Fault Simulation Basics

Fault simulation is a powerful yet not well understood tool for generating test vectors. This tutorial describes the principles of fault simulation, fault modeling techniques and the economic benefits of fault simulation. The section on principles of fault simulation includes serial, parallel, and concurrent fault simulation algorithms along with descriptions of digital faults, fault coverage, and fault mechanisms typically found in digital circuits. The modeling technique section presents the trade-offs of fault placement, fault collapsing algorithms, and the ability to discover different physical defects through fault simulation. Economic benefits are shown through empirical examples and through correlating fault coverage to average defect levels resulting after manufacturing test. FAULT SIMULATION OVERVIEW The primary types of simulation used in Computer Aided Engineering (CAE) are logic and fault simulation. Logic simulation uses computer models of an electronic design to imitate the behavior of a physical system. It allows the engineer to know if the design will work the way it is supposed to work before the product is manufactured. Logic simulation is the only practical alternative to the timeconsuming and costly practice of actually building and debugging physical prototypes. Once a design has been properly logic simulated, fault simulation is the next critical step. bad parts. In fact, it is not unusual for yields to average between 30% and 50% for integrated circuits. As Figure 1 shows, it is up to the test process to screen out these bad parts so they do not get to your customer. Fault simulation helps you screen out bad parts. I t is a tool that defines how well the test vectors used to verify a finished product will catch manufacturing defects. A perfect test vector set would be able to detect any defect in the physical chip. When you have verified a circuit and fully completed its logic simulation, you have a full description of how a defect-free version of the circuit operates. However, n4l you be able to know when a part is All manufacturing processes create good and CH1234-5/89/0000-T5-1 l$Ol.OO