Kitty,
While it is more comfortable for a fabricator to have these parameters you
mention pre-specified... really they are determined and design driven
requirements that depend on the intent and requirements for operation of the
circuit under the environmental conditions it will be exposed to and tested
against.
People tend to get caught up in specifications and rules... don't get me
wrong, I like rules and standards and specifications... but they do not
restrict what a designer can do. The design goals do. You have to determine
that...based upon the specific design and its needs.
If all things were designed based on past specifications and standards...
there would be no innovation or "anything new under the sun... " (to borrow
a quote from a somewhat famous passage...)
The power and responsibility the PCB designer has that the manufacturer does
not is in the designer's ability to balance the electrical, thermal,
mechanical, circuit board design requirements against the standards and find
a solution that can be manufactured and tested and assembled economically
while surviving the intended conditions the circuit must operate in for as
long as it is intended to survive. (Wow, I need a deep breath after that
one...) :) Believe me, there are many boards that are successful that do not
meet the specs. New ideas are being tried all the time that there is no spec
for.
While that really gives you nothing substantive to tell them, it should
indicate that you are not restricted in your designs unless it is self or
company imposed or contractually imposed on your project.
The limits are really... whatever can be fabricated and still work under the
conditions it must...
Some board houses cannot fabricate cutting edge technology, while a smaller
number of manufacturers can and do regularly and in fact set the 'cutting
edge'... Some assembly houses have trouble placing 0805 resistors while
others are happy to place 0201's or smaller... it just depends on what your
operating budget is and what vendors capabilities you want or need to work
with and how much your product can cost once it is built... etc... etc...
I really don't think IPC has put a restriction similar to what you appear to
suggest on board designs. The closest thing to a restriction would be a
voluntary compliance with IPC-7351 the land pattern standard for example. It
does set some spacing and land size standards that we can follow if we want
to avail ourselves of the common economic benefits that come from adopting
them.
However your manufacturing department may need to set guidelines for you to
agree with in order to guarantee that THEY can build your designs... or if
you exceed their abilities in your designs, you may have to have your
product built outside your facility at a more advanced assembly house or ask
your company to invest in more capable assembly equipment so they can build
the tighter, more advanced technologies... its all in the balance
criteria... if you can balance everything you get the best results for the
least cost... but not everyone is designing to those goals... some don't
care what it costs... it MUST WORK and ANY COST... so the designer needs to
understand that and design accordingly.
Sorry for the long winded response... your message caught me during lunch
time and well.. I could not help myself. :)
Best regards,
Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Datron World Communications, Inc.
3030 Enterprise Court, Vista, CA 92083
http://www.dtwc.com
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
e-mail:[log in to unmask]
_______________________________________
IEEE Member
Member IPC Designers Council
Instructor at Palomar College
http://dcchapters.ipc.org/SanDiego/
http://www.palomar.edu
http://pcbwizards.com
-----Original Message-----
From: Kitty Hines [mailto:[log in to unmask]]
Sent: Tuesday, August 28, 2007 11:53 AM
To: [log in to unmask]
Subject: [DC] Spacing Requirements
Hi,
I'm trying to revamp some of the spacing requirements that our
Manufacturing Dept. has decided upon and need some documented proof that
the spacing they "require" is unnecessary. I tried to find specific
requirements in IPC-2221, but wasn't able to find them. What I'd like to
find is spacing requirements for pad-to-pad, pad-to-trace, smd land-to smd
land. I'd like to find requirements for solder wave and reflow processes.
Does anyone know what IPC spec I should be looking at? Or a reliable
reference I can find? These requirements would only take Manufacturing
rules into consideration, disregarding any electrical rules.
Thanks for any help you can be,
Kitty
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