All of this is correct, but if you're really designing that close to the minimums, a small manufacturing allowance for wicking or other hole wall irregularities should also be added to the other allowances. Don Vischulis >I'm probably saying the same thing as George. The critical part of your >question is that your asking if you can remove the pads to increase wiring >density during layout. The answer is absolutely not. Remember what the pad >is for, to make an interconnect to the drilled hole. The reason it's larger >than the hole is to make sure the hole is still in the pad when all the >manufacturing tolerances are taken into account. The non-functional pad >means there's no interconnect on that hole on that layer, but the drilled >hole will still be there. The non-functional pads are friendly reminders to >keep traces out of that area. > >If you need more room, you need to ask two question of your board fabricator >and one of yourself. For the fabricator, the first question is "how much >larger than the drilled hole must a pad be and still capture the hole"? The >second is "what's the minimum pad to trace space they can etch?" For you, >"from a reliability viewpoint, what's the minimum conductor-to-conductor >spacing I want in the finished board?" Take the larger of the answers to >the last two questions and add it half the answer to the first. If you a >run a trace closer to a hole than that, your fabricator will be throwing >them away for shorts at electrical test or you'll have boards which don't >meet your reliability expectations. > >For example, your fabricator says I need a pad 0.010" larger than the >drilled hole and I can etch a 0.004" space. You follow IPC and want a >minimum 0.0035" spacing. You should put in the desired pad size (while your >at it, if you're cutting it this close, you might as well find out the exact >drill bit diameter they will use). Then rout traces with a minimum 0.004" >spacing. > > >> ---------- >> From: George Franck[SMTP:[log in to unmask]] >> Reply To: TechNet E-Mail Forum.;George Franck >> Sent: Tuesday, January 09, 2001 2:00 PM >> To: [log in to unmask] >> Subject: Re: [TN] Unused Inner Layer Pads: Remove or Not? >> >> Good afternoon, >> >> I have to provide another perspective on this. >> >> I assume that your pad stack is designed with the smallest pads possible, >> which allows for >> layer-to-layer registration errors, material shrinkage, drilling accuracy, >> annular rings, >> etchback, and all the other gremlins in the PWB Fabrication process that >> keep the drilled >> holes from being drilled exactly in the center of the padstack. Lets look >> at what removing >> the Non-functional pads can get you. >> >> Lets use an example, Your hole size in set up for a .0135 drill. You >> want 1 mil annular >> rings (internal), and you are allowing the PWB house .010 for their >> tolerance build ups. So >> the minimum pad size is going to be .0255. Lets steal the half mil from >> the Fab house..... >> and use .025 pads. >> >> Now, You, or the supplier, is going to put the .0135 hole and .025 pad in >> their test coupon. >> And they will use this coupon and make a cross section to verify that the >> drilled hole is >> where is should be, ie at least .001 from the edge of the pad (annular >> ring). >> >> The Cross section demonstrates that the edge of the hole stays inside a >> .023 diameter area, at >> least in that cross section. >> >> Lets go back to the design process. You are routing the board, and your >> design rules say you >> must maintain a .003 minimum space. If you run a trace too close to a >> pad, your CAD tool will >> register a "line to pad" spacing error. So all traces are kept .003 from >> the edge of the pad, >> and .004 from the edge of the hole. >> >> Now, lets remove the Non-functional pads, and rout traces. Some routers >> will now allow you to >> rout traces in the area once occupied by the non-functional pad. (This is >> the reason you >> wanted to remove the non-functional pads, i.e., to squeeze traces into >> places they dont >> fit..) If a trace is now routed .020 from the center of the hole, a >> spacing error is not >> generated. >> >> Remember the Allowances we gave the supplier. The hole can be anywhere >> within a .023 diameter >> area. The hole could be drilled thru this trace, and be within its >> mechanical tolerances. >> These dead shorts will be identified at electrical test. The reliability >> problems are the >> holes that are very close to the traces. They are not shorting out today, >> but given some >> humidity and some time.... ZAP! >> >> In my ever so humble opinion, (IMESHO) the non-functional pads can be >> reduced to a minimum >> size, (0 annular ring requirement) during the layout and rout design >> process. In this case, >> the pads are effectively a "keep out" area, representing the area where >> the drilled/plated >> hole may occur. The pads will help your CAD tool maintain your minimum >> spacing requirements. >> You should never rout a board without non-functional pads present. I >> suppose you could set up >> keep outs in your pad stack to do the same thing as non-functional pads, >> in which case, my >> concerns go away. >> >> After routing is complete, and all Design Rule Checks are completed, there >> may be some >> advantages to removing these pads. >> >> In my experiences as a PWB fabricator, we usually left the Non-functional >> pads in. There are >> agruments either way about non-functional pads in the fabrication process, >> and I am 100% >> behind any position my current boss takes. Before routing your traces, >> however, keep the pads >> IN. >> >> >> George Franck >> >> >> The reason fractions prevail and the Dewey Decimal system never caught on >> in America? >> Well...... that is because Truman beat Dewey... remember? >> >> >> Franklin D Asbell wrote: >> >> > Remove them all...provided of course your customer has no problem with >> > that. >> > >> > What you might learn later in fabrication is that the handful you >> > thought were not an issue has just bit your butt. Besides, if they're >> > unused, why use them ~grin~ >> > >> > Franklin >> > >> > >> -------------------------------------------------------------------------- >> ------- >> > 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 the following message: SET >> Technet NOMAIL >> > 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 >> > >> -------------------------------------------------------------------------- >> ------- >> >> -------------------------------------------------------------------------- >> ------- >> 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 the following message: SET >> Technet NOMAIL >> 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 >> -------------------------------------------------------------------------- >> ------- >> > >--------------------------------------------------------------------------------- >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 the following message: SET Technet NOMAIL >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 >--------------------------------------------------------------------------------- > > ***************************************** Get your N/Connect Internet account today. Call today toll free 1-888-644-0728. http://www.nconnect.net, or email [log in to unmask] --------------------------------------------------------------------------------- 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 the following message: SET Technet NOMAIL 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 ---------------------------------------------------------------------------------