Slippery When Wet: Plating High Performance RF/MW PCBs

Once again, summer has evaporated at record speed. This morning, my two teen daughters trudged sleepily out the door for their first day back to high school; one a sophomore and one a senior. Wow…it seems like the first day of summer was just last week, and my kids were just starting kindergarten!

I suppose this sense of nostalgia is why I am now drawing a rather quirky connection between plating high performance multilayer circuit boards and Slip n’ Slides. I loved Slip ‘n Slides when I was a kid, didn’t you?! Injury almost immediately ensued just after you got the darn thing stretched out and flooded with water—but it was still a blast! It was equally fun watching my daughters slide, belly-down, along the long yellow plastic sheet when they were small. Afterwards, they stood up wide-eyed, dripping and giggling, and glued, head-to-toe, with tiny blades of grass. I also recall my utter humiliation when my kids and their friends looked on, with a mix of horror and pity, when I tried to throw-down some of my “old moves”, thinking I’d be the “cool” mom. (Sorry, girls!)

Memories aside, let me explain how I’ve come to connect Slip ‘n Slides and multilayer high performance boards. Plating is an aqueous (wet) process in which metallic particles are electro-deposited onto the surface of circuit boards. When plating high performance boards, which often employ Teflon laden materials, it gets a bit tricky. Because, well…it’s Teflon! It is very smooth and slick. (Think of Teflon coated cookware and eggs)

So, imagine that the Slip ‘n’ Slide plastic is Teflon substrate, and that your kid is a metal particle afloat in liquid. Now! Quickly and permanently bond the metal particulate (child) to the Teflon material (plastic), as it goes screaming by in a stream of moving water (aqueous bath). Yeah–well, welcome to our world! Plating on Teflon is not as easy as you might imagine. However, if we were plating on FR-4, or a ceramic-loaded substrate, it would be more like your kid attempting to Slip ‘n Slide on indoor-outdoor carpet. (Ouch!) The surface is rougher and more porous, even within an aqueous environment.

Don’t get me wrong, we love Teflon! It has many desirable properties, so we simply take steps to treat the material surface before plating to make it rougher and more receptive to metal deposits. There are effective and ineffective ways to accomplish this task, and only an expert understands the critical difference. So always choose those with expertise in fabricating these boards.

A second concern, in regard to plating high performance multilayer boards, has to do with ensuring a good solid ground connection. Some designers accomplish this by putting a front-to-back connective band of edge plating around the perimeter of the board. Others opt for dense hole patterns around the surface edges of the board. Not all board manufacturers are skilled at edge plating. It comes with a small host of potential problems. So, if you are opting for edge plating, you may want to ask for some edge plating samples and discuss the processes and concerns with a new potential supplier.

The third issue to be mindful of is the challenge of plating successfully when controlled depth, back-drilling, or selective plating are required. From this side of our manufacturing facility door, I often feel Dorothy-like in the presence of Almighty Oz in the face of these processes; I don’t fully understand what they do back there, but it all seems like magic to me! Many steps, and focused attention to multiple processes must be taken to be successful. Make sure any supplier you partner with is well versed in these technologies.

The final concern, as related to plating, applies to all boards—not just high performance PCBs. Features are forever shrinking, particularly holes that require plating. This can be problematic in that smaller holes are prone to trap air bubbles inside the hole barrels, which create voids where the plating is prevented being deposited on the hole walls. Four factors must be present and religiously controlled for even, consistent hole plating: filtering, agitation, aeration and circulation. Filtering prevents contaminants from affecting plating purity and integrity. Tanks and filters must be dutifully monitored. Agitation occurs when the panels are moved, mechanically, sided to side within the tank to help push out any trapped air bubbles and to ensure even plating inside the holes. Circulation and aeration pumps make sure the water moves briskly and continually circulates, also helping to push out tiny air bubbles and aid in even plating.

Here is the gist of what I hope you will take away from my ramblings this week. Plating is an uber critical process when manufacturing High Performance RF/MW Multilayer PCBs. So be sure you engage with skilled and seasoned experts who frequently and successfully produce these types of boards.

Otherwise, you may end up feeling like you’ve been helplessly tossed onto a Slip ‘n Slide….downhill…on asphalt. Yikes!!!