David, My legs are kind of short, so let me open wide and see if I can't get one inserted up to the kneecap. We've faced this set of tradeoffs several times, and IMHO the short answer is that layers are much less expensive than moving to BoBoB (Blind or Buried or Both). Although you may be able to save a layer pair, and *possibly* a gnd/pwr reference structure to go with it, the number of fabrication steps and checks multiply so quickly that gains in materials related costs (layers) are quickly eclipsed. The real use of this technology, again in my humble opinion, is in order to make possible a product that can't be made otherwise, within given constraints. The constraints we've had force us to complex structures include X/Y dimensions, the Z dimension, and that nefarious 4th dimension, TIME. Interestingly, the design time saved by using the technology avail- able can easily be consumed by the handoff, engineering, and planning of the much more complicated process. If you go with the complicated approach, make sure you know when that n-week turn *starts*, and that you know exactly what data is needed and how to make it. Generally, costs for BoBoB are measured not in dollars but in gold bars. Having said that, let me explain how I've come to progress through the technology maze on a variety of complex products. For the 1st 3 dimensions, without looking at the exact logic, I've usually found it most comfortable to pack one side to the maximum reasonable density (yeah, right, define that!), and attempt to place the secondary side such that you overlap to the highest extent. That is, rather than using 60%/60%, use 90%/30%. When you get to 90%/70%, you're in the likely range of not being able to pin-escape, and this is the time to look for the "next idea, please". Speaking very empirically, if you can get through pin-escaping then you can probably succeed in routing with 3 non-attach layer pairs. Notice I didn't say how long this will take, and also that I used the word *probably*. As always, much depends on the specific circuit/ product/components etc. Of course, there are plenty of PWB's that won't require this many layers, but this discussion is geared towards the ones that do. If you lack enough Z-axis space for the number of layers that you believe you need, a buried via layer-pair goes some distance toward making things possible, as you can now use the space under the SMD lands. This effectively places one or more partially finished two-layer board in the middle of your finished board. When you can't achieve complete pin-escapes, the next step cost-wise would be blind vias one-layer-in from either or both sides. This minimizes the increase in fabrication steps, the effect is that of including semi-finished two-layer boards on the outside of your overall board. Sometimes this approach can save some layers, and may be the angle that best reflects your original question (remember your original question? I don't!). If you were really far off on the pin-escape step, bringing them down one layer can lead to the feeling that you've simply moved the problem in one layer, which you have (it's a type of sinking in the pit of the stomach). If you moved the problem without solving it, then you need the more expensive deeper blind vias. This is effectively placing semi-finished multilayer boards on the outside of your finished board. Once you've signed on for this, you've pretty well blown the wad and a buried via layer-pair in the middle will keep those blind vias from being lonely. This is how wedding receptions get big. We use this technology when the density hits 105%/105%, and try like crazy not to need two depths of blind vias combined with multiple buried via combinations. Now I've definitely run on far too long, and must go before I dis- appear into my own mouth with a loud metallic clank. But before I sign off I should warn you that blind and buried have to be mixed carefully, they can't overlap (with apologies to Ralph Hersey!) : I I I I U I U U U U U U U U I U I I I I blInd/bUried not possible possible Good luck, Jeff Seeger Applied CAD Knowledge Inc Yankee Technical Officer Tyngsboro, MA 01879 [log in to unmask] 508 649 9800 *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * ***************************************************************************