Okay, here we go, blog number 3; but first allow me to do a quick review of what we’ve covered so far:
1.) Not everyone who says they can make RF/MW PCBs really can.
2.) High performance substrates act NOTHING like FR-4 in the fabrication process, and a qualified supplier must be a “Material Guru.”
3.) Just as RF/MW engineering is a specialty, so is RF/MW PCB fabrication.
4.) Don’t be hasty in starting relationships with RF/MW PCB suppliers. Do your homework and ask important questions.
Now, moving along. Let’s talk about drilling holes. Automated drilling machines are incredible, when you think about it. The X-Y axis accuracy of hole placement, the throughput, and the speed of the spindles are all truly amazing! When drilling FR-4 material, the bits cut through material like a hot knife through butter. When you throw some Rogers PTFE, or Taconic in the mix, however, a dramatic shift occurs. The drill operators start throwing back Red Bulls, and all that mindless trust in the drill’s amazing technology vanishes.
Again, remember the Material Guru analogy: for every substrate brand, composition, thickness and copper weight, there is a specific recipe—in this case a drill recipe. (Thankfully, these recipes are supplied by the substrate manufacturers.) The speed of the spindles must be adjusted to keep them from tearing up the softer materials and leaving behind chewed up hole walls. The drill bits must be changed frequently to ensure optimal sharpness. The feed speed must be altered as well, to ensure a clean entry and exit of the drill bits. If you don’t have cleanly drilled holes with smooth hole walls, you will be in deep water once the boards hit plating (no pun intended).
In addition to these adjustments, talented design engineers continually delight us with their ever-so-complex designs that require multiple drill operations (due to buried and blind vias). Sometimes, back drilling or controlled depth drilling is required. All these factors serve to compound the, already complex, challenges. (Yes, there is laser drilling, but that comes with another set of unique challenges — and requires a separate post!)
Needless to say, drilling is a critical step in the manufacturing of RF/MW boards. If you mess it up in drilling, expensive laminates end up on the scrap pile, along with any hope a supplier may have of making a profit. So, here is what I hope you will take away from this brief post: Drilling RF/MW PCBs is dramatically different than drilling standard FR-4 boards. It requires knowledge, skill and experience. It naturally costs more (due to drill bit usage and added labor) and is far more risky, from a profit standpoint, for the supplier. It can be risky for you too, but only if you have inadvertently partnered with an unqualified supplier.
For all these reasons, when you get an opportunity to visit an existing or prospective PCB supplier, keep these things in mind as you ask questions about their drill operations. If you see wide-eyed drill operators, a heap of drill bits and Red Bull cans … you are probably in the right place!