Normaly used specifications do only focus on a certain set of parameters, which are of major importance for an application. But nevertheless there are many other paramaters, which don't seem to play a relevant role, but which can heavily influence what's going on in a circuit. Have a look for instance at a fuse. Major specifications are 'nominal current' and 'blow time'. But there's one important hidden parameter, which can dramatically affect performance of application: Ohmic resistance! This ohmic resistance is mostly unwanted and many people think about a fuse of having just a piece of wire, which melts when being overrated. Ohmic resistance is expected to be negligible. But that's totally wrong. I have seen fuses shining like a glimming lamp, because ohmic resistance was limiting short circuit current to such a low value, that fuse couldn't blow. If you take fuses from different manufacturers ohmic resistance can drastically differ, even if nominal rating is identical!

Another example for hidden parameters is the electrolytic capacitor. Many years ago it was difficult to fabricate types, that show very low equivalent series resistance. But this need not to be a disadvantage in all cases, because increased equivalent series resistance can dampen certain resonances. If such a 'poor' capacitor is changed against a modern type, which does show very low equivalent series resistance, problems can occur.
Low drop voltage regulators are an example for such a problem: Some of these regulators need a certain minimum equivalent series resistance of decoupling capacitor to keep intrinsic instability under control. If a 'poor' deoupling capacitor is changed against a modern type, which shows very low equivalent series resistance, then the regulator might tend to oscillate.

Other example: Today, there some highcaps arround made of ceramics, which offer an enormeous capacitance of up to dozens of microfarads. They are told to be an superior exchange for tantals. But what manufacturers do not tell is, that these capacitors show an extreme piezoelectric effect. I have measured dozens of millivolts, when slightly hitting the PCB. In a digital circuit this might be no problem at all. But a high amplifying analog stage handling very low signal levels might suffer from a drastical signal degradation!

More examples about hidden parameters?

Kai