John,
As I replied in some previous mails, I ended up creating a separate panel
layout drawing.
I found a panel of bare boards that was built for us by one of our preferred
fabricators. The low stress mousebite breakaway tab pattern they used was
closely related to the IPC-7351 (whose figure is the same as it was on 782,
cut and pasted into the new spec with one or two word changes).
I would imagine that good shops know the business of breakaway tabs pretty
well, so if there are items on my drawing whose design intent might conflict
with their standard practices once they CAM and DRC the panel, I'll be
getting a call. That's fine with me, so I can learn!
Thanks for your input!
Gary
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of John
Parsons
Sent: Wednesday, August 23, 2006 11:50 AM
To: [log in to unmask]
Subject: Re: [DC] Breakaway tab specification
Gary,
I don't have a copy of the current standard IPC-7351 which supersedes SM-782
so I don't know how tabs are detailed in there. My copy of SM-782 is pretty
old but it does show some examples of breakaway tab details. As you
suggest, I think that most shops have developed their own "standard" tab
designs which they use unless otherwise specified by the fabrication
documents. We certainly do. If you are determined to use the suggested
design you have found in the IPC doc's I would think there is nothing wrong
with that but would suggest that you go to the trouble of duplicating the
detail on your fab drawing rather than specifying the IPC document alone.
First off, the document you have referenced below (IPC-SM-782A) is obsolete,
having been superseded in 2005 by IPC-7351 and second, many fab shops may
not have a copy of that document so referencing it without providing the
detail on the fab drawing will only draw out the quote/fab process
unnecessarily.
John Parsons
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Gary M.
Koven
Sent: Wednesday, August 23, 2006 8:22 AM
To: [log in to unmask]
Subject: [DC] Breakaway tab specification
Gentlemen,
I'm looking for confirmation and/or correction on breakaway tab
specification.
On my drill drawings, this is my first note for a blanket default:
"FABRICATE PER IPC-6012 CLASS 2 UNLESS OTHERWISE SPECIFIED"
However, 6012 is uselessly vague about breakaway tabs...referring the user
to the acceptability requirements of the assembly.
In the Depanelization/Singulation section, I don't call out a specific IPC
standard for breakaway tab acceptability. The only statement I make is:
"BREAKAWAY TABS MUST SHEAR BELOW EDGE OF BOARD OUTLINE"
In the past, that has always been adequate.
However, I now have a board for which I've been tasked to pre-determine a
certain number of how many up.
On page 35 of IPC-SM-782A, Figure 3-31 "Breakaway (routed pattern)" shows
some better detail on their so-called Low-Stress (Mouse Bite) Breakaway.
The figure shows approximate center to center spacing of 75mm (3.0in)
between tabs and maximum indent of 0.4mm (0.016in).
Is it kosher to call out something like this:
"BREAKAWAY TABS TO BE LOW-STRESS MOUSE BITE PATTERN PER IPC-SM-782"
Or, is it not kosher because there exists a different standard in the
Acceptability series (like IPC-A-600, which I don't have here) which might
supersede a Design-series standard such as 782?
I've always in the past left arcana such as this to the assembly shop who
typically work with their chosen fabricator to pick a panel size, how many
up, tooling hole location, yada yada. Now that I must own this step on this
one board, I will have to issue some sort of panelization drawing, and I
want to be careful not to put too much information on drawing which might
add to the cost of the boards.
TIA and Best Regards,
===========================
Gary M. Koven, C.I.D.
Dynazign, Inc.
Charlotte, NC, USA
Veteran of the Marsh School
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