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14 DESIGN007 MAGAZINE I OCTOBER 2022 don't address that. Of course, 6012 needs to be modified as well to allow for thinner cop- per conductors because they're not doing 1 mil or 1.2 mils of plating on line widths that are 20 microns and below. ey're typically in the 12- to 18-micron thickness range, which doesn't meet 6012. Solder mask thicknesses may need to decrease when you have devices with 0.2-millimeter BGA pitches; you can't have thick solder mask or you get a standoff. Shaughnessy: What about EDA tools? Can they go down to the 20-micron level? Snogren: Yes. at's not a problem as far as EDA tools go, because a 25-micron line is just another D-code in the system. Some of the design rule criteria would be different, certainly, such as the annular rings. Some of the requirements that you would have on the drawing with conductor thickness or plating thicknesses would be different. Shaughnessy: It seems like, as far as signal integ- rity, there would be a lot of advantages, but still quite a few hurdles as well. Snogren: Yes. UHDI can be beneficial in one way: You can certainly get a higher precision line width and spacing when you're doing semi-additive processes because your etch loss is virtually nothing. You can be very precise, but you're dealing with very small features and any minor imperfection will cause an issue that you wouldn't see on a 3-mil line and space job. Shaughnessy: It seems like at this level you would want to simulate and run analysis every time? Snogren: Yes, and you need to do that in con- junction with the fabricator because there are so many different techniques now for manu- facturing a given feature. You can do it with subtractive, semi-additive, or modified semi- additive. ey will all give you a slightly dif- ferent result in maybe conductor thickness or tolerance. You must work in conjunction with the fabricator, understand their process, what the result will be, and design around their pro- cess. It's no longer just designing it, throwing it over the fence, then seeing who will snag it and try to build it. ere must be more cooperation between the designer and the fabricator. Shaughnessy: I mean, you're not going to start off an ultra HDI project without already having your fabricator selected. You've said that some vias are the thickness of a human hair? Snogren: at's right. ey're getting down below 3 mils in diameter. You're looking at 25- to 50-micron diameter microvias. Shaughnessy: at's really intriguing. What does the typical UHDI process look like? Snogren: It will be a buildup technology, where you start with a core and sequentially build layers on both sides as you go, using micro- vias as an interconnect between the layers. In some ways that's easy because registration is so simple. Each layer automatically registers to the layer below it, and you really don't have the kind of mismatch in registration that we have with regular multilayer boards and blind