Flex Talk: Demystify Flexible Stack-ups

The sheer number of flexible laminate materials and constructions can be a bit daunting for those new to flex and rigid-flex design. I sat down with Jeff Martin from Omni PCB for his insight into flexible laminates and his advice when working on a flex stackup. 

Tara Dunn:  Hi, Jeff, thank you for taking time to speak with me today. Before we dive into today’s topic, can you give a brief introduction to what you are doing now and what your background is with flex and rigid-flex? 

Jeff Martin: Hi, Tara. I am currently the chief technology officer for Omni PCB. I provide our customers with a high level of negotiation and project management support, technical consulting, and industry insight. I have been in the printed circuit board industry for 25-plus years. A number of those years were working as a front-end engineer and project manager for flex and rigid-flex manufacturing. 

Dunn: That is a long history with flex materials. For the many people who are new to designing with flexible materials, can you give us a basic understanding of flexible laminates?   

C_Dunn_Jeff_Martin.jpgMartin: Absolutely. Flexible laminates come in a variety of materials. Probably the most used and what people typically think of when starting to design a flexible circuit is polyimide dielectric. Breaking that down, the laminate will consist of a layer of rolled-annealed copper bonded to the polyimide and this is done in two different ways. Adhesive-based cores are made up of a polyimide core, a layer of adhesive and a layer of copper which is bonded together using heat and pressure. The adhesive commonly used is either epoxy or acrylic. A typical adhesive thickness is one-half to 1 mil. Adhesiveless based cores have the copper directly attached to the polyimide core without the use of any adhesives. Adhesiveless cores are typically recommended for applications that have a thickness constraint and also when working with rigid-flex constructions.   

Dunn: If the adhesive thickness is one-half to 1 mil, what thicknesses are available for the copper and polyimide? 

Martin: Although you can likely get nearly any combination of dielectric and copper thickness, the laminates will typically have some combination of half-ounce to 2-ounce copper, with anywhere from half-mil to 5-mil polyimide. When you factor in options for adhesiveless construction or adhesive-based, with or without flame retardant formulations, there is a wide variety to select from. 

Dunn: I imagine that can be overwhelming for those new to flex design and trying to select the best materials. Are there certain materials that are more common than others and likely to be in stock with a fabricator?     

Martin: This one is a little difficult to answer because it really does vary from fabricator to fabricator, but in general constructions using half-ounce and 1-ounce copper and 1 and 2 mils of adhesive are probably the most stocked materials. If a fabricator builds a lot of rigid-flex, they may be more likely to stock adhesiveless materials. It is risky to assume that materials may be in stock, so working with your fabricator as you are creating the stackup to understand what is in stock is very beneficial. If you are able to use materials in stock, that can greatly reduce your lead time and if you identify early in the process that there is a material that does need to be ordered, that material can be pre-ordered while the design is being completed. 

Dunn: I agree, working with your fabricator early in the design is always a good idea. Flexible circuit stackups also need to include any polyimide or FR-4 stiffeners that are being used. Can you explain the function of each? 

Martin: Sure. Stiffeners are used to create a support area where the flex will not bend and will support components and connectors. An unclad piece of FR-4 is a common solution. Another common type of stiffener is polyimide. This, too, can add support to component areas and is also often used to build up the termination area with ZIF terminations, thickening that specific area without increasing the overall thickness of the flex.   

Dunn: We have been speaking primarily about flexible circuits, what are the key things to keep in mind regarding materials and stackups for rigid-flex designs?  

Martin: First, as I mentioned earlier, check with your board manufacturer to see what material they stock. This will help when you are considering your stackup. With rigid-flex it is very important to keep acrylic adhesive outside of any rigid area that will have plated through-holes. So, selecting adhesiveless flex materials, and using bikini cut with the coverlays is highly recommended. From there, keep in mind the form, fit and function of the circuit. How will it mount? Where will it flex? How often does it flex? What will it plug into? And so on. 

Dunn: I often share my experiences with the “flex that didn’t flex.” Do you have any flex war stories and lessons learned you would like to share? 

Martin: I honestly do not have a recollection of one.   

Dunn: Wow, that is a great track record after working with flex and rigid-flex for 25 years. As we wrap up, please share your best advice regarding flexible circuit stackups.   

Martin: As I have mentioned a few times, I think my best advice to people new to flexible materials, or even those experienced flex designers, is to get in touch with your board house early in the process. Find out what materials they stock, all the way down to the adhesives they stock, and work with them to dial in the stackup. They are the experts and that simple step can prevent a lot of headaches later in the design. 

Dunn: Great advice. I couldn’t agree more. Thanks, Jeff, it has been great talking with you today and I am sure this will be informative for designers just getting started with flexible materials.

This column originally appeared in the January issue of Design007 Magazine

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2021

Flex Talk: Demystify Flexible Stack-ups

01-26-2021

The sheer number of flexible laminate materials and constructions can be a bit daunting for those new to flex and rigid flex design. Tara Dunn sits down with Jeff Martin from Omni PCB to hear his insight into flexible laminates and his advice when working on a flex stack up.

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2020

Flex Talk: Communicating Outside the Box Is Key to Flex DFM

12-17-2020

What do you do when you are designing a flexible circuit and need to go “way outside the box” to get the desired end-result? Tara Dunn looks at a few success stories, including gold conductors and complex rigid-flex, and emphasizes the power of communication.

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Flex Talk: Communication Goes Both Ways

09-30-2020

Technology, whether new to you or new to the industry, both require communication between the end-user and the fabricators. Tara Dunn explains how a collaborative approach benefits not only the end-user seeking the information but also the fabricator that will be providing the technology.

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Flex Talk: The Black Magic in the Business

08-25-2020

When you work with flex or rigid-flex, the communication between designer and fabricator needs to be impeccable, and the primary method of transferring information is through the fabrication notes. Tara Dunn gives a recommendation that, although not strictly required by the fabricator to build the product, will certainly benefit the end-user.

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Flex Talk: Additive and Subtractive—When Opposites Attract

07-27-2020

Market dynamics in the electronics industry are quickly changing. Some solutions add considerable cost to the PCB and often introduce reliability and yield concerns. Tara Dunn explains an alternative that has been installed in three U.S.-based PCB fabrication facilities: the A-SAP™ process, which is Averatek’s semi-additive process.

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Flex Talk: It’s the Little Things

05-15-2020

Let me share a personal moment. Shortly after we started staying home and socially distancing, I came down the stairs from my home office and noticed something colorful on the sidewalk. Keep in mind that I live in Minnesota, and anything colorful is extremely eye-catching while in stark contrast to the winter grass and leafless trees. Looking closer, someone had colored big, beautiful hearts all along my sidewalk and driveway, leaving a fun message at the end of all those colorful hearts. That simple act of kindness made me smile for days, anytime I looked outside and saw the chalk drawings and messages. It also reminded me that in the midst of uncertainty and life being unsettled, one small thing could make a significant impact.

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Flex Talk: Unintentionally Adding Cost?

04-16-2020

When designing a PCB, rigid or flex, there is a continuing series of choices and trade-offs, some with greater cost impact than others. Tara Dunn discusses a few things that can easily be overlooked when putting together the puzzle pieces of a design and that may have an unintended impact on the overall cost.

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Flex Talk: Mina—Enabling Soldering to Aluminum

04-15-2020

Averatek recently launched Mina™, a chemistry that offers exciting benefits over traditional methods of soldering to aluminum. Tara Dunn had the opportunity to speak with Divyakant Kadiwala, director of manufacturing at Averatek, to learn more.

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Flex Talk: Additive PCB Technology for Next-generation Electronics

02-18-2020

Semi-additive PCB processes help to enable very fine features, with trace and space down to 25 microns and below, significantly reducing space and weight for next-generation electronics. Tara Dunn speaks with Todd Brassard and Meredith LaBeau from Calumet Electronics about how the company is the first domestic PCB manufacturer to license Averatek’s A-SAP™ process and will be presenting information on the industrialization of this process at this year’s IPC APEX EXPO.

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Flex Talk: The Challenge of Change

01-20-2020

I recently kicked off a presentation on flex and rigid-flex by asking for a show of hands of those who had never worked with flex materials or considered themselves to be just learning how to design with flex. Over half of the room raised their hands, which excited me because I could help them learn something new. In my opinion, the best way to lessen the challenges and uncertainty of change is to arm yourself with as much knowledge as possible.

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2019

Flex Talk: Additive Electronics Momentum

11-14-2019

I have been involved with additive electronics for the past several years, and I have seen the discussion of and demand for sub-75-micron feature sizes slowly grow. Conversations, questions, and research about SAP and mSAP increased significantly when it was announced that the mSAP process was used to create the circuitry in the more recent versions of our smartphones. While this process is available in very high volume in some areas of the world, it is still in the early stages of development in other areas.

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Flex Talk: Don’t Build Flex That Doesn’t Flex

10-10-2019

One of the primary advantages of moving to a flexible circuit design from a rigid board is the ability to package the flex in three dimensions, bending or folding into imaginative configurations and saving precious space in the final package.

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Flex Talk: When You Do Everything Right and Something Still Goes Wrong

08-02-2019

This industry is full of tales describing the work and effort needed to overcome fabrication hurdles to produce a complex design. Tara Dunn shares a case study of one of those types of designs.

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Spark an Idea

07-11-2019

One of the favorite parts of my job are the days when I meet with a group of engineers and designers to talk about flex and rigid-flex. We might do a "lunch and learn" with a general overview of the technology or address a specific challenge. It is always helpful to bring samples to pass around and show different features. Usually, looking at a sample will spark an idea and the comment, "I wonder if we could do something like this.” From there, the brainstorming begins.

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Flex Talk: New Materials or New to You?

06-10-2019

There are so many new processes and materials in the PCB segment that it can be a challenge to keep up with all the new developments. It is fun to start chasing the next new thing, but it is important to keep in mind that even materials and processes that have been around for a while are still new to someone.

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Flex Talk: New Engineering Talent Joining the Electronics Industry

05-27-2019

Last spring, Ross Olson, an undergraduate student at the University of Minnesota and member of the U of M Solar Vehicle Project team, attended the Geek-a-Palooza event in Minneapolis and displayed one of their race cars. Tara Dunn had the opportunity to get to know Ross, and recently sat down with him to talk about his interest in engineering and his thoughts on the future of the electronics industry.

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Flex Talk: Old-fashioned Networking

05-08-2019

We live in a connected world. Information is collected at an astonishing rate, and people are working diligently to put this information to good use. It is new, fun, and exciting. But I sometimes wonder what is going to happen to the good, "old-fashioned" networking. Not networked devices, but the act of going out and meeting people in our industry, learning about their story and expertise, and sharing yours—mutually beneficial sharing of information and resources.

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Flex Talk: FlexFactor—Imagination and Innovation

01-23-2019

The ultimate goal of FlexFactor is to create a generation of students who use their critical thinking, creativity, communication, and collaboration skills to create the materials and devices that will address and mitigate the biggest challenges of the future.

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2018

Flex Talk: The Myth About Rigid-Flex Costs

12-12-2018

Do you cringe when you think of the option of rigid-flex? It is not an uncommon reaction when talking with designers and engineering managers about using rigid-flex to solve a packaging problem. Why? The most frequent answer is, “They are so expensive.” While it is true that a rigid-flex PCB is typically more expensive on the surface when compared to rigid-board solutions with cables and connectors, a lot is being missed with that mindset.

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Flex Talk: Mina—RFID, LED and What Else?

09-17-2018

“The science of today is the technology of tomorrow.” This Edward Teller quote is an apt description of the Mina product. This advanced surface treatment, recently developed to enable low-temperature soldering to aluminum in the RFID market, is not only finding success in that market, but quickly finding a home in other markets, including the LED market, where the incentive is both cost and improved LED performance.

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Raising the Capability Ceiling: SMTA Upper Midwest Chapter Expo

06-25-2018

An energetic and engaged crowd filled the recent SMTA Upper Midwest Expo & Tech Forum. The event, held in June 14 in Minneapolis, Minnesota, hosted 57 exhibiting companies and had over 100 pre-registered attendees. The underlying theme for the technical presentations was "Raising the Capability Ceiling!" Here's a wrap-up of the event.

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Flex Talk: E-Textiles—the Wild Frontier

05-23-2018

How many hours is your car sitting idle outside in your driveway or a parking lot? What if your car was used for solar harvesting—converting heat to energy? What about biometric sensors in automotives: skin sensors for preventing DUI, posture identification to monitor driver fatigue, monitoring exposure to hazardous materials in a load for truck drivers.

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Flex Talk: Something New for Everyone

05-02-2018

Whether you are new to single- and double-sided flex, moving into rigid-flex construction, thinking of using bookbinder technology, or investigating an additive process, working with new technology can be both exciting and challenging.

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Flex Talk: Invited Guests to the Party

04-16-2018

Hanging on the wall in my office is this quote from Jeff Bezos: “We see our customers as invited guests to a party, and we are the hosts. It’s our job every day to make every important aspect of the customer experience a little bit better.”

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2017

FlexFactor: Faith in the Future

11-27-2017

Take just a minute and read through this list of new product ideas. Can you identify the common thread? Yes, they are all enabled by advanced technology, but would you believe that these products were all pitched in the last year by high school students?

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Flex Talk: The Man Behind the Curtain

10-03-2017

“Pay no attention to the man behind the curtain.” This famous quote from The Wizard of Oz conjures up the image of Dorothy, the Tin Man, the Cowardly Lion and the Scarecrow discovering that the great Wizard of Oz isn’t as grand or as magical as he seems.

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Flex Talk: Knowledge is Power

08-31-2017

“What can I do to help drive cost from my design?” This is a question that I am asked routinely. That question is often followed by, “Can I get these faster?” Both questions are even more predominant when talking about flexible circuits or rigid-flex.

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Flex Talk: Mina—Trouble-Free Soldering to Aluminum

06-22-2017

Thinking about the RFID market and the significant growth projected in this market, I decided to do a little research on RFID tag manufacturing. During this research, I learned of a relatively new offering, Mina, an advanced surface treatment technology that addresses the common constraints of large scale manufacturing of aluminum on polyester (Al-PET) circuits.

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Flex Talk: Squink—Integrating Fabrication and Assembly in one Package

05-01-2017

When walking through trade show expos, I tend to be drawn into product demonstrations on the show floor. Recently, at the IPC APEX EXPO, I stopped in front of a piece of desktop printing equipment that was demonstrating with a flexible circuit.

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Flex Talk: Flex Material Handling—An Inside Peek

04-03-2017

As increasingly more designs move to flexible materials to take advantage of space, weight or packaging benefits, it has been clear that flexible circuits require a different set of rules than their rigid counterparts. We spend substantial time working through the design to ensure that he flex is as robust as possible.

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Flex Talk: Final Surface Finish—How Do You Choose?

01-26-2017

There are so many final surface finish options to choose from today. How do you decide which is best? HASL—both tin-lead and lead-free—immersion tin, immersion silver, ENIG, OSP, and ENIPIG are the primary finishes used in PCB fabrication.

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2016

Flex Talk: A Glimpse into PCB Sales

12-28-2016

Summarizing the feedback from both customers and manufacturers, the most successful PCB salespeople are organized, take a genuine interest in their customers’ needs and business challenges, have a better than average understanding of the PCB industry, fully understand the manufacturer’s strengths and capabilities and advocate for both to find the best solution.

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Flex Talk: Troubleshooting Flex Circuit Applications for Mil/Aero Projects

10-06-2016

I imagine that everyone has been in this position at one time or another: Despite everyone’s best attempt at creating the perfect design, PCB fabrication and assembly, something goes wrong and the troubleshooting begins.

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Flex Talk: Inaugural West Coast Geek-A-Palooza a Fun-filled Success

05-24-2016

Geek-A-Palooza kicked off the 2016 schedule May 12 in Irvine, California. Historically, Geek-A-Palooza has been held in Minneapolis but is expanding this year to include Orange County and Boston as well.

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Flex Talk: PCB Sourcing? One Size Does Not Fit All

04-18-2016

When analyzing a set of PCBs to improve yields and maximize profits, the first place to start is with a critical review of each PCB design. Are there any attributes that are pushing your manufacturer’s standard design rules? If so, is this necessary to the design or is there another approach that could improve the manufacturer’s yields, reduce cost, and ultimately increase profit?

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2015

Flex Talk: Thoughts on the IPC Flexible Circuits–HDI Forum

11-19-2015

As an attendee at the IPC Flexible Circuits–HDI Conference held October 28–30, I found myself in a room of people, all eager for technical information, with the opportunity to reconnect with industry friends and to make new connections. The audience was diverse with young people, new to our industry, sitting alongside industry veterans willingly sharing their knowledge and passion for HDI design and flexible circuit technology.

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Designing Flex Circuits for Domestic Prototyping

08-20-2015

Designing a flex circuit to be prototyped domestically? No problem. Designing a rigid-flex circuit for production offshore? Got it. Designing a part that will be prototyped domestically with a seamless transition to offshore production? That can be a little more challenging.

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Rigid Flex: Total Cost Comparison

07-09-2015

The transition to a rigid-flex design from the traditional approach of using cable assemblies to join two or more PCBs has obvious benefits—space, weight, packaging, reliability and increased currently carrying capabilities. Yet many times the perception that rigid-flex is a high-cost solution causes designers and engineers to hesitate.

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Primary Cost Drivers for Flex Circuit Designs

06-25-2015

Someone once told me that the potential applications for flexible circuits are really only limited by our imaginations. After pondering that a bit, I had to agree. In fact, one of the things I like best about what I do is that moment during a discussion when I can see the light bulb go off in a designer's head.

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The Flex-to-Fit Approach

06-04-2015

The flex-to-fit concept reminds us that creativity and engineering go hand-in-hand. When there is not ample space for a conventional approach, this process, which is the convergence of the mechanical world and the electronics world, results in the ability to design a flexible circuit along the contour of an existing, irregularly shaped structure.

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