PCB Plating Still Comes Down to Physics

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For this month’s plating issue, we spoke with I-Connect007 columnist Michael Carano, a longtime surface finish expert with years of experience at RBP Chemical and OM Group. We asked Michael to discuss the latest innovations in plating equipment and chemicals, as well as some of the drivers in this segment, and the biggest challenges and opportunities he sees in plating today.

As Michael points out, despite all of the technological advances in this industry, process engineers still need a solid understanding of Faraday’s Law and Ohm’s Law to successfully plate PCBs.

Andy Shaughnessy: This is our electroplating issue, and I’ve heard you say that you know how to plate anything. What's the latest in electroplating? What sort of innovations and challenges are you seeing in processes, equipment, etc.?

Michael Carano: If you go back 30 years, what's different in electroplating technology? Forget about electrodes for now. What's different about electroplating technology? I like to call it electrodeposition because we are attaching electrodes and we're running current through a tank. In the last 10–15 years, it hasn't changed much. But what has changed is the technology of the circuit board—smaller vias and thicker boards. Whenever you make a via smaller and you add more layers, what you do is you increase what they call the ohmic resistance through the via. 

With Ohm's Law, V = IR; you apply a voltage to a plating tank. If you have a 10:1 aspect 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 resistance increases significantly down through the hole. From a technology standpoint, you've got to make several adjustments, which the industry has. We’ve seen adjustments in tank design, plating rack design, process control, and the formulation 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. There 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.

That's significant. Then we went to periodic reverse pulse plating, which is basically electroplating 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. That has helped tremendously 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 technology—is they learned to plate copper in narrow trenches. They call it damascene plating; they actually fill a trench with electroplated copper. Of course, the trenches are very minute compared to a blind via of a circuit board, but the circuit board industry and the suppliers, in my experiences, have been able to adapt to chemistry and, again, the agitation and other adjustments, to fill blind vias by electroplating copper in a via from what's called bottom-up filling, super fill, without necessarily overplating on the top. That has helped create HDI and ultra-HDI and allowed that technology 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 saying: 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.

To read this entire conversation, which appeared in the June 2022 issue of PCB007 Magazine, click here.


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