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Jeff:

First step for a question like this should be your board house. They
should be able to tell you right off the bat whether there will be
any impact on their yield (read: cost to you) with the lower annular
ring.

Another alternative would be to drop your trace widths and clearances.
9/9 is very generous - most board shops will do at least 8/8 with no
cost penalty, while 6/6 is also not unusual for high performance shops
without yields suffering (bleeding edge at 4/4 and below is another
story...).

As an example, one of my local (ordinary quality) board shops has a standard
set of design rules (i.e. high yield expectations) that include:
0.15mm (0.006") track width
0.15mm (0.006") tack spacing
0.25mm (0.010") annular ring

With this shop, I regularly do 0.175mm (0.007") annular rings (by mutual
agreement), and with a consequent yield impact (i.e. I'm happy to pay
the extra cost).

If you do drop your annular ring below the board shop's accepted standard,
then you should restrict the use of these pads/vias to only the locations
necessary. The more instances of "exceptions to rules" that exist on the
board, the higher statistical probability of a board failing in mfr.

Remember that the principal reason for min. annular ring specifications
is to guard against drill breakout. There are alternative design techniques
that can allow you to shrink the annular ring, by allowing controlled
breakouts (usually on interior layers) without sacrificing board
performance. For example, teardropping to a pad provides a guard against
breakout on the side of the connection. (If you use this or other
techniques you'll need to give your board shop explicit permission to
allow breakouts)

Hope this helps,

John Haddy


> -----Original Message-----
> From: Jeff Adolphs [mailto:[EMAIL PROTECTED]]
> Sent: Thursday, 4 October 2001 6:15 AM
> To: Protel EDA Forum (E-mail)
> Subject: [PEDA] Annular Ring question, via question
>
>
> Cheers Protel Group!
> 1. Since I am using a small via to make routing SMT easier. Small via
> having a 10 mil dia. annular ring (pad of 31_21). Am I free to reduce
> the annular ring on other parts without any harm. Specificly, reducing a
> Pad of 75 with a drill of 43, to a Pad of 61 with a drill of 43? A pad
> of 61_43 would make a connector much easier to route.
> Board info:  So far I'm trying to keep the PCB at space 9 mil minimum,
> trace of 9 mil minimum.
>
> 2. Will I save money by using a Via of 31_21 only when necessary and
> using a Via of 50_28 everywhere else.
>
> Thanks in advance!
> Jeff Adolphs
> Lake Shore Cryotronics, Inc
> Westerville, Ohio, USA
>

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