I sent this some time ago in reply to George Franck's question about 
"Via Hole and Pad Size" (see far below), and I'm pretty sure it never 
made it due to an E-mail problem I had. So here it is again, aged 
somewhat but hopefully still useful:

1A) Since we usually specify only finished hole sizes, we would define 
a .0135 drilled hole as .010 +/-.003 or we may make the size tolerance 
+.003/-.010 to indicate that it is a via and we don't care about the 
minimum size (as long as the vendor meets our plating thickness 
requirements).

1B) As far as I know, the smallest inner pad we have used with this 
hole size is .024 dia. with a .005 line to pad space and a .035 G/V 
plane clearance diameter. I consider this to be very tight spacing and 
we would only use it in very dense boards if required to get the board 
routed.
For a more reasonable, medium density design I would use a .027 dia.. 
inner pad with .006 line to pad and .039 G/V clearance.
For a fairly low density board I would use a .030 dia.. inner pad with 
.007 line to pad and .044 G/V clearance.
We typically make outer pad diameters .002 larger than inners 
(assuming SMOBC). On the bottom outer layer we would use .035 diameter 
to facilitate testing.  All the above inner layer sizes assume 
impedance control is required.

2)For .062 thick boards I would try to go no smaller than .020 
finished hole size unless required for density.
We have used .014 fairly often and .010 if necessary.

Nick Wander
Printed Circuit Engineering
Unisys, Roseville, MN.

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From: 	George Franck Jr[SMTP:[log in to unmask]]
Sent: 	Wednesday, February 05, 1997 2:51 PM
To: 	[log in to unmask]
Subject: 	Des: Via Hole and Pad size.

Good Morning:

I would like to take a quick survey of designers/fab shops about thru 
hole vias for comparison with our internal practices.

1) For a via DRILLED with a .0135" drill in a .062 MLB:
   A) How is the hole defined?
   B) What Pad sizes would you typically use/expect for Inners and
Outers?

2) What size via holes do you typically use/expect for .062 MLB?

Any comments like: I would be OK for my Gateway 2000, but not in the
Boeing 737 I fly on?

Based on my understand, I would answer the survey as follows:

1A) Define hole as .012 - .007. (if you care about a minimum size)
1B) From IPC-D-275 Class 3 Level B,  (Para 5.3.2.1)
    30 inner and 32 outer. ( a+2b+c = 12+2*1+16 = 30.. )

I think designs typically use smaller pads, hence the survey.

Thanks for your input!

--

George Franck Jr
Raytheon E-Systems
Falls Church Va.