David-- I agree with Susan. For the last three years, in the Design-through-Acceptance of printed boards workshop, we cover the various levels of wicking acceptance for the three classes of products. [Comment: IMO, the existing dimensional requirements are not based on technical requirements, rather the acceptance criteria is graduated to "look good" for very little, some, and a-lot; respectively for Class 3, 2, and 1. There may have been some consideration for a combination of minimum annular ring and dielectric spacing (conductor spacing).] During in the workshop discussion, I mention the real requirements are to maintain a minimum electrical (dielectric) spacing between "electrically different" conductive patterns (conductors, plated-through holes, or lands). >From a design point of view, by requiring lands on all layers (functional or not) coupled with internal land-to-conductor spacing ensures/controls the electrical spacing because designers will not be able to create a design with a minimum plated-through hole -to- conductor spacing, which could be compromised by manufacturing registration. >From an inspection/quality/MRB action, for most applications, wicking is almost a "don't care", from a practical point of view, as long as the minimum dielectric spacing is maintained. Wicking is some what a "process indicator" that the drilling process should be investigated or there may be a material problem with the bonding of the resin to the reinforcement fibers. A couple of thoughts: 1) A caution is the direction (angle) of wicking with respect to a nearby conductor. If the wicking is angled (along the fibers in the yarn) toward the conductors on the planar surface of the fabric, then this is a "high risk" condiction because there may be "microvoids" along the fibers that could reduce the electrical spacing below minimum, or create such an electrical stress on the dielectric to stimulate cathodic/anodic filament growths. 2) Conversely, if the wicking is angled away from the conductor, then it is a reasonable (very low) risk, and should be acceptable for most low voltage applications (<50 Vdc or ac peak). I also mention that a good design, where long-term reliability is a serious design requirement, does not have closely spaced (~1mm edge-to-edge) of plated-through holes on the same grid lines to where the reinforcement yarns bridge the spacing between holes. It is better to off-set the plated-through holes either diagonally or such that plated-through hole centers are on a grid that is greater than half of the sum of their hole diameters. Hope these thoughts add to your info. pile. Ralph Hersey Ralph Hersey & Associates Phn: 510.454.9805 FAX: 510.454.8905 [log in to unmask] ------------------------------------ Subject: FAB: Wicking in PTH Resent-Date: Tue, 15 Apr 1997 12:43:58 -0700 Resent-From: [log in to unmask] Date: Tue, 15 Apr 1997 10:44:17 -0500 (CDT) From: David Bergman <[log in to unmask]> To: [log in to unmask] I received a call from a member inquiring whether there is any IPC activity taking place on wicking. With PWB designs becoming increasingly dense, spacings on internal layers can be at risk with minor misregistration and solder wicking. I know that ITRI is working the registration issue. Is anyone actively working to study wicking. Looking at: o Material differences o Alternate metallization techniques o Different glass styles o Other? Anything will help. Best Regards __________________________________________________ David W. Bergman, V.P. of Technical Programs IPC 2215 Sanders Road Northbrook, IL 60062-6135 847-509-9700 x340 Phone 847-509-9798 Fax email [log in to unmask] www http://www.ipc.org faxback support 800-646-0089 --------------------------------------------------- *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To subscribe/unsubscribe send a message <to: [log in to unmask]> * * with <subject: subscribe/unsubscribe> and no text in the body. * *************************************************************************** * If you are having a problem with the IPC TechNet forum please contact * * Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask] * ***************************************************************************