>Hi,
>My customer is curious as to what would cost more a 4 layer board or a six
>layer board. Here are the details:
>
>Board has 1 gnd layer an 1 vcc layer and is primarily surface mount and a
>digital circuit without any critical paths. These will remain constant. The
>board was autorouted for both a six layer version and a four layer version,
>so here is the difference of 2 signal layers or 4 signal layers . The 4
>layer was routed to 96% and 2500+ vias and the six layer routed to 100% with
>1450+ vias. We all can surmise that the six layer version will be more
>reliable in the long run.  I am told that the cost is drilling alone is
>about a $100 difference per board. Can any body help with the cost analysis?
>
>Thanks in advance,
>
>Scott Shepard
>Interactive Design Solutions
>also at [log in to unmask]
>

Comments on the drill costs: a delta of 1000 holes for $100 !  Wow!  Thats
ten cents per hole.  I don't think holes cost that much...However, hole
density is another issue.  I have seen a factor chart based on holes per
square inch.
# of holes per sq. inch       factor
up to 40                        +0%
40 - 50                         +20%
50 - 60                         +50%
60 - 70                         +65%
70 - 80                         +80%

100 holes per sq inch is having a hole every 100 mils.

As far as reliability goes, the simpler design is usually the more
reliable.  The simpler the design, the lower the cost.  So, if you are only
changing # of layers, routings, and trace/hole density, (and knowing
nothing about the design), I would quote the two designs from the same
shops and take the lower cost design.  If I haven't said 'usually' enough,
I want to add that this usually works pretty good.  (Don't forget to add in
the cost of assembling 4% of the connects in jumper wires.)

George Franck