Fatigue failure of printed circuit board chemically etched copper traces in multifunctional composite structures

Abstract

One class of multifunctional composite structures is one that is capable of load-bearing while transferring electrical current across a given span. An example is large antenna systems integrated into composite skins of aircraft and spacecraft. Current aircraft missions that require large amounts of sensing equipment can potentially involve heavy and bulky wiring systems. The bulk and mass inefficiency of these systems can be avoided by using multifunctional structures that mimic printed circuit boards to carry both load and electrical current for power or data. The fatigue life and failure modes of these types of systems were experimentally investigated to understand the capability of embedded metals inside composite host materials. ‘Dog bone’ specimens with copper traces were used to measure the fatigue life as a function of loading level. The specimens were fabricated by mass-production printed circuit board methods using woven glass/epoxy composite and chemically etched copper traces that were embedded or surface mounted. Functional failure, described as failure of the copper trace while still maintaining load-bearing capability, was of primary interest. This occurred when the specimen was loaded at levels below 55% of static ultimate tensile strength. Fracture of the copper was caused by cracks forming in the composite surface and propagated through the copper trace. Completely embedded copper traces exhibited longer life than traces that were surface mounted on the composite. An elasto-plastic analysis and summary of experimental results in the form of copper strain amplitude allow results to be applicable to other configurations, i.e., host structures of different stiffness than woven glass/epoxy.