"So, under these circumstances, Paul and I 'sing from the same sheet.' Werner That is a great relief Werner. I am not comfortable being on the wrong side of a Werner technical discussion. All this is very interesting and we should consider testing NFP on some of those variables like hole size, grid size, low flow resin, thin vs. thick. In all honesty I did not expect to have such a lively discussion on NFP from a column written about corner cracks. I would have explained more of the factors affecting NFP if the column was on that aspect of reliability. The columns I am writing now are limited to the failure modes for which I have created animations. To take it one step further, there is the consideration of "glass lock" and "glass crunch" which is inherent to high layer counts with NFP in and thin dielectrics. There can be so much copper in the "pancake stack" of NFP that the area of the PTH is resin starved and the copper is resting directly on the glass fibers on the B-stage layers. Breaking materials is generally stress relieving extending cycles to failure. Low resin due to squeeze out and cracks in the dielectric between pads may extend thermal cycles to failure. You have to consider both copper and material when you talk about reliability in lead free applications. We need to study the effects of design on reliability. I met with Gary Ferrari and Deter Bergman when they were in Ottawa giving training classes last month and we had some lively discussions on reliability vs. design. I think the consensus is that there is less objective evidence and more "common sense knowledge" than we would like on the implications of design's effect on PWB reliability. This concerning in lead free applications. Sincerely, Paul Reid Program Coordinator PWB Interconnect Solutions Inc. 235 Stafford Rd., West, Unit 103 Nepean, Ontario Canada, K2H 9C1 613 596 4244 ext. 229 Skype paul_reid_pwb [log in to unmask] <mailto:[log in to unmask]> -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier Sent: July 12, 2010 5:42 PM To: [log in to unmask] Subject: Re: [TN] non-functional pad removal Hi Paul & All, that is the problem/danger with Technet responses which have to be a fortiori short. One has to assume some underlying understanding-I assumed everybody knows that 'small PTV diameters tend to barrel crack' and 'large PTV diameters will tend towards innerlayer [I did not write 'interlayer' anywhere] separations.' To prevent the later, remove NFPs from large diameter holes and leave them in place for small diameters. PCB thickness has also a significant influence, as do a whole number of other parameters. So, under these circumstances, Paul and I 'sing from the same sheet.' Werner -----Original Message----- From: Paul Reid <[log in to unmask]> To: [log in to unmask] Sent: Mon, Jul 12, 2010 5:23 pm Subject: Re: [TN] non-functional pad removal Hi All, It is very hard for me to be in disagreement with Werner but we are not as far apart as it would appear. We do agree that lead free applications increase the effect of non functional pads (NFP) in or out. The data becomes tighter, and cycles to failure reduced, if the test vehicle is exposed to lead free assembly and rework. My response reflected the findings we have on PTV (plated through vias). In smaller holes and grid sizes the degree of pad rotation is reduced due to the proximity of adjacent holes. Because smaller holes are on a smaller grid, it appears that there is less stress relieving by material squeezing out between PTV with each thermal excursion so the force of the z-axis expansion is concentrated (for lack of a better word) in the Cu barrel. In larger grid sizes the material easily stress relieves by squeeze out between the PTVs and as a result, decreases the stress in the barrel and, at the same time, increases the degree of pad rotation. With larger hole sizes the grid is larger and the amount of pad rotation is increased, in part due to the increased distance between holes. At the same time larger holes have barrels that are easier to plate and the larger hole size means the barrel are inherently more robust (even Cu distribution, small uniform crystalline structure). Interconnect failures (I think Werner miswrote interlayer separation) is exacerbated when pads are removed. Interconnect separation is not a wear out failure mode; usually interconnect failure reflects a process problem. Although larger holes on larger grids shifts the areas were stress is expressed, (from barrel to corners) I expect that if the interconnections are not compromised, then NFP out increases cycles to failure. All things being equal, NFP out increases reliability. I do not know the effect on thin coupons or low flow resin systems. It would be interesting to test those two variables. Please understand that variables like hole size, surface finish, type of material all count when you are dealing with a third level influence like NFPs in or out. It is easy to have other factors be covertly influential in a NFP In vs. Out study. Nickel protection of the PTH and degradation of material, like delamination, (in lead free applications) would be two major confounders to this type of study. It is disappointing to find, at the end of test, that the data is confounded because another factor dominated the results. Steve Kelly from PFC Inc. in Toronto read my comments in the iconnect007 column and is interested in testing my contentions in a rigid flex, 12 layer construction. I do not know the effect of NFP in vs. out on rigid flex or normal flex for that matter. Steve is planning to fabricate test coupons with clad polyimide and low flow resin. Our agreement is that PFC will fabricate the test vehicles and we will provide testing. If we get compelling data, confirming or refuting our historical findings, we plan to publish the results. I expect we will make this a lead free study. Sincerely, Paul Reid Program Coordinator PWB Interconnect Solutions Inc. 235 Stafford Rd., West, Unit 103 Nepean, Ontario Canada, K2H 9C1 613 596 4244 ext. 229 Skype paul_reid_pwb [log in to unmask] <mailto:[log in to unmask]> -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson Sent: July 12, 2010 11:53 AM To: [log in to unmask] Subject: [TN] non-functional pad removal 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 ______________________________________________________________________ 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 ----------------------------------------------------- = ______________________________________________________________________ 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 -----------------------------------------------------