Chip Type SMT Electrolytic Capacitors
by.Zezo Mohamed
We can see in the picture
above several silver coloured can capacitors. They all have a black base which
allows them to be placed by machine onto the top of the PCB. Therefore they are
surface mount or SMT capacitors. The manufacturers often call these capacitors
Chip Type.
Unfortunately they are often
mistaken for Polymer capacitors. This is incorrect because polymer capacitors
do not have electrolyte. You can see that the big ones in the photo have K or X
vent stamps. If the capacitors have vents then they are not polymer because the
vents are to allow the gas to escape if the capacitor fails and makes a
chemical reaction inside. The smaller ones do not have vents because their
small construction or different type of electrolyte will not result in a
significant build up of gas when the capacitor fails.
So it is possible that these
capacitors will fail. It is also possible that mishandling of the product may
result in the user breaking the capacitor off the base. That would also require
replacement of the capacitor.
Does the PCB have a
Position for SMT caps only?
If you look at the lower
capacitor on the left you will see to the right of it that there is an empty
position where an SMT capacitor has not been installed. You can see that there
are two solder points there. One is for the negative and one is for the
positive. There is no thru holes on this PCB to allow you to use a radial
capacitor instead of an SMT one. The SMT caps have two little metal tabs
sticking out from their base. So if you needed to replace one you would have to
heat the tabs one by one in order to desolder the old capacitor from the board.
Then you would place the new capacitor in position and heat the tabs one by one
and add solder. Yes it is possible to do this job with hand soldering.
Pay attention to the
Polarity of the caps
Here we can see the negative
stripe for these chip type capacitors. The can type capacitors used in computer
equipment have polarity so it is very important to install them with the right
polarity otherwise they will blow. The SMT chip type capacitors have a
hemisphere mark on the top which you can see in the photo. This indicates the
negative lead. If you refer to the above photo there is a + marked on the
board. So you would install the cap there with the hemisphere on the other side
because that lead is not +ve it is -ve.
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