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14 SMT Magazine • November 2017 he says. "Over the last three-year period, flex circuits and rigid-flex had a little bit of growth, but as far as percentage of the business, it was very, very low. The bulk of the business was HDI and so the writing on the wall was clear. That's why we made the shift in equipment and in up- front engineering, and it's paid off." Does HDI technology lower the cost of boards? "It is a mixed bag," explains Tor- res. "Generally speaking, moving to HDI technology and keeping layer count down will ultimately wind up with a lower cost for our cus- tomer. However, there are times that, with standard through- hole technology and lower layer counts, simply adding two more layers may be more cost-effective than moving to an HDI design." Material Matters Besides the lines and spaces, an- nular ring, and holes to fill with cop- per, one the important issues that Torres's team review is the materials to be used. "I was very interested in listening to the discussion on what's coming. The first thing that we have to do in order to satisfy customer needs is to come through with quality and high- reliability product. So, we give a lot of feedback about what materials to use. We would be very, very open in that conversation," he explains. "If a customer has the design already in hand and wants us to use a certain type of material, if we think that the design will be better and go through easier with a different type of mate- rial, we'll certainly suggest that. We have part- nered with different material suppliers and we often go out with our sales force, with our sup- ply partner at our shoulder, and start the design discussion of HDI work with material. It's very important that the materials work for both the OEM and the PCB manufacturer. That's pretty critical." Finisar's Bird wears two hats: one for be- ing the technical lead for the EEs and the CAD designers at the company, and the second for leading their organic substrate roadmapping and development. "Maybe 18 months or two years ago, we were working with a fabricator. We start- ed cheating on the FR-4 design rules, and to their credit they built over 1,500 of these over a period of a year or so, and finally gave it up, and they said 'No bid.' Of course, the upper management team came to my group saying, 'You're violating the design rules." And I said, 'Yes, but not without an engineer sitting next to the CAD designer. What we have here really is not a design rule violation, but a technology limitation," explains Bird. "That spun off an effort to find a substrate that could sup - port the design rules that we're cheating to. Our PCB technolo- gy team was tasked with the ef- fort. We got one new substrate developed and released, but now those new design rules are being violated; we have to con - tinue development. For that pre- vious generation, once we got off of FR-4 and onto alternate materi- als, we were still in the subtractive process. Now , we're looking very heavily at the semi-additive process. Our trace with normal FR-4 is going from 4 mil trace and space down to say, 3.5 mil, roughly, and that's what earned us the no-bid. Now, we're at 2 mil trace and 3 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., and so what we're looking at is thinner and thinner lami - nate, and hybrid stack-ups. We can certainly do all of our power delivery and control sig- nal distribution on FR-4 layers, but what lies on that surface is really 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-lev- el surface layer that can support those types of densities and mount chips. That's what we are working towards right now." The Assemblers' Perspective "We're listening to the designer and the fab- ricator talk about getting layers thinner and smaller," says Burns of MC Assembly. "But one COMMUNICATION STILL THE BEST TOOL Steve Bird

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