Electrochimic capacitor are equivalent to a capacitor with a resistor in parallel and an there is an inductance in serie with this.
So, the impedance is :

ZC = (R/1+jRCw) + jLw
with w=2xPIxf

So when f is increasing, w is increasing and the impedance becomes very high as an open circuit, so without any effect against HF signals.


Ceramic and plastic capacitors havn't this serial inductance. Their impedance is :

ZC = R/1+jRCw
with w=2xPIxf

So, when f is increasing, w is still increasing, but the impedance is decreasing making thus a short-circuit for high frequencies. That's why this kind of capacitors are used for HF filtering and electrochimic capacitors for LF filtering.

It's the use in digital boards to implement ceramic or plastic capacitors on the supply of each digital chip. Because the spectrum of a switched signal may have some high frequency's harmonics due to the high slew rate. Unfortunately, this HF noise is conducted on the wires and power supply and may alter the digital chips operation.

The electrochimic capacitors are used for example to filter the output of a power supply by decreasing the ripple and taking over the current peaks due to the switched loads.


The tantal capacitors are similar to electrochimic capacitors for the impedance but have the disadvantage to be very fragile during SMD soldering operations. The high temperatures of the oven may break these capacitors and it's often invisble for human eyes. They're usually avoided on SMD boards using refusion soldering.

Regards
Stephane