RE: [SI-LIST] : why .062?

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From: Cusanelli, Tony (Tony_Cusanelli@mentorg.com)
Date: Thu Mar 09 2000 - 03:08:27 PST


A few people asked for the "old guys" to comment on why boards are .062.
Back in the dark ages (30 yrs ago) the boards I first designed were merely
mechanical devices that supported parts. Connectivity was accomplished via
wire-wrap. Parts were heavy and boards had to be thick enough to prevent
breakage, but thin enough to carry. 1/16 inch became the right compromise
and card guides, standard socket boards and chassis were all designed to fit
that thickness.
 
Even with today's lighter parts, tracks are thin and need some support.
.032 boards are okay if they are small. Flexing and the pressure from
connector insertion/extraction will fracture thin traces.
 
PS - for you young guys - wire wrap was accomplished with a device similar
to a hollow drill bit that would wrap 6-8 turns of 30 awg wire around a
square or rectangular post. The edges would bite into the wire and make a
gas-tight connection that was tested with a pull device (strain gauge). We
had some PC boards, but they were mostly 2 and four layer. Draftsmen just
could not lay out a board as fast as I could write a wirelist for the wiring
machine.
 
Yes now I feel like a dinosaur.
 
Tony
 
 
 -----Original Message-----
From: Alex Li [mailto:alexl@ati.com]
Sent: Monday, March 06, 2000 9:36 PM
To: 'si-list@silab.eng.sun.com'
Subject: [SI-LIST] : different 4-layer board Stack up (S-P-G-S) ?

Recently I saw a 4-layer mother board with 100 Mhz 128-bit memory bus.
This board has unusual signal-power-ground-signal stack up. I talked to
one of their engineer for this kind of arrangement. They said since most PC
motherboard has several power plane split and on the top level there are a
lot of components with pads. they think if they route all the 128-bit memory
bus on the back and put it close to ground plane, they have much routing
area and this will help to keep the signals clean.
 
    This is kind of new idea to me, does anyone see any drawback by this
arrangement ? Will this decrease the decoupling caps performance ?
 
                                                Alex
 
    

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