Dear T-Nagers (PCB Design and Fab variety), I am experiencing considerable difficulty in getting a number of fab houses to produce our project's PCB's "right first time". Confident two week turn-around quotes result in 4 and 6 week deliveries as they find they have to re-fab two, three and even four times. Maybe we're using rubbish fab shops, but they are certified MIL or IPC houses. The only feedback on general points that I've had, and that's common to them all, concerns our pad:hole ratio vs. 4 mils annular ring requirement. Here's what we're asking them to produce at the moment: -9" x 6" approx VME std boards (0.63 +/- 0.008" thk) class 3 for military avionics. -10 and 12 layers -All layers are 1 oz copper except for layer 2, two middle layers and the second last layer, which are 2 oz for planes. -Material is FR4 IPC-4101/26 -Pad diameter (generally) = finished hole diameter + 12 mils -Trace width = 10 mils signal, 15 mils power, spaces = 8 mils, reduced locally to 8 mils where they have to pass through confined spaces such as between fine-pitch connector pins. -Via holes = 18 mils, via hole pads = 30 mils -Outside layers start as 1/2 oz and are plated to minimum 1 oz -Solder mask is 4 mils thick LPI (probably too thick, but I've no idea what to specify) -Finish plating is either HASL or ENIG -Patterns of thermal via holes are filled with silver-loaded epoxy. -Nothing particularly fancy otherwise on most of the boards, although one type did have 3 blind via groups on each side as well as the through-hole group. Our designers do not include any allowance for manufacturing tolerences in their selection of hole:pad ratio (I am arguing strongly for them to include something, hence my proposal below). Pad diameter minus annular ring requirement minus thickness of copper and finish plating equals zero. I have proposed to our technical team a reduction in via hole diameter from 18 mils (aspect ratio 1:4) to 14 mils (aspect ratio 1:5), as a means of giving the fab houses some manoeuvering room, but have received a reply including the following arguments against it: 1) 1:5 aspect ratio is too great for long term reliability. Expansion causes hole copper plating to segment under thermal cycling and break contact with layers. IS THIS TRUE? Any evidence or can a voice of experience out there answer this one? 2) Reducing trace widths further results in "weak traces and very weak connection between traces and pads." HOW THIN CAN TRACES RELIABLY GET THESE DAYS ON MILITARY BOARDS? The boards are very densely populated, and a general increase in via hole pad diameter of even 5 mils means losing 9% of our board area, such is the number of holes. This we cannot afford, hence the proposal to reduce hole diameter, especially for vias. I could really use some support regarding what via hole aspect ratios, plating thicknesses and minimum pad:hole ratios are reliable for military use given teh baord info above. Our Tech Team are even proposing to remove non-functional pads to free up real estate in some areas - a move I'm strongly resisting. MTIA, Peter [This e-mail is confidential and may also be privileged. If you are not the intended recipient, please delete it and notify us immediately; you should not copy or use it for any purpose, nor disclose its contents to any other person. Thank you.] --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d 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 delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------