Jack, The IPC-2221 committee has addressed this issue in the working draft. They have discussed removal of NFP's and made a recommendation similar to what you and Werner have summarized. The designer may then choose the best note path for his/her application requirements. As has been mentioned, I do not believe that one could develop a single generic note, that would cover every application issue. Gary On 8/3/2010 8:06 PM, Werner Engelmaier wrote: > Hi Jack, > Here are my comments > > > So here is what I have been told about the subject of non-functional pads (NFPs): > > - Drill bits last longer if NFPs are removed—YES > > - In some cases NFPs create anchor points for stress, and barrel cracks occur—I THINK YOU MISUNDERSTOOD THIS. > > - Reliability is more relevant to hole diameters less than .020", remove NFPs from larger hole diameters—RELIABILITY IS > OF COURSE EQUALLY RELEVANT, YOU JUST GET DIFFERENT FAILURE MODES DEPENDING ON DRILLED DIAMETERS: > BARREL CRACKING FOR SMALLER DIAMETERS, INNERLAYER SEPARATION/SHOULDER CRACKS FOR LARGER DIAMETERS. > REMOVING NFPs FROM SMALLER DIAMETER HOLES HAS LESS RELIABILITY CONSEQUENCES. > > - Yield may increase by reducing possibility of inner layer shorts by removing NFPs—??? > > - NFPs only play a role with higher aspect ratios—NO > > - Divide the hole length into three zones, and remove NFPs from the middle zone—YES > > - High layer count with thin dielectric creates resin-starved pancake stack, remove some NFPs—??? > > - Keep the stack symmetrical, if a connection is made on layer 8 of a 16 layer, leave NFP on layer 9 to balance the stress— > A SYMMETRICAL STACK IS A GOOD IDEA TO REDUCE WARPAGE DURING SOLDERING; YOU CANNOT "BALANCE" [WHATEVER THAT MEANS] STRESSES THIS WAY > > - NFPs reduce voids in thin low-flow prepregs—POSSIBLY > > - Don't remove NFPs from layers 1,2,3,n-2,n-1,n—FOR LARGER DIAMETER HOLES > > > I assume that those working on high-tech high-reliability designs know what > they want to do and have customized their fabrication drawing to get what > they want. On the other hand, I'm concerned about the average designer who > wants to know best practices, but isn't sure how to form the note. > > Here is my attempt to cover the most scenarios with the least complication: > FOR CLASS 1, NON-FUNCTIONAL INNER-LAYER PADS MAY BE REMOVED, UNLESS OTHERWISE SPECIFIED > FOR CLASS 2 AND 3, NFPs MAY BE REMOVED FROM DESIGNS WITH LESS THAN > 5:1 ASPECT RATIO, OTHERWISE, NFPs MAY NOT BE REMOVED FROM LAYERS 1,2,3,N-2,N-1,N OR FROM HOLES GREATER THAN .5MM > > > Werner > > > > > > > > > -----Original Message----- > From: Jack Olson<[log in to unmask]> > To: [log in to unmask] > Sent: Tue, Aug 3, 2010 12:19 pm > Subject: Re: [TN] non-functional pad removal > > > I have re-read all of the responses related to removing non-functional inner > > layer pads (or not), and this will be my final attempt to condense what you > > guys have taught me. I usually get in trouble trying to summarize other > > people's posts, but its important because somehow we have to tell the > > fabricators what we want. > > How can we neatly describe a broad set of considerations into one simple note > > on a drawing? > > > > So here is what I have been told about the subject of non-functional pads > > (NFPs): > > > > - Drill bits last longer if NFPs are removed > > - In some cases NFPs create anchor points for stress, > > and barrel cracks occur > > - Reliability is more relevant to hole diameters less > > than .020", remove NFPs from larger hole diameters > > - Yield may increase by reducing possibility of inner > > layer shorts by removing NFPs > > - NFPs only play a role with higher aspect ratios > > - Divide the hole length into three zones, and remove > > NFPs from the middle zone > > - High layer count with thin dielectric creates resin- > > starved pancake stack, remove some NFPs > > - Keep the stack symmetrical, if a connection is made > > on layer 8 of a 16 layer, leave NFP on layer 9 to > > balance the stress > > - NFPs reduce voids in thin low-flow prepregs > > - Don't remove NFPs from layers 1,2,3,n-2,n-1,n > > > > In additon, there was a direct conflict between two comments, one > > recommended removing NFPs for thick power planes (BEllis), the other said a > > DOE testing 24 layers of 2oz was more reliable with NFPs removed (GGagnon). > > > > I assume that those working on high-tech high-reliability designs know what > > they want to do and have customized their fabrication drawing to get what > > they want. On the other hand, I'm concerned about the average designer who > > wants to know best practices, but isn't sure how to form the note. > > > > Here is my attempt to cover the most scenarios with the least complication: > > FOR CLASS 1, NON-FUNCTIONAL INNER-LAYER PADS MAY BE REMOVED > > FOR CLASS 2 AND 3, NFPs MAY BE REMOVED FROM DESIGNS WITH LESS THAN > > 5:1 ASPECT RATIO > > OTHERWISE, > > NFPs MAY NOT BE REMOVED FROM LAYERS 1,2,3,N-2,N-1,N OR FROM HOLES > > GREATER THAN .5MM > > > > The IPC is interested in eventually developing a NOTE GENERATOR as a > > companion to the IPC-2610 documentation series, and this seems to be a > > perfect example of the type of subject that designers would want help with. > > > > Do you have a better suggestion for a good note, or care to present a > > note "framework" for non-functional pads? > > > > so close, and yet so far... > > Jack > > > > > > . > > On Mon, 12 Jul 2010 10:52:51 -0500, Jack Olson<[log in to unmask]> > > wrote: > > > >> I know this topic has been discussed here before, but an article I just read > >> by Paul Reid put a different twist on it > >> http://www.pcb007.com/pages/zone.cgi?a=69569&_pf_=1 > >> > >> I wanted to ask about a paragraph where he says: > >> > >> "We know from many years of reliability testing that a board with > >> non-functional pads removed tends to be more robust than the same board > > with > >> non-functions at every layer. Occasionally, designers will have > >> non-functional pads at every internal layer. In most applications this > >> produces a reduction in reliability with an increase in barrel cracks in the > >> central zone of the PTH. It appears that this produces a number of anchor > >> points along the PTH and failure occurs in the barrel. Customers who remove > >> non-functional pads for increased PTH reliability reduce the "anchor" point > >> and stress is transferred to the knee of the hole." > >> > >> This is a very important point for me, because I have always heard it > >> explained a different way. My (unfounded unscientific) understanding was > >> that fabricators wanted to remove them to save drill bit wear (especially > >> for high-volume boards in benign environments), but designers often want to > >> keep them in because the extra ribs provide more support (especially for > >> harsh environments). > >> This article suggests that keeping inner layer pads is LESS reliable. > >> The reason it is important to me is that our boards are expected to survive > >> 20 years in an automotive environment, we have been allowing unused pads > > to > >> be removed, but some have suggested we retain them for lead-free > > processing > >> temperatures. > >> We haven't cared about inner-layer pad removal until now, but soon we will > >> be required to design for RoHS compatibility, and we were about to start > >> specifying that they be retained. Am I misunderstanding these results? > >> > >> Jack > > > > --------------------------------------------------- > > Technet Mail List provided as a service by IPC using LISTSERV 15.0 > > 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 > > ----------------------------------------------------- > > > > > ______________________________________________________________________ > This email has been scanned by the MessageLabs Email Security System. > For more information please contact helpdesk at x2960 or [log in to unmask] > ______________________________________________________________________ > > --------------------------------------------------- > Technet Mail List provided as a service by IPC using LISTSERV 15.0 > 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 > ----------------------------------------------------- > ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________ --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 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 -----------------------------------------------------