Instead of picking one of you choices, here is one scenario that seems to
work well:

1. Since you have 1mm to work with, divide this into .1mm increments
2. Use .5mm pads for your vias, which leaves .5mm space between
3. use .1mm traces and spaces, and two will fit nicely between pads
4. let the board house decide what drill size to use, and don't restrict
   the smallest hole size (since you probably don't care what the finished
   size really is, right?)

Since we would be doing this on a .093 thick board, we would also allow
for teardropping the pads, and specifying that break-out is ok (the
teardrops
will prevent opens in breakouts) so the fabricator can use a larger drill
size, but on your thinner board this may not be necessary.

good luck,
Jack


-----Original Message-----
From: Stuart Adams [mailto:[log in to unmask]]
Sent: Tuesday, January 09, 2001 11:00 AM
To: [log in to unmask]
Subject: [DC] two traces between 39 mil spaced via's ???


I have a dense design with many 1mm pitch
BGAs on a small board. The fanouts from
each BGA form a grid of via's spaced 1mm (39.3 mils) apart.

In order to be able to route the board I need to be
able to run two signal traces between the vias.

        39 mils
   <------------>
        |  |
 (VIA)  |  |  (VIA)
        |  |
     (two traces)

Which is the most manufacturable way to do this ???

Option #1
---------
  5 mil traces and spaces
 10 mil drilled via hole in 14 mil pad (6-8 mil finished hole size ?)
  7 mil from drilled hole edge to unrelated copper

Option #2
---------
  4 mil traces and spaces
 11 mil drilled via hole in 19 mil pad (7-9 mil finished hole size ?)
  8 mil from drilled hole edge to unrelated copper

Option #3
---------
  4 mil traces and spaces
 12 mil drilled via hole in 19 mil pad (8-10 mil finished hole size ?)
  7 mil from drilled hole edge to unrelated copper

Is there a better option than one of these ??

(This is a standard .062" 12-14 layer board)

Thanks,
  Stuart

---------------------------------

Stuart Adams
Bright Star Engineering Inc.
19 Enfield Drive
Andover MA 01810 USA
Tel: +1-978-470-8738
Fax: +1-978-470-8878
Email: [log in to unmask]
Web: http://www.brightstareng.com/

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