Re: [SI-LIST] : Different oz copper for grounds

Chris Bobek (cbobek@cadence.com)
Tue, 21 Sep 1999 09:59:07 -0700

Hi all,

Thanks for your responses. First, I need to address the Cadence issue. Yes, I work for Cadence, but just because I work for a tool vendor doesn't mean I know how to use all of their tools (I wish!). I try to learn as much as I
can about our tools, but that takes time and we are very busy trying to design boards. Besides, tools are just half the answer. I would much rather learn some theory from experts like all of you than how to use a particular tool
to throw some numbers at me that I don't know how to interpret. You'll notice people like Howard Johnson don't spend a lot of time talking about tools because he wants people to understand *why* things happen, not how to punch
buttons and get some results. My goal is to understand *both* sides of things, the theory, and the tool to help me solve my problem.

Whew, sorry for the bantering, but I just had to say it!

Here's a more detailed description of my problem:

We are building a conduction cooled VME card. Our customer is providing ALL of the thermal data and constraints to us (ie., they did the thermal analysis). They recommended an electrically isolated thermal plane as layer 2.
This plane will conduct heat from underneath hot components through thermal vias that will reside under each hot part. The plane will be exposed at the two card edges where it will bond to two wedgelocks. The wedgelocks will
then couple heat to the cold plate (and to the card cage) that holds the card. For whatever reason (customer says so), there can be no electrical connection between the card cage and the VME card.

Because they did the thermal analysis, there is no question about the "best" way to sink heat out of the parts on our board. They already figured that out for our application. I don't think they are going to go the 2oz copper
route, so the last concern seems to be the SI/EMC problem with a floating plane. I would appreciate it if someone could ellaborate on that for me. Do you think we could make the thermal plane a ground plane, but have some kind
of electrical insulator underneath the wedgelocks? I know that will reduce the effectiveness from a thermal perspective, but it might be worth it, no? Btw, here's what our preliminary lamspec looks like:

S1
Thermal
Gnd
S2
3V
S3
S4
Gnd
S5
5V
Gnd
S6

Thank you very much,

Chris

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