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
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