Bill,
The Courtyard comment was in reference to some members of the IPC-7351 executive committee not wanting to publish any guidelines on silkscreen outlines because they are against the use of silkscreen outlines in PCB library parts. Our push back was to at least have some guidelines on silkscreen outlines and the response was to create some general rules:
- No silkscreen outline under the component body; these get covered up during assembly and don’t provide any useful purpose (waste of ink)
- Silkscreen outlines visible after assembly process and provide a functional use as alignment marking for assembly registration accuracy
- Silkscreen outlines should be inside placement courtyard. Because part placement allows courtyard to courtyard bumping, if silkscreen is allowed outside the courtyard boundary it could easily be confused with neighboring parts.
- Silkscreen outlines should be mapped to the Maximum Component Body with one exception, the Silkscreen to Pad spacing rule “overrides” the Component Body Mapping
- Silkscreen outlines should map the component body and not go around pads. Excess silkscreen outlines should be avoided to make room for ref des locations. Silkscreen outlines should perform a “hatch” outline along the component package body.
- Pin 1 is identified by extending the silkscreen along Pin 1 length of pads when component leads extend outward. Bottom only terminals Pin 1 is identified by a missing line.
However, the IPC Calculator has a preference switch to allow silkscreen outlines outside the courtyard if the user wants that feature. All drafting outlines have on/off features that the user controls as preferences. Also, the IPC Calculator does produce a Pin 1 dot that is sizeable by the user and has an on/off switch because some PCB designers like and others don't. The industry is changing very rapidly toward micro-miniature component packages and new concepts regarding silkscreen and polarity marking must be addressed. But sometimes, it's hard to teach old dogs new tricks (or rather some won't accept change, even if it's for the better). :-)
Your message indicates that you do not use the IPC Calculator and that you are making assumptions based on a presentation. Before you comment again, download and install the free IPC Calculator with 21 CAD tool interfaces and tell us what you don't like about it - www.pcblibraries.com
Tom
Tom Hausherr CID+, CIT
President
PCB Libraries, Inc.
13730 Sorbonne Court
San Diego, CA 92128
858.592.4828 Office
858.859.5371 Cell
www.pcblibraries.com
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Brooks, William
Sent: Tuesday, February 24, 2015 7:13 AM
To: [log in to unmask]
Subject: [DC] A note and comment for the IPC 7351 land pattern footprint revision committee at APEX.
Hi Anne Marie,
I enjoyed seeing you and the gang at the convention yesterday. :) Great to see all the awards handed out to some great people who give of their time and talents to help us be successful in this industry. Also awesome to see old friends again and the presentations in the Design Forum were terrific. (Oh, and BTW... Your amazing introductions to the speakers were epic...) :)
I thought since I can't be there that I would offer a couple of thoughts to Tom Hausherr and the education/executive committee overseeing the footprint/land pattern standard this morning.
Tom's presentation in the design forum yesterday about the potential changes to the land pattern tool hit upon some items that I would 'beg to differ' with. Just thought I would put my two cents in if you can get this to them this morning.
One thing I don't see the4 value in are the pin one designation schemes presented at the meeting. My vote is for the 'Polarity Dot'. The assembly houses already know it, expect it and it is already incorporated into our libraries. Just saying. Don't make more options that people won't recognize... especially just because they want to keep the polarity symbol within the courtyard area of the footprint land pattern. That's really not a justification for them.
Second there is a fundamental disconnect with the 'courtyard philosophy' which led to this idea that requires all elements of the land pattern to reside within it. That's really not its function or reason for being a part of the footprint. Here's why...
My understanding and personal use of the courtyard's addition to the total land pattern and its elements is the visual benefit we get as a designer who is placing components into a design to see quickly if the part will be able to be placed, tested and reworked by automated machines and standard rework tools with the placement of the land pattern or footprint we put on our board design.
Obviously others need access to the courtyard area of the board around the component and it must be kept clear of obstructions by other parts or components to allow, for example, the test equipment to probe the pins on a part or the rework tooling to be able to adequately heat the part to remove it. The Courtyard is by definition intended as a visual aid to the designer to keep the area in the courtyard clear of other physical objects. The area allowed is much larger on BGA's than most components to allow the rework heating nozzle which is much larger than the body of the BGA to get close enough to the board and direct hot air under the edges of the part to melt the solder balls to remove the part for repair or replacement. If we get to close with other parts they cannot rework it. Cell phone manufacturers probably wouldn't bother with the use the courtyard because their rework procedure is to throw away the whole board... but many of us who do higher end low volume expensive equipment rework or repair our boards. Another purpose for the courtyard is to remind us that component placement machines have pick up heads that need to be able to pick the part and place it on the board for soldering accurately and they have positional accuracy tolerances and features that come close to the board when placing components that could hit other parts if they are too close together. The courtyard is supposed to indicate a positional deviation boundary or tolerance for pick and place, auto insertion machines, flying probe tester machines, and other tools, etc.
The Courtyard's purpose is not a corral to keep all the cows that belong to the farm in... so to speak... which is what it seems that the team has somehow got the idea that no element of the footprint can be outside the courtyard... that idea patently unrealistic and has little or no value to the designer since we can already see the elements of the footprint just fine without the courtyard. The courtyard is a reminder to the designer that they better give the part some *extra clearance* for the 'other guys' that need to get at it and not just cram them all together next to each other and expect the assembly folks to be able to figure out how to get it built with no clearances.
Have fun today, I'll be down there tomorrow to explore the show floor and see what new things are happening out there in the industry.:) Best regards,
William Brooks, CID+
Senior MTS
2747 Loker Ave West
Carlsbad, CA 92010-6603
760-930-7212
Fax: 760.918.8332
Mobile: 760.216.0170
E-mail: [log in to unmask]
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