[SI-LIST] PDS Capacitor Mounting Details for Lowest Inductance?

Si Gurus,

This subject has been discussed at length on this list and I agree on the
need to minimize the mounted capacitor inductance but have a few questions
regarding the actual application. Befor I get started please realize that I
am a circuit designer with limited knowledge about actual PCB design. So far
most of our designs fit on standard 4 layer PCBs (signals on the outside and
power and ground in the middle).

1. Is it possible to do via-in-pad (prefered) or via-next-to-pad without
incurring extra cost or manufacturability penalties? At one time I believe
that one of our vendors didn't like this approach for a reason that I don't
remember. What is the current thinking on this approach among PCB
manufacturers? 

2. What about thermal reliefs for the connections to the internal power and
ground layers? These originated for solderability reasons on through-hole
boards. Are they still required on SMT mounted bypass caps? I know that
these are not wanted due to the increase in mounting inductance that they
cause. Is it possible to get rid of these without incurring
manufacturability problems? If not is there a recommended geometry?

3. Does anyone have a prefered stack up for 4 layer .062" boards? I would
like to have as much plane capacitance as possible. For high plane
capacitance you need close pwr-gnd plane spacing but this then leads to
large spacing over planes for the signal layers. Added to this is the fact
that needed dielectric layers are only available in certain standard sizes.
Does anyone have any recommendation for a compromise stack-up that would
work good for these conditions?

If would also appreciate any links that you could give me where I could
learn more about the practical aspects of implementing this PDS strategy in
a PCB. It would also make it easier to send known good information to our
PCB designer.


Thanks for your thoughts,

Steve

Steve Lund
Sr. Design Engineer
OPW Fuel Management Systems
114-300 Mackenan Dr.
Cary, NC 27511
(919)460-6000 x2133
(919)460-7595 FAX
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