Ken, Personal experience with a specific circuit for a specific application, migrating from a 2-layer board to a 4-layer board resulted in something like a -10 to -15dB reduction in emissions. That was probably the most dramatic reduction in emissions with regard to migrating the same circuit from one layer count to another.
The following is an extremely rough estimate ... Board cost is almost directly proportional to layer count. So, as a *very rough* rule of thumb, use the ratio (new layer count)/(old layer count) So roughly speaking, migrating to a 4-layer board from a 2-layer board should cost 4/2 of a 2-layer board or 200% increase. Migrating from a 4-layer board to a 6-layer board by adding 2 layers would result in the 6-layer board being 6/4 or a 50% increase. IOW, more layers has a mitigating effect with additional layers. If you remove layers, the ratio works the other way. Removing 2 layers from an 8-layer board will result in you paying 6/8 the cost of the 8-layer board or 75% the cost of the 8-layer or a 25% reduction. The trick with this ratio is dealing with 2 layers at a time. In order to keep the board constrution symmetrical, you can only deal with even numbered layers, as you probably already well know. So, increases or decreases can only be estimated multiples of 2. One thing follows from this. Suppose you were given the magic wand to set any layer count you wanted in the beginning. BUT, afterward you're only allowed a 20% increase in cost for modifications, You must start with 10 layers anticipating an increase to 12-layers resulting in roughly a (12/10) = 20% increase in cost. I think this is something that's missed in most prototyping considerations. As far as management is concerned, IMHO, the decision should be to maximize layer count as soon as possible. This has these effects in my opinion: 1) it's far better to overkill for EMI on first spin prototypes, 2) removing layers has more of an appeal budgetwise than adding, 3) overkill leaves available real estate for any future mods which are usually bound to happen. The whole, point in my opinion, is to set up a simple board construction (layer and stackup) that will survive as long as possible even through all sorts of modifications. Therefore, board costing can be anticipated and stabilized as best as possible for a long time into the life of the project. Undercounting layers which might result in adding layers later in the process can have a much more disastorous effect especially with large quantities of boards are in play; time-to-market cost, ECR/ECO cycle cost, design cycle cost, MRB/scrapping material cost, added burden on budget, ... etc ... I certainly understand all too well we in business don't live in a perfect world. But, given the chance to incorporate some of the ideas that I consider important, it's far better to overkill for EMI/EMC right off the bat than having to scramble around a week or so before final product release. The above all strictly my opinion ... Regards, Doug McKean ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
