I always read about decoupling CAPs should be near to MCU pins, but how much near? If we are talking about very small caps of 0201 dimensions, wich distance is good from MCU power pin?

Look at this example:

enter image description here

Each grid block it's 5mil of distance, decoupling cap was placed at 5mil from MCU power pin and this it's minimum distance I can get from my PCB manufacturer. As you can see I found some difficults to layout adjacent pins, also on reflow this small cap can slide(that happens to me) on MCU pins and this is problematic to solve. (note that big via is a 13mil size and seems very big near 0201 package)

But this 5mils I can get are really necessary? Can I go to 10mil or 15 mils? or place in vertical position this cap?

  • \$\begingroup\$ The reason to minimise the distance is to limit the parasitic inductance and avoid transmission-line effects. 5 mil is such a small distance that for all practical purposes it is negligible out to extremely high frequencies. I will bet if you take the tenth harmonic of the clock frequency of your microcontroller and calculate the impedance of this tiny inductor and the wavelength in the trace you will be able to see for yourself the practically nonexistent effect it will have. For any given frequency and maximum impedance you can calculate what is a tolerable length. \$\endgroup\$ – Oleksandr R. Feb 17 '16 at 13:49