Fascinating question Bruce. I had to turn this one around in my mind for
sometime before responding...
The IPC design specs were developed from DOD-STD-275 and we always
referenced them in our document packages when doing DOD work in the past...
I know the military cancelled the Mil Stds but that doesn't mean we don't
still use them... LOL They just evolved into the IPC standards.
The big reason we used the standards then and now is the cost savings and
reliability enjoyed by all who use them. In using the common format and
language we set the goal to standardize things to make our products easily
manufacturable, easier to assemble, and reliable. Multiple vendors can tool
up for the same criteria and not have to be custom tooled for every job that
comes through... Parts makers standardize for similar reasons, in fact I
think the SMT folks are still trying to rein in Japan's penchant for making
smaller packages that are not standard. I think they would like them to
settle in on a few sizes to reduce the amount of different tooling needed.
The electrical tables in the IPC specs came from the Mil-specs. They work...
and have some margin built into them. This tolerance margin covers things
like contamination on the surface of the board, etching tolerances, etc...
The industry establish minimum spacing requirements to ensure that failures
are less likely. Gary made a good point about the assembly processes and
procedures being essential knowledge when designing a board and product. We
'preach' this to designers to drive home the importance of knowing ALL the
requirements for the board up front, end product use and environment,
manufacturing, assembly, test, tooling, fixtures, panel details, you name
it.
It all affects the design and therefore the designer must be aware of these
things in order to accommodate them in the design assembly and in their
board, and review them all in the very beginning of the process of design
with all of the key members of the product team.
J-STD-001 in the very beginning of the General Requirements in table 3-1
ties the two specs together. The board is an integral part of the whole
assembly and as such must meet the spacing requirements that are governing
the overall assembly as well. The minimum spacings called out in IPC-2221
Table 6-1 page 39 only apply to the bare board... and the assembly values
are mentioned there on the right as well. Notice that the Assembly values
are larger than the bare board values. I would think the spacing between
component leads and their relationship to the different voltages on the
board and in the wires and leads and chassis of the assembly are superceded
or governed by the agency that you are applying for approval from, such as
TUV, CE or UL, FCC or NASA or any other regulatory commission you must
comply with in order to sell your product in the country your market is in.
The IPC values are the minimums but other agencies will require more voltage
related spacing than the numbers you see in that table.
I don't know if I have hit the specific question you had, but I hope I have
clarified the relationship between the 2 specs and how they would be used to
help you qualify the product and its design. I think Gary pointed out the
need for the relationship as well.
Best regards,
Bill Brooks - KG6VVP
PCB Design Engineer , C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
e-mail:[log in to unmask]
http://www.dtwc.com
http://pcbwizards.com
-----Original Message-----
From: Bruce D Stilmack [mailto:[log in to unmask]]
Sent: Tuesday, January 04, 2005 3:19 AM
To: [log in to unmask]
Subject: [TN] Minimum Electrical clearance
I know this has been discussed at length in the past, but I can not find an
answer to this question in the archives. I am hoping one of you ( or maybe
even Jack) can answer this. J-STD-001 talks at length about not violating
minimum electrical clearance. I know that IPC-2221A has requirements for
both the PWB and the assembly with regards to minimum electrical clearance.
However, how does one make the connection between the two? There is no
direct reference that I can find that pushes you to 2221 to look for this
information. Also, and this is for Jack or those that were involved with
creating these specs, why are the assembly requirements located in the PWB
design spec?
Bruce Stilmack
GDLS-TO Manufacturing Engineer
(850) 574-4773
[log in to unmask]
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