Issue link: https://iconnect007.uberflip.com/i/1526666
60 PCB007 MAGAZINE I SEPTEMBER 2024 and can undercut there. It's not the resolution of the etching, it's the complete yield impact including trace variation, top to bottom, not just sides of the trace. ough being able to do less than 60 µm lines and spaces subtractively has already started for PCBs, we currently use carriers. We create them, and then encapsulate them aer their process. ey're so much eas- ier to handle aer the signals are encapsulated. Matties: What about power distribution in the BGA in this type of configuration? Inside the BGA, you have a real challenge. We try to use low copper weight on sub-0.9 mm pitch BGAs with half-ounce, 1-ounce, or 2-ounce copper for power. To get the power inside the middle of the BGA, due to the den- sity and pitch of interconnects, the 2-ounce web is the limiting factor to get the power deep inside the BGA. is is magnified by large BGAs. Imagine this is a BGA with several rows and columns of antipads, and you only have a 1-ounce or 2-ounce copper foil. e available current that can get through this area to get to a connection is the width and thickness of the foil (called the power layer web). Most num- bers I see for GPU power are around 700 watts total for today and that's at 0.9 mm pitch BGAs. We're on to nearly 2,000 watts at sub-0.9 mm pitch. is technology has taken nearly three years to develop, with four partners collabo- rating on equipment, materials, and process testing. e problem is that half-ounce copper, while it has very high resolution and you can get close to straight sidewalls, does not have a lot of height, so there's not a lot of mass to be able to get there. When you go to 1-ounce copper, you have better mass, but you lose resolution. When you go to 2-ounce copper, you lose even more of this resolution. Even with vacuum etching, you lose resolution on 2-ounce. In fact, it isn't worth moving from 2- to 3-ounce as the subtractive post-etch difference is nearly the same. e post-etch actual mass of copper (in 1-ounce and 2-ounce) in the web is not that much different, but don't tell that to a power guy. We have a method to not lose any web width independent of copper thickness. is requires not only new manufacturing equip- ment and processes, but also a collaboration with key laminate suppliers to create what Wus calls "liquid prepreg." We need this not only for our power solution, but to encapsulate pas- sives and CTE resistance materials. We have it now in Asia with one fully developed supplier and two others in R&D. For 224G+ applications, it is even more important to maintain the Dk throughout the via connection. Most other methods must use advanced SI materials in the power layers to achieve this. Wus uses FR-4 materials and still has low Dk near the via. With subtractive power processes, you must stack redundant power layers to get to the mass copper the power engineer needs. In any BGA PCB or substrate cross-section, you'll see that about 60% of the layers are signals or references to the signal, and the other 40% are power dis- tribution layers. Wus can achieve high resolu- tion power webs inside the BGA, which means less redundant layering and lower thickness packages. is is a 3-ounce high resolution web with virtually no reduction. is is at 0.9 mm, " is technology has taken nearly three years to develop, with four partners collaborating on equipment, materials, and process testing. "