1 nm/sec and was attributed to the disruption of secondary interfacial bonds by absorbed water molecules. Although Mode II loading, which may be generated under more complex thermomechanical conditions, is known to affect fatigue crack growth rates [4], the goal of the present study was to characterize the subcritical crack growth behavior resulting from Mode I cyclic fatigue. The application of cyclic loads results in a marked acceleration in crack growth rates due to mechanically induced damage accumulation. Behavior resulting from variations in the fatigue load ratio is presented along with unique near-threshold growth rate behavior where evidence of the low growth rate plateau was again observed. EXPERIMENTAL PROCEDURE Specimens were comprised of a model underfill polymer, diglycidyl ether of bisphenol F epoxy cured with 2-ethyl-4-methyl-imidazole ha
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