FEDEVEL
Platform forum

wenyuan , 09-05-2024, 03:30 PM

I know that I need 22uF and 10V. But I dont know when to choose 0603, 0805, 1206 or whatever footprint

SirJames , 09-05-2024, 03:49 PM

Well, I usually try to have resistors and capacitors in same size - 0603. But that's just because it looks better on board.

SirJames , 09-05-2024, 03:59 PM

I never specify MPN of generic passive components in BOM because manufacturers have their own stock of them. But when I'm not sure if the capacitor is manufactured I use filters on any distributor site and see if there is at least like 10 or 20 items with my specification.

Artarka , 09-05-2024, 05:35 PM

Larger ones tends to have better dielectric (C0G instead of X5R)
There are also other minor differences, but the difference doesn't matter 98% of the time.
We tends to choose either smallest/cheapest/available.
Roll a dice and pick anyone you like would just be fine.

Robert Feranec , 09-06-2024, 02:03 PM

in most projects, you can choose based on the size of the other components or use the same capacitor you use across multiple projects.

If this capacitor is very important, you can compare characteristics - there may be big differences in real capacitance vs. the capacitance on paper. Especially have a look at characteristic which shows capacitance if you apply DC voltage across the capacitor. Also, if this capacitor is for power delivery, then smallest size may have lowest inductance (that is good). This video may help https://youtu.be/bu4CWYFlOwg

QDrives , 09-06-2024, 07:33 PM

For capacitors there are so many specifications that can influence the decision in which to choose.
For ceramic capacitors the following is also very important:
- ESR
- DC bias
- AC bias
- Temperature tolerance
- Initial tolerance
- Aging
It has been a while since I have seen the video of Robert.
In general, the bigger the package, the less DC bias effect you see for same capacitance + voltage.

When you say "*I know that I need 22uF and 10V*" is that including the effects mentioned above or excluding?
People that say "*I never specify MPN of generic passive components in BOM*" do not know the specification mentioned above of 'their' components.

In the attached picture are the capacitance and impedance graph of the 22uF 16V 1210 X7R capacitor (Samsung CL32B226MOJNNNE).
It does not get above 20uF with a frequency above 1kHz. As example 8V and 100kHz it is about 10uF.