When discussing RF/MW PCBs, starting with base materials seems like a logical place to start. However, the topic of advanced circuit materials is … well … complicated, especially for a single blog post. I’m sure this is obvious to you, but it took me the better part of this week to come to this conclusion with the help of Dale Doyle of Rogers Corp. and Denis Boulanger of Ventec. (Thank you both for your help, and graciousness!) In the end, I have resolved to leave the “heavy lifting” to the experts. Rogers, Taconic, Arlon, and Isola all have information-rich websites and employ amazing professionals like Dale and Denis who are invaluable resources (as are certain industry blogs).
Nevertheless, I did discover that I have a thing or two to contribute when it comes to this subject, as it is related to printed circuit boards.
There are a wide variety of high performing substrates on the market ideally suited for RF/MW applications. At Transline, we use all of them — because you specify them on your blueprints. In fact, we stock almost every part number of Rogers material, and many of Taconic and Arlon and a few Isola. We do this to shorten lead times and because approximately 60% of our business is in the RF/MW industries. Due to our fluency with these materials, I feel qualified to give you a snap shot of what happens after your order hits our shop floor.
First off, RF/MW materials act NOTHING like FR-4 materials in our manufacturing process! They don’t even behave like each other or one part number to another, or one material supplier to another. That is because they are all made differently and have unique compositions: Teflon, ceramic, duroid, PBD, hybrid mixes, and so on. Further, some are reinforced, some aren’t. Some are reinforced with crushed fiberglass, some with woven fiberglass. The highest-performing materials, with no reinforcement, can have dimensional stability issues so severe that they make your board fabricator want to start parking cars for a living.
A capable, qualified RF/MW PCB manufacturer must be a virtual guru when it comes to materials. They must be experts at knowing how each substrate brand, each composition, each part number, at each copper weight and thickness responds to … (taking a big breath) … etchant, plating chemicals, heat, lamination, moisture, and a whole host of processes met in fabrication. These laminates can be moody and fragile … nothing like good old predictable, robust FR-4. So, just as a good RF/MW engineer brings some art and magic to the science of their design process, so it is with the board manufacturer.
Why is this important to know? Because many an excellent PCB fabricator has made the innocent, though faulty, assumption that because they can make extremely complex boards with FR-4, that this RF stuff will be a cake walk. They may have even enjoyed success with some RF boards made on a specific material, but unable to succeed on another. (Shortly thereafter is when you get that embarrassed phone call informing you that they can’t make your boards after all.)
What I am proposing here is that RF/MW PCB manufacturing is a specialty, just as RF/MW engineering is a specialty within the general discipline of electrical engineering. Far too many PCB suppliers and engineers appear to lack this awareness. Why do I believe this? Because I work with RF engineers daily who have the scars to prove it! I believe this because after having 16 years of experience working with very complex FR-4 boards, and a one year working with RF/MW boards–I still feel like a rookie when it comes to RF boards. I also hear evidence from materials suppliers and buyers. I hear it from engineers on LinkedIn. It is for these reasons that I was compelled to create this blog.
So, here are a few possible solutions I hope may be helpful:
When you evaluate a new RF/MW board supplier, consider asking what percentage of their business is RF/MW, and how long they have been doing RF/MW PCBs? Which materials are they accustomed to working with? Ask questions about their quality and test records that verify their ability to successfully hold the tough impedance tolerances you may expect. Ask for RF/MW customer references. Ask your substrate rep for recommendations — in some ways, I think they have the best seat in the house, often offering some much-needed objectivity.
My advice is this: Don’t rush, headlong, into a relationship with a new supplier because they can save you 10%, because by doing so they may, unwittingly, cost you far more — like the loss of an important customer. Think more along the line of long courtship and marriage, rather than one-night stand in Vegas (a tall order when we are all so price driven!). Finally, look and listen for signs of true expertise. Look for that rare mix of knowledge, skill and experience mingled together with a twist of art and magic.
Blogs are designed for dialogue, so please offer your feedback and comments. If you have more ideas or input on this topic, please share it. We have much to learn from one another and I look forward to hearing from you!