Issue link: https://iconnect007.uberflip.com/i/1471044
12 PCB007 MAGAZINE I JUNE 2022 ratio board and a 20:1 aspect ratio board, which one is going to be more difficult to plate? Very simple: the 20:1, because the resis- tance increases signif- icantly down through the hole. From a tech- n o l o g y s ta n d p o i n t , you' ve go t to make several adjustments, which the industry has. We've seen adjustments in tank design, plating rack design, process control, and the formula- tion of new additives that enhances throwing power and overall plating distribution. It's always funny when someone says, "Why can't we just do this?" Well, you're violating Faraday's Law. ere are only so many things you can do and you can't violate certain laws of the universe. Faraday's Law is one that you don't mess with. So you've got to manipulate the amperage, the voltage, and all the things that make a difference in getting chemistry into the hole. at's significant. en we went to periodic reverse pulse plating, which is basically elec- troplating except it uses a reverse current as well as a forward current. It makes adjustments in the wave shape using square wave vs. more turbulent type waves. at has helped tremen- dously to plate and improve the reliability of the thicker through-hole printed circuit boards. Finally, one of the things that was borrowed from the semiconductor industry—we've all talked about what's new because sometimes it's not new, it's just re-cloaked in a new tech- nology—is they learned to plate copper in nar- row trenches. ey call it damascene plating; they actually fill a trench with electroplated copper. Of course, the trenches are very min- ute compared to a blind via of a circuit board, but the circuit board industry and the suppli- ers, in my experiences, have been able to adapt to chemistry and, again, the agitation and other adjustments, to fill blind vias by electro- plating copper in a via from what's called bot- tom-up filling, super fill, without necessarily overplating on the top. at has helped create HDI and ultra-HDI and allowed that technol- ogy to really blossom. How do you fill blind vias when you're not going to put a component in them? You need to fill them with something. It's like the say- ing: A man digs a hole, and then he wants to fill it. Well, you must fill the hole in a blind via because you can't have air in a blind via if you're not going to put components in it. You can use via fill paste, a polymer paste, or you can electroplate. I happen to be partial to electroplating because it can be done in some of the same equipment by just making adjust- ments. at's really what has allowed HDI and ultra HDI to truly blossom, in addition to all the imaging. Dan Feinberg: As part of that, what do you have to control the most? Is it the wattage that's coming out or is it the voltage? It would seem that if your voltage was not in control and it was a quarter-volt or half-volt higher, you would have a problem with the evenness of the level of a plating. Carano: Interesting, Dan. ere are always two schools of thought and I've learned this from the metal finishing industry as well. People talk about two types of what you just mentioned— constant current and constant voltage. I hap- pen to be a fan of constant voltage and letting the current rise. But there are other factors. e additives themselves must be significantly altered in order to adjust so you can suppress plating on the surface. ink of a blind via as a vase: It goes wide on the top and gets down to a narrower base at the capture pad. You want the plating to fill that, whether it's 3 mils or 4 mils deep. You want it to come up faster and fill that via with- out leaving a void in the center of that area, all while minimizing overplating or mushrooming on the surface. Michael Carano