Issue link: https://iconnect007.uberflip.com/i/886239
18 The PCB Design Magazine • October 2017 tant than exactly what the discontinuity was looking into the transmission line. So I start to wonder more about pad sizes, backdrill preci- sion, and the fact that I can't get big enough anti-pads on my vias to get the impedance of my via to begin with. At least from my point of view, I'm used to thinking that those via dis- continuities are by far a larger problem. Would you say yes or no, in the work you do, Mark?" "Oh, absolutely, Mike," Thompson said. "You raise a good point. In fact, 10−15 years ago, all the fabricators removed any non-func- tional pads. Today, that is rarely ever done— mainly because of board geometries, because of what you're talking about, us cramming more and more stuff on any given area. Because we're ending up cramming more stuff on there, there's less room for error for us for removal of non-functional pads. "If we do that same kind of a thing today, let's say you had 5,000 drilled vias on this par- ticular board, and on a given internal signal layer you had only two terminations, two traces where you had terminations, everything else would be considered a non-functional pad. If I remove all those non-functional pads, have I not just created an absolute registration night- mare for myself in lamination? I've removed so much additional metal from the internal layer, that now I've created a mismatch and I'm set- ting myself up for registration problems." Steinberger shared his thoughts on pads. "From my end of it, I'd like to remove all the pads. Every single one of them. If I could get away with a pad diameter equal to the drill diameter at my rout- ing layer, where I actually did want to connect to the trace, I'd do it. It's just that you guys wouldn't exactly buy it." "That's just technology in general which has yet to catch up to what we require," said Sunny. "The new type of layup, back straights, calcula- tions, algorithms, is all try- ing to get back to what you're saying, to remove those non- functional pads and get that alignment to where we need it to be for perfect manufacturing." Happy Holden mentioned a recent article he wrote that discusses landless via processes. "There are four different ways of doing it, one of which is the positive electrophoretic resist. But there are three other ways: the direct imag- ing machines, the Hewlett-Packard method that they got from the Japanese (which doesn't re- quire any change in imaging registration), and then there's the Russian method. Anybody can use the Hewlett-Packard and Russian methods; they are free and no change of equipment or process is required—just a change in artwork. I've made more landless vias than everyone in the world put together, but the article mentions all four methods. "Landless vias were never adopted by the IPC, though we (HP) have data that shows land- less vias are 10−times more reliable than vias with lands. The landless via and its reliability blows a hole in Class 1, 2, and 3. HP did ex- tensive testing: on different hole diameters, dif- ferent thickness of boards and different sizes of vias; and we found out that our Japanese part- ners apparently knew a lot more than we did, and they were less subject to influence from the IPC. You needed a pad 50 years ago because we used to crimp actual leaded components, not to the whole barrel but to the pad, and because we've been doing it for 60 years, the industry thinks we have to do it forever, even though it's detrimental to reliability, density, and signal integrity. Hewlett-Packard used them exclusively once we dis- covered the secret, and nobody bothered to look at our boards closely enough to realize there weren't any lands on those vias. They're covered with sol- der mask, so you don't really see it." The conversation then turned to IPC and its process- es for creating and amending standards. Yogen wondered how much luck he would have getting IPC to officially recog- nize the landless via. SIGNAL INTEGRITY: THE EXPERTS WEIGH IN Yogen Patel