Keepin’ it Smooth: How Surface Roughness Impacts High Performance PCBs

First a disclaimer: I am not an electrical engineer. I am only fluent in my knowledge of PCB fabrication. In other words, much of what I am about to share is borrowed from well- educated and experienced EEs who don’t share my aversion to math. I am, in effect, jumping into the deep end of the pool, but only because I am wearing my (virtual) brain floaties! For this reason, I ask for your forbearance as I attempt to translate this left-brained subject matter into my right-brained mother tongue. Here we go…

Substrate copper application methods. I have been discussing surface finishes for the last couple of posts, and I would be remiss if I didn’t cover the crucial topic of copper surface roughness and how it specifically impacts high performance PCBs. Most substrates are copper clad with either rolled annealed (RA) copper, electrodeposited copper (ED) or reverse treated copper (RT). I have put some links below should you want to learn more about each type of copper and the resulting surface roughness of each.

When the copper surface is rough, even at microscopic levels, the effective conductor length grows and the resistance increased as the signal must move up and down with the topography of the copper surface. To the naked eye, copper-clad substrate appears very smooth but when you view the surface under magnification, the copper can look like something akin to the Himalayas! For this reason, some choose RA copper that is both smooth and consistent in thickness. RA can be more costly, however, and not an option for all. ED copper has the roughest surface but, depending on the application and speed requirements, may be perfectly adequate. RT copper is smoother, and doesn’t cost more than ED. Yet with RT copper you need to be careful about potential delamination and poor peel strength. Once again, we are back to trade offs and specific design needs!

Rogers has a “LoPro” laminate series that is extremely smooth while Taconic offers an extremely smooth “reverse treated/ED” clad laminate. These laminates aid in good, sharp etch definition as well. In some cases, these are smoother than RA copper and can be less costly.

Skin Effect Considerations

  • Michael Ingham of Spectrum Integrity shared the following illustrations with me in regards to the skin effect and how a signal flows through a conductor. In the first illustration, he shows a rough cross-section of how a very high frequency signal tends to run on the outermost areas of a conductor—creating the skin effect. However, he notes that this is only true when there is not a ground plane underneath.
  • In the second and third simulation illustrations he used a full 3D field solver for a top layer trace that has a ground plane beneath.
  • The second illustration shows how a current will flow through the entire trace cross section area at low frequency.
  • Finally, in the last illustration, he shows the current distribution at 20GHz, which is mostly at the bottom and sides of the trace.

Fig 1 Current Flow

Michael’s point is that when it comes to surface finishes and texture, the most critical issue appears to be the smoothness of the copper. He has raised the question that if the high-frequency current mainly flows on the bottom and sides of a trace—is using nickel really causing the unwanted losses, or is copper roughness the culprit? Many fear using ENIG-plated traces and go to great measures to avoid and resort to using costly mixed plating, etc. Below is measured data of a RF trace with standard ENIG plating. The overall loss may be surprising.

Fig 2 Insertion Loss

This is just scratching the surface (no pun intended!) in regards to the topic of conductor losses and copper profile. When dealing with coupled trace structures, the effects of nickel could have a big impact due to having adjacent trace walls interacting. But I will leave this topic for Michael to cover at another time!

Spectrum Integrity has been very successful in high-speed/high performance designs, and has done so focusing more on design technique and paying great attention to all the properties of materials such as the smoothness of the copper rather than focusing on surface finishes or restricting to very low dielectric loss materials. They have enjoyed very good success with avoiding costly mixed plating and with the use of smooth copper laminates like the Taconic RT/ED material.

When it comes to the smoothness of outer layer copper traces, a board fabricator can go a long way to hurt or help your desired results! The higher the speed/frequency and more critical the application, the more you need to be working with a company like Transline Technology who understands high performance board fabrication. For instance, when we clean the outer surface of the boards, we do it with chemical cleaners that are non-abrasive and maintain the smooth copper outer surface. Additionally, there are many points throughout the manufacturing process where standard practices of PCB handling can also compromise the otherwise smooth copper outer surface that can create havoc for your design performance. High performance boards can appear deceptively simple in their design, but there are many intricate details that must be considered when making a sound high performance board. Many a board fabricator does not possess the knowledge or the trained eye for the subtleties of high performance PCB fabrication.

Conclusion. Skilled and well-informed partners are the key to success when it comes to choosing surface finishes and materials for high performance designs. These issues are complex and have many nuances that must be considered to create successful products. As such, it is critical to forge strong working relationships with both your advanced material suppliers and your board suppliers. By doing so, you will save much time and money and avoid a host of needless headaches. I have listed some additional resources below. Many thanks to Michael Ingham of Spectrum Integrity, who is always teaching me something! I highly recommend Spectrum Integrity for RF/MW and high performance design. Their website link can be found below. As always, I welcome your input and comments! judy@translinetech.com

I also invite you for lively discussion regarding High Performance board design and fab on Linked In: http://tinyurl.com/85ymddk

–Judy

 

Additional Resources

http://tinyurl.com/7he2lmv

http://tinyurl.com/7wnewcq

http://www.polarinstruments.com/support/si/AP8155.html

http://tinyurl.com/7yblg4m

http://www.spectrumintegrity.com/

http://www.rogerscorp.com/documents/1183/acm/RO4000-LoPro-Laminates.aspx

http://www.taconic-add.com/en–products–material-view.php

Getting the Right Finish, Before You Start

If you are a fan of John Wooden, the celebrated UCLA coach, you will know that he had more than his share of clever sayings. My husband, who played basketball all his life, and who went to college on a full-ride scholarship—nearly worships Coach Wooden. For this reason, I have heard many of these wise and witty Wooden-isms over the years. One that comes to mind is: “Be quick, but don’t hurry.” With the speed of advancing technologies, we all have to be quick, but we cannot afford to hurry—or be hasty—putting revenue and market share opportunities at stake.

In regards to RF and microwave printed circuit boards, there seems to be some confusion about PCB finishes and their affect on the high performance requirements of these applications. Too often, when considering the available finishes and the potential impact they have on performance, many engineers become both quick and (unwittingly) hasty when they make finish choices based on information, which is conflicted, at best.

I think it’s time to clear up some of these issues, so I am going to spend the next few blog posts talking about these issues. Hopefully, by the time I’m through you will have a much clearer understanding of finishes and which to choose for your product, before you start! I will be drawing from our real-world experiences, as well as looking to experts in the substrate and RF design industries.

Today, I am just going to cover the major available finishes, and which ones seem to be preferred by those with high speed applications:

  • Tin (Lead free)
  • HASL (Hot Air Solder Leveling) Tin/Lead 63/37
  • ENIG (Electroless Nickel Immersion Gold)
  • ENEPIG (Electroless Nickel/Electroless Palladium/Immersion Gold)
  • Hard Gold
  • Soft Gold
  • Immersion Tin
  • Immersion Silver

In high-speed applications, the prevailing wisdom suggests ENIG, ENEPIG, hard gold, soft gold and immersion silver are the best choices. Gold is a natural choice due to the fact that it does not oxidize and that it is wire-bondable. Immersion silver is gaining some traction due to the excellent conductivity, but it oxidizes and it is not wire-bondable, which keeps many from choosing this option.

Unfortunately, I must leave us barely posed in the starting blocks, in regards to finishes! In two weeks, however, I will sound the starting shot, and we will be off to the races. I will discuss each finish in more detail with the pros and cons of each.

If you have specific questions you would like to submit about this subject, please post them here in the comment section or email me at: judy@translinetech.com.

I’m looking forward to tackling this complex and critical subject together!

 

–Judy

Under Pressure: The Sticky Business of Laminating High Performance Multilayer Boards

Bummer! Now, I’m going have Freddy Mercury camped in my head all day singing “Un-dah Presh-ah.”  Well, let me see if I can drown him out with discussing the Wonderful World of PCB Lamination. Even I want to run screaming from my own brain after contemplating this topic for very long!

Many times as I have thought about the various challenges of manufacturing High performance boards, my mind is helplessly drawn to make a comparison to bread making. When you think about it—they are both born from chemically based processes and formulas, right? Okay, maybe I need to cut down on the Food channel, but stick with me anyway!  For instance, there is grocery store bread that is mixed in towering vats, and baked in loaves by thousands, and then there is Artisan bread. Mmm…that warm, wonderful, crusty bread, that is made in small batches by passionate bread makers.You know the ones; they waft with the tangy fragrance of things like fresh rosemary or garlic. These breads often boast of secret recipes.  Some areas of Italy have famous breads, whose bakers claim that it is the water, unique to that region that makes it special.

Regardless, both types of bread have value and a place in our lives.

In my visually, and apparently culinary driven mind it goes like this:

FR-4 boards = Wonder Bread 
RF/MW/High performance boards = Artisan Bread

With this in mind, let’s consider multilayer designs that include high performance materials—either on all layers or on selective layers.  What is required for successful production of these boards?  Once again, we need material gurus who are fluent in the knowledge of high performance materials and how they behave.  In this case, specifically, how they respond to lamination; because, as I’ve said many times, they all act uniquely.  Each high performance material comes with its own lamination profile, a recipe of sorts, which specifies the temperature rise rate, as well as the cooling rate.  These are provided to us by the material manufacturers, however, this recipe must be “tweaked” for the Press being used, and the environment in which it is being produced.  In other words—we are back to that intangible, though critical aspect of Art and Magic—like Artisan bread making!

A second concern, during lamination is the surface treatment of the layers.   All boards must be cleaned thoroughly and put through a scrubber prior to lamination.  This ensures that the surface is free of all contaminants and debris that would prevent strong, even adhesion or create de-lamination in the future.  High performance boards require special TLC at this stage of fabrication.  They cannot be treated like standard boards due to the material composition.  A little known secret, at this stage of fab,  makes all the difference—but if I told you what it was, I would have to kill you.  (Sorry, it’s part of our secret recipe!)

When standard multilayer boards are made they are “booked” by stacking the layers together with pre-preg placed between each layer to act as a bonding medium (epoxy-resin loaded fiberglass sheets).  When you have varying substrates on various layers, that formula goes out the window.  Farewell, Wonder bread.   Every high performing material has a corresponding bond ply that matches its properties in order to bond properly and also for performance reasons.  This information is supplied by the material manufacturers as well. It is critical that a RF/MW/High performance board manufacturer be current and well-versed on bond plies. Hello Artisan bread.

The last item of interest, when it comes to lamination, is the impact of the environment on High Performance materials. All substrates are somewhat impacted by humidity, this is especially true in some types of High Performance substrates.  They are very vulnerable to humidity and the environment. Awareness of which materials are most vulnerable and how to treat them are crucial to success.

It all comes down to this:  Only Master Bakers make Artisan Bread! Artisan bread makers are able bake Wonder bread, but beware of the baker who makes Wonder bread everyday and tells you he can make you a great loaf of Artisan bread!

Master Board makers successfully and consistently make RF/MW and High Performance boards because they have the knowledge, skill, experience and all the secret recipes that make for a top-notch high performance product. Therefore look for the qualities of a “Master” when you evaluate potential suppliers.  It will save you much time, frustration and headache if you do.

Bon appétit!

–Judy Warner

The ‘Hole’ Truth about Drilling PCBs

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!

Judy

Are You Living in a ‘Material’ World?

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!

Best wishes,
Judy

Getting ‘Mixed Signals’ from Your PCB supplier?

A few weeks back, I was having a chat with a national PCB broker who has a unique and broad perspective of the printed circuit board and electronics industry. We were waxing nostalgic about the “old days” and philosophizing about the days to come.

When she asked about our company, Transline Technology, I told her that we manufacture a wide variety of boards, but that our strength and focus lay in RF and microwave products, which account for about 60% of our business. I was bragging about our work and our customer base when she abruptly interrupted, “You know, Judy, not all board suppliers who say they can make RF boards really can.”

I was caught off guard and asked her to elaborate. She recounted several horror stories whereby she had placed RF/MW PCB orders for her customers and the suppliers failed, in one form or another, leaving her embarrassed and in search of a more qualified supplier.

This puzzled me. Although I have been in this industry for over 17 years, only this past year has involved RF/MW PCBs. Transline is a relatively small shop, and we are very successful with RF, so I assumed most others were as well. I wondered why, in some cases, much larger, far more well-recognized suppliers were failing? I tucked this question away for later consideration.

A week or so later, I was talking to the owner of a RF/MW design firm, a new prospect, who tends to take on very complex RF boards. It was like déjà vu. I was bragging … then interrupted … and the tales of woe poured out like an overdue confession on Sunday morning. This time I dug deeper. By the end of the conversation I had a new friend and customer.

Same week … different RF engineer … same story. Déjà vu, squared.

By the end, of these exchanges I was left with this conclusion: Not all PCB suppliers that say they can build RF/MW PCBs can!

So what makes RF/MW boards uniquely challenging to build? Why do some otherwise excellent board suppliers, have trouble with RF/MW PCBs?

I sat down with the owners of Transline, Larry Padmani and Chris Savalia, and asked them to share some of the inherent challenges in manufacturing RF/MW PCBs. They looked at me with cocked heads and compassionate smiles, as if a wee toddler just stumbled into their offices and asked them where babies come from. Clearly, these were loaded questions.

Their patient answers came and I soon experienced the proverbial sensation of “drinking from a fire hose.” It soon became clear that the answers were many and complex. I soon understood that there existed a chasm between most RF/MW engineers and their board suppliers that sorely need to be filled. It was then, I decided, I wanted to somehow help fill this gap … for a couple of reasons. Altruistically, I wanted to spare RF/MW engineers from needless suffering, before they found a well qualified PCB supplier. More selfishly, I knew if I could adequately inform the RF/MW community, they could more easily discern between a qualified and non-qualified supplier — and I knew it would become clear which camp we fall into.

So, I decided I would write an article for one of the trade magazines.(I may not be able to do complex RF calculations … but I can write!) I began by putting out questions to the RF/Microwave professionals via LinkedIn, asking what they would specifically like to know about RF/MW PCB manufacturing. There came a small flood of questions that continue to flow. I felt suddenly naïve and ill-equipped for the job!

Then two small miracles happened. I met a colleague who spent most of his career working for Rogers and Taconic on a nationwide scale. He offered his help in educating me on material properties and helping understand both the engineering issues and manufacturing challenges. His help, along with patient tutelage of owners of Transline, promise to bring me up to speed.

The second miracle was meeting Pat Hindle from Microwave Journal, who was commenting amidst the mix of my brewing discussions on LinkedIn. He pointed me to helpful resources and, he astutely observed, that this topic may be too broad for one article. In the weeks ahead I will take on the challenge of addressing the broad range of questions and issues surrounding RF/MW PCBs one-by-one. I know I will learn much, and hope to teach a little and help bridge the gap between RF/MW engineers and board manufacturers.

Keep the questions coming, both here and on the LinkedIn RF and Microwave community. I will do my best to address each one!

— Judy

http://www.translinetech.com/

Ed.: Judy Warner is director of sales and marketing for Transline Technology, Inc., in Anaheim, CA. She has been in the printed circuit board industry for nearly two decades. Her career began with Details, Inc. (later to become DDi). She was a top-producing sales professional for 10 years for Electroetch Circuits (later to become Tyco, then TTM). She also spent several years as an independent sales representative, including time as the owner of her own rep firm, Outsource Solutions.

Inverted QFN Land Pattern

Ever experienced the heartbreak of inverted land pattern? It’s not supposed to happen, but every now and then, it does. Maybe something happened when creating a custom footprint. Maybe, somehow it got
inverted in the CAD software and then placed on the wrong surface layer.

Maybe it was a subliminal attempt to make up for those giant open vias in the thermal pad. Who knows. But, it happened, so now what?

You could re-spin the whole board. Ugh. That’s, like, wasteful and stuff. Certainly, if this is a production build, you’ll have to re-spin. For some prototype applications, like if it’s a high frequency or RF thingy, you may very well have to get a new set of PCBs fabbed, too.

But, sometimes in the prototype world, you may be able to salvage the board run. We used to do stuff like this all the time with through-hole parts — need an extra chip, just dead bug hang it on up there.

Flip the chip over and use some small gauge wire — maybe wire-wrap wire — and hand-wire to the upside down chip. Gluing it down first may be helpful. Just keep in mind that since the thermal pad isn’t soldered to the board, you will lose some thermal performance. Maybe solder a small heat sink on it or something. And don’t forget to wire that pad to ground too (if it’s supposed to be grounded).

Duane Benson
Just put it on the seventh surface of your tesseract and it will fit right.

http://blog.screamingcircuits.com/