Sounds like a bad idea to me; but perhaps the board house can handle that.  It's pre-preg on the outer layers for this layer, right -- layer 7 to layer 8?  If so, I'd think you could just do a single blind via, going from layer 1 down to layer 7.  That would leave bottom layer totally untouched, and the copper pour could run over it.

Alternatively, do the blind via, and then put a hole over it (may cause DRC's); along with the obligatory note as to why you have two holes on top of each other.

Shawn Upton, KB1CKT
Test Engineer
Allegro MicroSystems, Inc
[log in to unmask]
603.626.2429/fax: 603.641.5336


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Thursday, September 01, 2011 4:36 PM
To: [log in to unmask]
Subject: [TN] Need for "Alost Though Holes"

Hi,

 Can anyone suggest a technique that provides a 7 layer though hole yet the
net is insulated from PCB bottom of an 8 layer board?

 I have an 8 layer board where the customer wants a large copper coated GND
section on the bottom (layer 8). There are through holes everywhere as this
is a fairly dense board. The customer does not want vias or barrels exposed
on this bottom side in the GND section YET there are many other routes and
vias in other section of the bottom (layer 8).

 When I suggested an insulating layer of plastic film the customer would
rather pay more money for the PCB and not bother with the application of the
insulator.

Any suggestion?

Thanks,
Bob K.


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