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JULY 2019 I PCB007 MAGAZINE 61 Shaughnessy: That was interesting. I like the part of your presentation about the resin-starved areas that were "thirsty." Munson: That weave had no way of get- ting the resin to flow into it, whether it was due to compression and density or bad resin that was overheated or had been stored out of a controlled situation. Shaughnessy: With some of the things you showed in the presentation, do you have any advice you'd like to give to designers on any things they should or shouldn't do? Munson: We are going to see designers be- ing challenged by circuit needs. We are creating blind and microvia structures, and we're building a lot more architecture and cir- cuitry into the board itself making it a three- dimensional structure. From a design stand- point, one of the biggest things we suggest is giving as much space between power and ground as we possibly can so we don't have a 2-, 3-, or 4-mil separation on the inner lay- ers where we're creating an opportunity for a short to occur. The more space we put in there, the greater the opportunity for the resin to flow effectively and at least block parts of that pathway. Microvias are always going to be a challenge. From a designer standpoint, microvias are the necessary connections to the rest of the whole circuit board. We need to spend more time with the fabrication process to understand what the equipment limitations are and how to help them. We think the press and the lami- nation process are both critical—how long to press, how hot the press is, how much to allow the resin to flow into the weave, and then how fast to cool. The faster it cools, the quicker the flow stops. It becomes a dance, and a lot of fabricators are in the process of trying to put as much product through those time-consuming processes as possible. Pushing the fabricator to understand this process and its limitations, where the stackup limitations are, how many panels you can put into it, and the differences between electric heating and oil heating are all important. All of those variables become critical, and the de- signers need to understand that those issues may affect 2–4% of the hardware. Some of the boards processed in the same stack might all perform extremely well, but on the same pan- el, there might be nine out of 16 boards, all with the inner layer weave issue on that pre- preg. To me, it is not just a prepreg issue; it's a prepreg and press issue. Shaughnessy: What would be one or two big takeaways from your presentation that you would you want everybody to remember? Munson: As density and circuit sensitivity con- tinues to change, we are going to have greater opportunities to have these types of failures and risk issues. Understand what the equip- ment process limitations are so that we can look for those things and design them out. The second takeaway is to understand the cleanli- ness of your process. How clean are the inner layers of your board? How can you assess that on a normal, regular basis? That helps. Shaughnessy: Very good. Thanks, Terry. Munson: Thank you. PCB007 Figure 3: Close-up showing CAF shorting in a layer of resin- starved prepreg.

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