From: Larry Smith ([email protected])
Date: Fri Mar 03 2000 - 09:55:16 PST
Chris - The effective inductance associated with a discrete decoupling
capacitor is an extremely important parameter in determining the
effectiveness of a the capacitor. We find that most of the
inductance associated with typical decoupling capacitors is associated
with the pad and via layout which conducts current to the power
planes. Reverse aspect capacitors can help but may not be address
the dominant inductance. After the via and pad layout has been
optimized, there is some value to using reverse aspect capacitors.
The same comment goes for AVX LICA parts that have many bumps for
attachment to power and ground. The most effective pattern is a
checkerboard where Vdd/Gnd alternates on every adjacent bump. But we
must follow up this capacitor terminal pattern with a similar pattern
of pcb vias to the pcb power planes. If that is done, the LICA
capacitor is very effective at reducing the effective inductance of the
capacitor. But we must be willing to chew up the wiring channels with
Vdd/Gnd vias on the pcb in order to make LICA's effective.
> From: [email protected]
> Date: Fri, 3 Mar 2000 11:58:55 -0500
> Subject: RE: [SI-LIST] : Adding inductors to ground?
> Along these lines, does anyone have any experience with the new low inductance
> caps giving better performance in the power system decoupling network? In
> particular, AVX advertises several types of low inductance caps. They have
> their reverse aspect ratio 0508 and 0612 type that are supposed to has 1/2 the
> inductance of their 0805 and 1206 counterparts. They have their IDC family of
> parts. These parts give a designer 8 connection points on the chip so ideally 4
> short traces instead of 1 could run to VCC and ground, thereby reducing the
> inductance contribution of the connection of the device to the power system and
> therefore the whole inductance of the system. On the super low inductance side,
> AVX offer their LICA parts. These are actually little BGAs that contain an
> array of capacitors and are supposed to reduce the inductance down into the 100
> pH range. Before committing to the expense of totally switching over to any of
> these devices, it would be nice to know if anyone has had experience to show a
> justifiable improvement in system performance in going from the traditional 0805
> or 1206 package style ceramic chip caps that we currently use, to one of these
> other low inductance varieties.
> Chris Hansen
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