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Mon, 01 Jul 96 07:28:03 EST
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        (Between the 4 vs the 6 lyr)
        Are the drilled hole sizes the same?
        Are the pad sizes, line widths and spacing the same?

        There may be some other variables that could be a major complexity
        factor on the 4 lyr.

        Groovy
______________________________ Reply Separator _________________________________
Subject: Re: 4-layer vs 6-layer
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    7/1/96 9:57 AM


>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
     
     



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