Damage mechanics of electronics on metal-backed substrates in harsh environments

Abstract

Increased use of sensors and controls in automotive applications has resulted in significant emphasis on the deployment of electronics directly mounted on the engine and transmission. Increased shock, vibration, and higher temperatures necessitate the fundamental understanding of damage mechanisms which will be active in these environments. Electronics typical of office benign environments use FR-4 printed circuit boards (PCBs). Automotive applications typically use high glass-transition temperature laminates such as FR4-06 glass/epoxy laminate material (T/sub g/=164.9/spl deg/C). In automotive underhood application environments, metal-backing of PCBs is being targeted for thermal dissipation, mechanical stability, and interconnections reliability. In this study, the effect of metal-backed boards on the interconnect reliability has been evaluated. Previous studies on electronic reliability for automotive environments have addressed the damage mechanics of solder joints in plastic ball-grid arrays (BGAs) on nonmetal backed substrates and ceramic BGAs on nonmetal backed substrates. Other failure mechanisms investigated include delamination of PCB from metal backing. The test vehicle is a metal backed FR4-06 laminate. Metal backings investigated include aluminum and beryllium copper. Three adhesives have been investigated for metal backing including arlon, pressure sensitive adhesive, and pre-preg. The use of conformal coating for reliability improvement has also been investigated. Component architectures tested include plastic BGA devices, C2BGA devices, quad flat no-lead (QFN), and discrete resistors. Reliability of the component architectures has been evaluated for hot air solder level and electroless Ni/Au finishes. Crack propagation and intermetallic thickness data has been acquired as a function of cycle count. Reliability data has been acquired on all these architectures. Material constitutive behavior of arlon and pressure sensitive adhesive has been measured using uni-axial test samples. The measured material constitutive behavior has been incorporated into nonlinear finite element simulations. Predictive models have been developed for the dominant failure mechanisms for all the component architectures tested.