Denis,
>From Section 5.4.1 of IPC-2221:
Diameter true position (dtp) callout, also known as positional tolerance
zone, allows 57% more tolerance area than bilateral tolerance callout.
>From Table 9-3 of IPC-2221:
Level A = 0.25mm dtp = 0.010in
Level B = 0.2 mm dtp = 0.008in
Level C = 0.15mm dtp = 0.006in
Let's say you have a board with a 1mm pitch BGA and 0.025in (0.625mm) vias
between the balls. If the allowable dtp = 0.003in (0.007mm), you'd still
short out your board. Other callouts on drawing such as manufacturing
allowance, minimum spacing, and minimum annular ring end up trumping Table
9-3 any day of the week on a BGA board.
I have never understood why the industry is satisfied with these
producibility values in Table 9.3 when the reality of building BGA boards
requires much tighter dimensional controls on drill position/drill drift.
But the purchasing agents at the fab shops would salivate if they saw Level
C/Class 3 on drawing. Assuming your umbrella spec on drawing is IPC-6012
Level B/Class 2/industrial, if you put 0.003in (0.007mm) dtp on drawing,
you'd very likely pay a fortune for that board fab. So you have to put
0.2mm (0.008in) to keep it as a Level B/Class 2/industrial and out of the
realm of Level C/Class 3/mission critical and keep the price reasonable.
Other situations may invalidate the above umbrella ... or else I'm growing
ever more daft in my quest of the truth.
Best Regards,
===========================
Gary M. Koven, C.I.D.
Dynazign, Inc.
Veteran of the Marsh School
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Denis
Lefebvre
Sent: Tuesday, April 25, 2006 11:18 AM
To: [log in to unmask]
Subject: [DC] Diameter True Position
I must admit, this is a drawing callout that I have never fully
understood. Back in the early days, per MIL-STD-275 I'd callout Diameter
True position at 0.010", then at some point in time it was reduced to
0.008". Now I have someone telling me it should be 0.003". Huh?
Can someone help me understand what this (Diameter True Position) means
and how dropping the value to 0.003" might be a good thing or a bad
thing?
Denis Lefebvre, CID+
Sr. PCB Designer
Finisar Corp <http://www.finisar.com/index.php>
(408) 542-3832
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