Digital Testing
Timing circuits or Oscillators not working: Some types of capacitors exhibit 'aging' or a change in capacitance value based on age. Capacitor 'Aging' is the natural process wherein X7R, X5R, Z5U, & Y5V capacitors exhibit a change in capacitance after their temperature is raised above the Curie point for their particular formulation, during soldering, or during some life time or temperature cycling tests. When the capacitor is heated above the Curie point, a change in the crystal structure occurs, and the capacitance increases. This increase in capacitance is called "deaging." As the temperature is later reduced below the Curie point, the capacitance gradually returns to its previous values. The decline in capacitance is called "aging" and occurs at a rate that decreases roughly linearly with the log of time. So circuits working one day, which rely on a capacitance value may not work the next day. C0G capacitors have a different formulation which does not display any aging characteristic. C0G dielectric capacitors remain constant with time. Tantalum capacitors are stable over time and do not suffer from aging.
Additional Digital Logic Pitfalls, Dictionary of Capacitor Terms.
