Ceramic capacitors lose a portion of their capacitance value over time as a function of their construction. This loss cannot be avoided, but it can be measured and specified. Manufacturers often use the decade hour as a unit of measurement for this loss.
As an example, let’s examine KEMET part number C0603C105K8PACTU. This is a 10uf part with an X7R temperature coefficient. Kemet specifies this part to be within its design tolerances for a specific time window after reflow, and uses the decade hour to do so. A ‘decade hour’, in this instance, refers to a period of time ten times longer than the previous measurement, i.e. a logarithmic scale of time as opposed to a linear one. For a visual example, please check these handy graphs from KEMET.
As you can see, the capacitor loses about 5% of its total capacitance value every decade after its last heat cycle. An hour after the reflow process, it reads at about 12uf. Ten hours, or one decade, later, its capacitance will have dropped 5% of its capacitance. A hundred hours, or two decades, later, it drops another 5%. A thousand hours, or three decades, and it drops another five percent and conforms to its intended design tolerances.
This 5% drop every decade continues indefinitely over the life of the capacitor, until it’s subjected to another thermal stress event over 130C. This resets the capacitance back up to 12uf and starts the decade timer over, ‘de-aging’ the part.
For more information on ceramic capacitors, including aging, please check out this excellent Ceramic Capacitors article on our EEwiki.