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12 PCB007 MAGAZINE I NOVEMBER 2020 ments in your system. What are some of the aspect ratios that you're hitting now? Stepinski: It depends on the diameter. It's a gradual curve, so the larger the diameter, the easier it is to do the aspect ratios, while the smaller the diameter, the harder. But on through-holes, we can metallize 40:1 on blind microvias depending on the diameter. If it's re- ally, really tiny, it's around 1:1. If it's larger, we were doing 10:1, 15:1, or things like that with slots. On through-hole filling, we've dem- onstrated up to 8:1. One of the challenges is that the suppliers present a strategy to do these things, and it doesn't work on the higher aspect ratios. It was kind of like the bridge-plating strategy that's common with through-hole fill. You have to do things a little bit differently. It's hard to get things done with one electrolyte. You may need two or three electrolytes in a row to shape the structure the way you want. As Happy indicated before, throwing more in the hole than on the surface is okay. However, what generally happens is if you throw more in the hole than on the surface, the solutions that are on the market tend to compromise the knees of the hole, and you have to come back and do something about the knee. Then, you also have a situation where if you bridge plate, you can plate the knee shut before you finish filling from the bottom in four sequences. You have to analyze each electrolyte, see what the possibilities are with it, and then find out, "I'm going to use this electrolyte for Step A, and this electrolyte for Step B." It's a multi- step approach in plating. It's different than what it's been traditionally where people just want to drop it in one or two baths and are done. As you go toward these higher aspect ra- tios, this is where we find ourselves now—un- til the time that people develop new electrolyte formulations that are more sophisticated to do it in one step or two. Whenever you're on the leading edge, this is probably where you are with multiple electro- lytes, no matter what year it is. This is the way to go when you're on the leading edge. If you want to be a few years behind, you can deal with one or two until the suppliers catch up with their R&D. Day: Although, the time fac- tors are a different scale now- adays. You can do it, but as Alex says, you can't expect to do this in one hour. These cycles are unrealistic now. You have to go for bridge plating, then you need mul- tiple electrolytes, and sometimes on that, you shift the onus from the plating to the etching because the ideal situation is you get more in the hole than on the surface. But more realistically now, you get the clus- ters—the high-density areas of holes that you have to fill, these end up as robbers—but then you end up with bad uniformity. Then, you have to look at a good leveling perform- er to bring back the leveling performance. If you start with a non-uniform copper surface, it doesn't matter how good your etching is— you will not be able to etch well. The copper management scenario becomes good leveling, which means actually more on the surface, and you put more constraints on your etching. Holden: Last time we were there, you had three electrolytes in line. Has that changed? Stepinski: We have three sets of electrolyte packages, but they don't all run at the same op- erating ranges. You can run one organic pack- age but have different chloride levels, different copper levels, and different sulfuric levels. Holden: I meant you had three different special- ized copper plating solutions for their own par- ticular capabilities. Stepinski: In the shop in total, we have seven electrolytes going. Matties: That's what it takes to be on the lead- ing edge. Stepinski: If you want to plate complicated structures, that's what you have to do. On Rick Day

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