Component Size selection

[57bel]

Hello,
I saw a previous thread where it ended on a question that I would like to know the answer to: https://www.edaboard.com/threads/component-selection-for-rf-circuits.280238/ . It ends wondering when you should switch to a smaller component. The general consensus seems to be always chose the smallest component that can handle the power. This minimizes size and parasitic effects, but is there a general rule? At 20 GHz a 0805 would cause a lot more effects then a 0201, is there a general chart? Or just always select the smallest that can handle power?
Thanks,
-57bel

 

[barry]

You might also want to think about manufacturability, testing, rework (0201components are pretty tiny)

 

[FvM]

At 20 GHz, even 0201 resistors can't be assumed as lumped components. Better model PCB structures and all discrete components in a suitable analysis tool.

In the lower frequency range, at least lumped models of parasitic component parameters and PCB trace segments are necessary for a realistic circuit analysis.

 

[biff44]

0201 components are hard to handle.
for instance, if you unsolder a 0201 resitor, and try to install a new value to those pads, you often find the solder not wetting, or the pads lifting from the solder iron heat.

one size bigger makes a big difference.

this is not an issue if you NEVER replace any parts, like in a mass produced product.
but in a product that has a small volume production, you should try to use bigger parts. Or at least ONLY use the 0201 part size were it is absolutely necessary considering parasitic inductance and capacitance.

So, if you have a Pi attenuator at 20 GHz, sure, use 0201 size. If you have a bias resistor to limit current, do NOT try using such a tiny part there!