Issue link: https://iconnect007.uberflip.com/i/1471044
JUNE 2022 I PCB007 MAGAZINE 17 of things going on. You can either have mass transport or diffusion control. When you have a conventional setup, you're really relying on diffusion control where the ions diffuse from the high concentration down to the low con- centrations. But when you get into higher aspect ratios, you've got to switch to higher convection which we call mass transport control. at makes a big difference in the type of throwing power you're going get, so keep that in mind. Mass transport vs. diffusion control—what are you going to do, how are you going to do it; is it going to be governed by your current densi- ties, your board designs, your tank setup, etc.? I know there are companies, particularly on the semiconductor side, that make instrumen- tation that actually monitors the plating solu- tions constantly. ey can analyze for copper, acid, additives. Why not do that for printed cir- cuit boards? ere are companies out there that have these types of instruments that actually sample the working chemistry constantly and give you a concentration of all the additives, including the organic additives, and then makes adjustments based on set points. It's done in the semicon- ductor sector because obviously with semicon- ductors, it's very expensive to have any scrap in the semiconductor because it happens so fast. But why can't that be adapted to this industry? at's part of the problem. We tend to want to make things manual, just go do a titration. at's not enough today. I think that's what you should really look at: the whole package. Look at training the workers about the mate- rials, and then the equipment, the machines, everything, whether it's the tanks, the anodes, the controls. Do the fishbone diagram to show where all the bad things can happen, all the good things can happen, and you'll understand plating pretty well. ey need to realize that it's a com- plex process. ere are a lot of working parts. Holden: Another thing I'm going to write about in a future column is palladium on copper. I was looking over an older issue of e PCB Magazine, and there happens to be an ad in there for PaleoBOND. Carano: at is direct palladium over copper. Again, people are looking to eliminate that nickel layer if they can and get good solubility, also looking at eliminating gold. Unfortunately, the price of palladium has gone up, but again, if you can just put three to four micro-inches of palladium directly over copper, in my opinion, you'll have good solderability, you'll be able to attach your BGAs and almost any surface com- ponent QFNs to that and reduce your cost sig- nificantly. Remember that the density of gold, Happy, is 19.32 grams per cubic centimeter. e density of palladium is 12.1. So, for every microinch of any of those metals you lay down, you're laying down a heavier amount of gold than you are palladium because of the densities. Obviously, the cost goes way up. Palladium does have an advantage, particularly for flex. Holden: I visited Photocircuits in the early '70s when they were supplying palladium on copper for the automotive industry. With the growth of electric vehicles and everything, I thought I'd mention that you can get away with pal- ladium on copper, and it has advantages over immersion tin on copper or silver on copper or SMOBC. Carano: Yes, because it plates directly on without any displacement reaction, whereas you can plate immersion tin or you can plate immersion silver but you also remove some copper. You have a galvanic cell going on. But Palladium does have an advantage, particularly for flex.