TABLE OF CONTENTS The decision to rework electronic devices or assemblies depends on the cost of reworking versus the value of the part. Consumer electronics, due to their low cost, are considered throwaways and seldom reworked. For some, such as plastic-encapsulated microcircuits (PEMs) that are produced in high volumes, rework is impractical because of the difficulty in removing the plastic encapsulant without destroying the components. However, for high-value assemblies, such as densely populated PWBs and MCMs, where the final value may be as high as $10,000 or more, the ability to rework is essential. In such cases, rework may be necessary to meet schedules when small numbers of assemblies are produced, as for example for space and many military applications.
Rework generally consists of removal and replacement of a failed electronic component that has been detected after initial electrical testing or testing after burn-in. The actual rework procedure is more of an art than a science. Since each component or assembly is different, considerable experience and care are required to selectively remove one device without damaging adjacent ones.
For adhesive-attached components or chip devices that are electrically connected by wire, TAB, or ribbon, the leads must first be excised. In the case of solder-connected components, the solder is heated to its melt temperature and wicked off the part.
Most electronic components are attached with epoxies. Epoxies, being thermosetting, do not melt and resolidify as do thermoplastic adhesives and solder. At sufficiently high temperatures,...
