Radial capacitors have two leads which stick out from the bottom of the capacitor. They are a different configuration to axial capacitors in that sense because axial have one lead that sticks out from either end.
Wet Electrolytic Capacitors
Rubycon MCZ Radial Capacitors
Since Polymer capacitors came into use it was necessary to find a term to differentiate between the older technology of using a liquid electrolyte and the newer of using polymer crystals. A typical reference to capacitors with liquid electrolyte might call them wet electrolytics. Both the Panasonic FC in the first photo and the Rubycon MCZ in the second photo are wet electrolytics.
Radial Electrolytic capacitors have polarity and therefore it is important to solder them in with the correct polarity. Failure to do so and they vent (bulge at the top) after some minutes of operation. Positions for capacitors are marked with a circle that has one part shaded white. This marks the negative lead position. The capacitors also all have a stripe down one side. This marks the negative lead. So you need to line up those marks.
Not all PCBs follow this rule though. ASUS actually uses the white shaded part to indicate the positive lead on their PCBs, which is quite confusing. Occasionally there may be an error in the marking of the PCB also. So the best thing to do when replacing capacitors is to trust the way the original capacitors were positioned on the board and not the PCB.
The type of electrolytic capacitors that are used on motherboards are Low ESR (Equivalent Series Resistance). The lower the ESR, the faster the capacitor can discharge and fast discharges are what the computer circuits need.
Not Very Easy to Obtain
It is important to understand that there are many differences in specification between capacitors and although you may find capacitors in a local shop they may not be the Low ESR capacitors that are required for stable operation of the computer. Chances are that you will have to order them. There are several sources for Low ESR capacitors however.
One of the problems with replacing capacitors on motherboards and in power supplies is that the manufacturers use capacitors with diameters that are a little smaller than those offered to the general public via the big distributors. Chances are that the 1000uf and lower are 8mm and the 1500uf-2200uf are 10mm whilst some sources may offer 10mm and 12.5mm respectively for those sizes. On motherboards that is not too much a problem as some capacitors can be installed leaning over but in power supplies it can on occasion be a problem as spacing can be tight in all directions.
Wet electrolytics have a habit of failing either due to their poor manufacture or due to their operating condition exceeding their rated ripple current, voltage or other factors. Typical failure modes are High ESR, Low Capacitance, Shorting or being Open. They can also on occasion give visual indication of their failure by bulging and leaking electrolyte from the top or pushing out their bottom bungs and leaking electrolyte from the bottom. But that is not always the case.
Affect of Capacitor Failure on Computer Equipment
If capacitors have failed or are performing below specification, this can have various effects on computer equipment such as making the computer start only after several tries, instability due to CPU or memory errors or various strange problems which appear impossible to diagnose. We will discuss all that in another article.