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 ---------------------------------------------------------------------------------