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PCBD-Nov2017

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14 The PCB Design Magazine • November 2017 THE HDI ROUNDTABLE EXPERTS DISCUSSION roughly, and that's what earned us the no-bid. Now we're at two mil trace and three mil space, and that's still not good enough. So, we're continuing to increase the density, and for us, our whole game is no vias, no stubs, and no parasitics. Everything's predictable: no glass weave, etc. So what we're looking at is thinner and thinner laminate, and hybrid stack-ups. We can certainly do all our power delivery and control signal dis- tribution on FR-4 layers, but what lies on that surface is important to us. We can do small modules, system-type modules for our ICs and such, our chip-on-board and so on, but really what we need is a full system-level surface layer that can support those types of densities and mount chips. That's really what we are work- ing towards right now. We're now looking at 20−25-micron traces and 25−40-micron gaps, and seeing if we can't do that. If we can, then we can support all our chip mounting tech- nologies. We can support our system-level traces that get from the origin to the destination and so on. When we do that, then of course these traces get very lossy, and that becomes a huge concern. So our antidote to that is keep them short. Get them shorter. And so that's kind of our roadmap in a nutshell. We're starting with surface density, because the propagation is the fastest, keeping it stubless, and then making the net as short as possible. We run everything that's important differentially for noise immu- nity. So, you can imagine the types of stackups we're looking at in terms of hybrid stack-ups and so on. Shaughnessy: What sort of materials are work- ing out best for you? Bird: When we first started this effort, my boss asked me, "What would you use? What's the first step that you would do?" My response was, "Thin, glass-free laminates. Something that's just as thin as you can make it." Let me put it this way: There is a slide that shows FR-4 with traces and spaces in the 4 mil range. With the help of HDI, we then took the through-hole off the surface and stacked the blind and bur- ied vias. We call this technology level modified FR-4. This got us to 3.5 mil trace/space. There is another slide called the constant impedance funnel for differential pairs. You can imagine this funnel that starts out in the FR-4 at 4 mil trace/space and goes all the way down to be something like chip-on-glass or ceramic at 0.6 mil trace/space. One can place a large bracket in the gap with a label saying "This gap is too large. This gap is responsible for chewing up real estate because things are not scalable." The things that you do on an IC or on glass, you have a lot of overhead in terms of the splay angle, trying to get the sig- nals out (escaped) and getting them routable. We are trying to solve that. Be- cause once you solve the trace to be similar to the pin or pad pitch, then your channels become much more scalable on the surface. By the way, as we're going up in frequency, we're also going up in channel count. We're getting whacked from both sides. We need to step back and look at what design rules we need. If we're at 25-micron trace and 40-micron gap, things such as channel count and speed become much more manage - able. Now the trace has become much shorter. From a signal integrity standpoint, the first thing to do is get rid of all your parasitics, get rid of all your stubs, trace, tie-bars, etc., as well as your parallelism and propagation delay; get rid of ev- erything that will kill a signal, and then go for improved material. That's where we're at right now. We're searching the world for the best ma- terial with the best copper with the best adhe- sion, which means the best processes in terms of subtractive, additive or semi-additive. Then of course the vias stick out like a sore thumb, and even 25-micron vias stick out like a sore thumb to us. So it's very important that we go vialess for all our high-speed traces. If we do that, then we can get to where we want in organic material. My whole point is, let's be or- ganic if we can, and I don't think we're done being creative here. Steve Bird

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