I'm trying to choose the right size of smd mlcc ceramic capacitor.
Two of the biggest values of my capacitors are, 4.7uF and 10uF which I first chose to be 0603 because my pcb space is very limited.
Those capacitors are output capacitor in power manager IC (battery charger with power path), and decoupling capacitors, for GPS module, MCU, etc.
The voltage on their terminals does not exceed beyond 4.2V in worst case.
I was wondering if there is any significant reason (Apart from cost) to choose larger footprints for an smd MLCC especially in my case.
EDIT: What is the main difference between two sizes considering same capacity, voltage and dielectric?
Best Answer
High-K ceramic dielectrics (X7R, Z5U etc) have voltage-dependent capacitance, ie the capacitance drops as voltage increases.
This does not actually depend on the voltage: it depends on the E field which is voltage divided by thickness of dielectric (V/m), and of course the dielectric type (those that cram the most capacitance into the tiniest space for the cheapest price tend to have worst capacitance variation).
A physically larger capacitor (more volume) will have thicker dielectric, thus lower E-field inside the dielectric, thus it will lose less capacitance as voltage is applied. You can find the info in the datasheets. This graph shows X7R vs X5R, the lower-grade dielectrics like Z5U are worse, so watch out.
See also this answer.
This will bite you if you forget to account for it, as your 10µF cap will actually have less µF when voltage is applied to it. If you designed for 10µF and the cap actually gives you 4µF then you may not meet your specs. But if you do account for it, then there is no problem.
Note that as capacitance loss depends on the E-field and thus the volume of the cap, two caps of the same footprint (like 0805) and same dielectric (like X7R) then the thicker cap should lose less capacitance at working voltage.
There could be an issue with ripple current if it's high enough (you don't want to overheat the capacitor with RI^2 losses). A large ceramic cap will allow a higher ripple current, due to having lower ESR and lower thermal resistance to the board. Ceramics are surprisingly good at this, ESR is very low, and a tiny cap can take 1 amp ripple without problems, but maybe not 10 amps due to losses being in I^2... good thing they're cheap and easy to parallel. Try googling "ceramic capacitor ripple current" for a bunch of useful links, like this one.
Inductance is also slightly higher for larger packages, but this also depends a lot on mounting inductance (ie, pcb, stackup and vias). In your case, you're not decoupling a huge CPU with 50 caps around, so layout is a lot more important than the difference between 0603 and 0805.
EDIT --
Peter says: I find Kemet K-Sim very useful: ksim.kemet.com
See also Murata SimSurfing