NON-DESTRUCTIVE ANALYSIS OF TO-247 STRUCTURAL DEFECTS

摘要

A single pulse of ultrasound fired into the polymer encap-sulant of a TO-247 device travels through the polymer at a speed around 3,000 m/s. Its mission is to characterize any internal material interfaces that it encounters. It will be partly reflected by any material interface, but most strongly-virtually 100%-by the interface between a solid and an air gap, and less strongly by solid-to-solid interfaces. When a pulse encounters a solid-to-solid interface, a portion of the pulse crosses the interface and travels deeper, where it may be reflected by a second interface. A solid-to-air interface reflects virtually 100% of the pulse, and no ultrasound crosses the interface.