On 12 Mar 01 at 12:51, Roger M. Stoops wrote:
> (Matthew Lamkin wrote)
> [...]
> I suppose that I could do that and then use the other 2 available
> layers for routing the signals. [Yup.] Why would the powerplane be a
> middle layer? if it has groundplaning/emc reduction effects wouldn't
> it be better nearer the outside? [Ground is typically more effective
> when used on the outer layers as an EMI shield. Has to do with
> common-mode rejection of noise and other such technical stuff. Also,
> power/ground planes are easier to use and create on inner layers,
> also make for smaller Gerber files.
... if done with negative planes, yes. But often it is easier to make
them with positive, poured copper, especially split planes - depends
of the CAD system..
> When power/ground planes are
> placed near one another (0.1-0.2mm), the design benefits from the
> resulting distributed capacitance, but this would not be a benefit
> for EMI reduction
Yes, it would! Better power/Ground decoupling = less switch noise on
pwr/gnd sy stem (think about ground/power bouncing with fast CMOS
circuits:-( ) = less radiated emission from cable shields etc.
> (8 layer designs work really well to get both
> benefits, but your management probably isn't ready for that yet...).
*sigh* many customers are not ready for that - they MUST fall on
their noses with 6 layers in EMI compliance test before they
understand this....
The EMI benefits from planes more towards the outer layers are
often not so big as you may assume - at least seen over the whole
system.
The shielding effect for signal layers between the planes has two
sides: You can keep a noisy signal or component away from other
"receiving antenna traces" on other layers with a plane between. BUT,
concerning EMI with anything in the outer world (CE legislation...)
it may be as bad as before or worse - The RF energy is
not radiated in all directions, it is just guided to the board edges
and then it finds the edges as sometimes perfect slot antennas (and
you have a mostly quiet system with an interesting antenna gain
diagram - high gain in two directions, CE limits exceeded about 10
or more dB)
The same system with another layer stackup might have a perfect
omnidirectional aerial diagram with the same RF energy distributed
equally, no terrific "gain" in specific directions and the limits not
exceeded.
You can have real good EMI compliance only in two ways.
-Design the circuit to avoid the generation of unnecessary RF energy
(signal slopes only as fast as necessary, avoid known noisy ICs
if possible, good power/gnd decoupling)-
- RF energy which cannot be avoided must be absorbed = converted to
heat in anything with ohmic behaviour, like speciall ferrites,
filters etc.
Pure metal shieldings etc. can only reflect the RF energy, it
will disturb anywhere else, but not disappear.
If all that stuff sounds very complicated - with 4 layers, you would
not have many chances to try different stackups - and much homework
must be done from the circuit designer, and if this is not done well,
it cannot be our job (and won't be successfull) to try to rescue an
ugly circuitry with a perfect layout.
Just my 2 Euro-cents
Regards
Matthias
-----------------------------------------------
Matthias Mansfeld Elektronik
* Printed Circuit Board Design and Assembly
Am Langhoelzl 11, D-85540 Haar, GERMANY
Phone: +49-89-4620 0937, Fax: +49-89-4620 0938
E-Mail: [log in to unmask]
Internet: http://www.mansfeld-elektronik.de
---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
To set a vacation stop for delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
