Issue link: https://iconnect007.uberflip.com/i/1507356
76 DESIGN007 MAGAZINE I SEPTEMBER 2023 What is UHDI? As soon as we cross below the 1-mil (0.001") line-width threshold, we need to stop talking in terms of mils and ounces and begin talking in terms of microns. For reference, a 3-mil trace is 75 microns, so a 1-mil trace is 25 microns. In general terms, ultra high-density interconnect (UHDI) refers to lines and spaces on a printed circuit board that are sub-25 micron. As elec- tronics continue to shrink, so does the printed circuit board, not only in the X- and Y-axes, but also the Z-axis. Designers are challenged with reducing the form factor as well as the thick- ness of printed circuit boards to meet these demands. Subtractive vs. Additive Subtractive technology is how we have pri- marily produced printed circuit boards since their inception. e subtractive process refers to the selective building up of traces and features on a copper-clad substrate through plating processes, and then removing, or sub- tracting, the base copper to leave the circuit pattern. e limiting factor of the subtractive process is the thickness of the base copper, typically 5-micron, or 2-micron ultra-thin foil. is base copper thickness defines the mini- mum trace that can be achieved through the etching process. at is the technology that got us as an industry to the 3-mil line and space. e etched sidewall of the trace is tapered, not straight, with the taper getting larger the closer you get to the base substrate. e amount of taper is defined by the copper weight; the higher the weight, the more the taper. is limitation is what drove the development of UHDI technology. UHDI additive technology starts with unclad substrate and adds an ultra-thin 0.2-micron layer of liquid ink to the substrate. e selective trace pattern is applied and the circuit pattern is then built up through the plating processes. e game-changing difference here is only hav- ing to etch 2 microns of base material, which results in perfectly straight trace sidewalls. SAP and mSAP e semi-additive process (SAP) and modi- fied semi-additive processes (mSAP) have Source: Averatek Corporation