TABLE OF CONTENTS Failure is defined as the inability of a system to perform its intended function. Failures in circuit card assemblies (CCAs) result from a complex set of interactions between the stresses that act on a CCA and the materials of a CCA. Failure mechanisms are the processes by which a material or system degrades and eventually fails. Three basic categories of failure include overstress (that is, stress-strength), wearout (that is, damage accumulation), and performance tolerance (that is, excessive propagation delays). Overstress and wearout failure categories are commonly related to irreversible material damage; however, some overstress failures can be related to reversible material damage (that is, elastic deformation). Overstress and wearout failure are characterized by a damage metric. In general, damage metrics for overstress failures are defined in terms of a pass or fail criteria. The damage metrics for wearout failures define how much useful life is consumed by an imposed environmental condition. Performance-tolerance failures may be associated with stress conditions that produce reversible changes in material properties. As a result, failures occurring at elevated stress levels may not occur during normal operation. Performance-tolerance failures are characterized by a performance metric. Failure occurs if the performance metric is outside of a specified tolerance range. Performance-tolerance failure mechanisms are usually associated with the failure of integrated circuits and not the CCAs (for example, excessive propagation delay in integrated circuits due at high temperature or excessive thermal transients due to inadequate diffusivity).
In the physics of failure...
