My first transmittal was done in error before the message was complete. Sorry
about that. Here is the whole thing.
Mike
*******
My comments on the past/subject exchanges
From Ron Miller
Does anyone have published articles or books that document this use prior to
1989?
[MLC] Basic capacitance with thin films was of cource used many years prior
to 1989, but (as one individual pointed out) it's the manufacturing processes
that are patented. The processes achieved a reliable way to obtain higher
voltage breakdown using PCB processes.
[MLC] The "4 mil" thickness originally quoted actually refers to an original
military specification minimum requirement and also to the Bell Core
specifications prior to 1989. The Zycon (then Hadco, now Sanmina) process was
subjected to a rigorous qualification process and approved (as reflected in
the Bell Core specifications) as the only source qualified for use of less
than 4 mil thick PCB dielectrics. The thickness used by Zycon was indeed 2
mils as someone else observed.
********
From Chris Cheng
I've used Zycon before thinking they can lower EMI or power noise but the
reality is they don't and the reasons are stated above.
[MLC] Sorry, Chris, but you are WAY off on this one. Check out the literature
from 1989-1991 and the electronic "Product of the Year" award given to Zycon
for the ZBC 2000 product (the original name) for EMI reduction. I know you
are practicing some good design to achieve Class B certification, but good
power/ground plane decoupling plays a major part in that success. Many papers
demonstrated attenuatin of 20-30 dB over all frequencies above 40 MHz when
using BC and DELETING over 100 0.1 Uf decoupling capacitors. (Check with Dr.
Jim Howard at Sanmina, Santa Clara, CA if you doubt this.) Lee Ritchey
commented correctly on the contribution of the planar decoupling.
*********
From Chris Padilla
I use plenty of sub nf caps to help curb emission problems in a lot of my
systems and they do work just fine and are necessary.
[MLC] Where specific frequencies need to be suppressed, selecting
sub-nanofarad capacitors WITH THE APPROPRIATE SELF-RESONANCE is very
beneficial to the control of EMI. I have successfully employed this technique
in three classified military designs to achieve >20 dB velow MIL-STD-461C
levels (which are already WAY below FCC Class B).
**********
From Chris Padilla
EMI is a system level phenomenon and the size of your PCB most certainly can
resonate at frequencies well above 100 MHz.
[MLC] Absolutely, and well said. This is the primary phenomena that appears
to be overlooked by Chris Cheng.
*********
From Chris Cheng
I suggest you do the following experiment (I've done it so many times to toy
with EMC engineers). Take a system board with your favorite sub nf caps, do
an EMI scan, replace every single sub nf caps with .1uf or .01uf on the same
PCB and redo your scan. Let's see if they make any difference.
{MLC] You may "toy" with inadequately educated EMC engineers, but any
seasoned EMC engineer knows (as do SI engineers) that it's the chip capacitor
inductance that is a primary limiting factor in decoupling (and therefore
suppression). Therefore, as has been correctly recommended by many of the SI
List contributors (and apparently correctly practiced by Chris), we should
use the largest capacitance value we can for a given package style/size to
achieve the best decoupling.
*********
From Lee Ritchey
It's the plane capacitor that does all the work. Lots of good reseach that
backs this up.
[MLC] Right on.
*********
From Chris Cheng
Clarification, its the plane capacitance between the power/gnd pair that
sandwich the signal layer that provides the return path for the signal image
current. Nothing to do with Zycon planes. BTDT.
[MLC] Chris makes a key point in identifying the return path for any
high-speed currents. However, the reference to the Zycon planes seems to be a
slam at the benefits of that technology. If one studies RF techniques in
depth, then the fact that for a given resonance frequency the Q of that
resonance
is decreased with increased capacitance. This is the unheralded forte of the
buried capacitance concept. I have performed spectrum analyzer tests (with a
tracking generator) on circular and square PCBs (11" diameter/side) employing
BC that clearly demonstrated (relative to identical PCBs without BC) NO
RESONANCE effects at high frequencies (>40 MHz). The effect of this
characteristic is LOWER EMI that is (many times) caused by PCB dimensional
resonances. This benefit is particularly useful for High-Tg FR-4 boards and
higher frequency designs (>500 MHz) as the FR-4 losses play a significant
role at the higher harmonics.
*********
From the above comnments, I wish only to convey that we are all still
learning, and an open mind is critical to solving the design challenges of
the future.
Mike
Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
*** Serving Your Needs with Technical Excellence ***
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