Re: [SI-LIST] : Decoupling capacitors

Larry Smith (ldsmith@lisboa.eng.sun.com)
Tue, 11 Aug 1998 13:02:17 -0700 (PDT)

> What do you think about the "common" practice to place bypass capacitors
> on the solder side of the board right under the IC located on the
> component side and use vias to connect to VCC and gnd the capacitor. I
> certainly don't recommend to do that but I have seen it so many times I
> wonder if in some cases it is justified.
> Any comments
>
> --
> Philippe Poulet

The objective is to get the least amount of impedance (inductance)
between the power consumer (asic) and charge storage (capacitor).
That is usually accomplished by placing vias from the cap to the power
planes, and then vias from the power planes to the asic. Traces
make a lot of inductance. Manufacturing soldering rules generally
do not allow you to put a via directly under a surface mount pad
for an asic or a capacitor (at least with IR reflow).

regards,
Larry Smith
Sun Microsystems