Re: [SI-LIST] : high-power board

D. C. Sessions (dc.sessions@vlsi.com)
Mon, 05 Apr 1999 09:22:04 -0700

Pat Zabinski wrote:
>
> We're looking into a board that has much higher power requirements
> than we're familiar with, and I'm looking for some advice. Here's
> a summary of the board:
>
> * roughly 1000 Watts (1 KW) off a single +5 V DC supply
> * full-swing CMOS
> * ~50 ASICs
> * 200 MHz clock and data paths
> * up to 300 simultaneous switching outputs per chip
> * between 12x12 and 24x24 inch multi-layer PCB
> * ASIC packages have yet to be determined/designed
>
> To some extent, we have a good handle on the standard SI issues, but
> we're looking for input in two areas: decoupling and power.
>
> With 300 full-swing switching outputs per chip, what's the best
> (or a good) way to decouple the supply? First, at the chip-level.
> Second, at the board-level.

Chip-level decoupling won't help with your SSO, although it will
keep the onchip (core) supply in better shape. I most
profoundly hope you're using reasonable packages such as the
copper-slug cavity-down ball grid parts (what we call HBGA).
You REALLY need the low theta(jc) and low pin inductance.

My first suggestion is to ditch the 5v supply. You're paying a
premium for obsolete parts and then having to pour in more than
twice the power compared to 3.3v parts -- and in your case that
power density is a show-stopper. Of course, you probably can't
change it by now. Bummer.

> At this high of power level, what are the primary concerns?

Desoldering the ICs. Delaminating the PWB.

> How
> do we best address them? Is there a limit on how much current
> a half-ounce copper sheet in FR4 can tolerate?

Yup, and you're probably way past it. Pay particular attention
to the power path through your via field around the ICs; we've
seen some parts suck enough juice to blister the FR4 on much
lower-power applications than yours. If at all possible go
to multiple power planes with 2 oz copper. Not only will you
get better power distribution but it'll spread the heat somewhat
as well.

> How much current
> can I push through a standard via before it melts?

More than you can afford in terms of IR and Ldi/dt drop.
The vias aren't the worst of the bottleneck; it's the gaps
between them that are likely to kill ya.

> How can
> we effectively remove the heat?

Water jackets. Freon immersion.

Look, I'm not joking. 1000w is the kind of power that you use
for heating a small room. With a high-velocity fan behind it
my little ceramic heater still glows cherry red with 750 watts;
you are going to need a large external chiller to keep under
150C junction temps with anything aircooled.

-- 
D. C. Sessions
dc.sessions@vlsi.com

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