Paul, To answer your question, as long as you have specified the tolerance you have an "accept/reject" criteria... weather through the notes on your drawing or through the IPC Specification that is referenced. It's up to you to determine if the tolerance is acceptable or not. I have seen wide variations in tolerance schemes for specifying line widths and land areas on boards. Percentage deviation from nominal is common... some put a plus/minus range on specific features like lines under a given size... Care should be taken in trying to drive the design by the tightening tolerances. A designer does not "improve a design" by 'tightening tolerances'. You only end up running the cost of the board up and the yields down. The tolerances defined let the manufacturer know 'what range of variances from nominal are acceptable' and that you will reject any that fall out of that tolerance because they will not work in the design. A designer should know what will fit or work in the design and what will not. Tolerances are useless if the designer has not reviewed them for 'fit form and function' and set them to allow the MAXIMUM variances in the design... thus realizing the maximum cost savings, yields and reproducibility. If the design will tolerate feature variances from the 'ideal' or nominal as designed, these variances need to be communicated to the vendor by correctly defining the tolerances for the features of the board. This also means knowing the capabilities of the vendor you choose to do the manufacturing. Some vendors do not possess the machines and processes to support tighter tolerances... Obviously, you would have to send boards to them that can still work with less restrictions on things like over and under etch, plating thickness, drill accuracy and run out of their machines, etc... The tighter the tolerance... the more difficult to manufacture... thus the more expensive to manufacture and better machinery required. Save your more difficult designs for more the capable vendors but set your tolerances to the level that is 'just' acceptable. 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 Quote "Anyone can design a strong bridge, but it takes an engineer to design one that will 'just' not fall down..." - Anonymous -----Original Message----- From: O'Connor Paul [mailto:[log in to unmask]] Sent: Tuesday, January 11, 2005 7:05 AM To: [log in to unmask] Subject: [TN] Surface Mount Land 'over' Etching Hello, I have a question that someone may have had some experience with, my PCB's are fabricated to IPC 6011 / 6012 class 2 as per IPC-A-600. Question is on the minimum conductor width, the standards define & measure the minimum width of the conductor at its base (IPC-A-600 Section 3.2), but in the case of a fine pitch surface mount pad (I'm assuming conductor as defined in IPC6012 Section 3.5.1 - relates to both traces & surface mount lands) its possible to have the measurement at the base of the device within spec yet the etching process can reduce the width at the surface of the land by so much that there are serious assembly difficulties, I've seen variations of between 10 & 50% reduction on the surface of the lands between different batches. I'm aware that this is usually only an issue when there is a combination of fine pitch & heavy copper, my proposed solution it to tighten the spec to specifically state that the point of narrowest conductor width can not reduce below in my case 25%, I'm wondering has anybody else had a similar experience & if so what was the approach to resolving it ? Thank You. --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------