Final answer:
Heat and voltage fluctuations are primary causes of premature diode failures, not excess resistance or voltage drops, with heat-related failures often stemming from excessive current leading to thermal issues.
Step-by-step explanation:
Premature failures of diodes can be caused by several factors. Among these, heat is a major cause because it can lead to thermal runaway resulting in diode failure. Also, voltage fluctuations can stress the diode, leading to breakdown if the voltage exceeds the diode's maximum rated peak inverse voltage (PIV). Moreover, voltage drops are not a typical cause of diode failure unless they are indicative of an underlying issue such as improper circuit operation. It's important to note that an excessive current flow through a component can cause undesired heating effects which can lead to failures, demonstrating that diode failures are often heat-related.