Getting a Handle on Flex Cost Drivers

Judy Warner: You've been in the board fabrication business for over 20 years. What factors do you see as contributing to the rise in flex and rigid-flex designs?

Tara Dunn: I believe that space, weight and packaging are the primary factors driving designs to flex and rigid-flex technology.  For customers that are already using flex and rigid-flex, I am seeing an increase in their percentage of new designs using flex and I am also noticing an increase in the frequency of requests that I receive from people that are new to flex technology and looking for guidance.  Overall, electronics are becoming smaller and lighter, and for those that are not becoming smaller, we continue to increase the density and functionality of electronics within a static package size.

Warner: What are a few key factors designers need to be aware of that drive cost and reliability?

Dunn: There are three primary drivers of cost:  materials, panel utilization and technology.  I also like to include a fourth: understanding your fabricators capabilities and making sure that your design fits in their sweet spot for manufacturing.  Understanding and utilizing the most commonly stocked materials will reduce cost and lead time. The most common materials are a combination of 1/2 oz or 1 oz copper, with 1 mil or 2 mil polyimide.  There are many other options with laminates build with 1/2 oz to 2 oz copper and 1/2 mil to 6 mil polyimide. Understanding the common materials stocked at your specific fabricator will assure that material is available and that you are not incurring extra expense for minimum lot charges of unusual materials.

Flexible circuits are often designed with unusual shapes; review designs for the opportunity to reverse nest parts and adjust to best utilize the manufacturing panel.  The two most common panel sizes for flexible materials are 12" x 18" and 12" x 24". Either size typically requires a 1" border on all sides for coupons and processing holes.  If provided a 1-up with no array requirements, your fabricator will work to nest parts and maximize material utilization. If you provide the array, keep in mind both the material utilization with the array and how that array will fit on the production panel.  

Also, as the technology level of designs increases, work with your fabricator to ensure that you are not adding unnecessary cost.  As an example, it is entirely manufacturable to build a rigid-flex with plated through holes in the flex portions. But, moving the vias outside of the rigid area of the stackup results in added processing and added cost to your design.

When looking at reliability from a design perspective, the devil is in the details.  Be aware of bend radius guidelines, add tie downs to anchor the pads and prevent lifting.  Via locations are critical, avoid placing them close to fold or stress points and mechanically reduce stress points.  Route conductors uniformly, perpendicular to the fold lines, and avoid sharp angles.

Warner: How did you learn about flex and rigid-flex technology, and where would you suggest designers go to learn more about the design, fabrication and assembly of flex products?

Tara Dunn - President, OMNI PCB

Dunn: It is a funny story actually.  I landed in the electronics industry by accident, not knowing anything about flexible circuits other than a vague reference to the cable in most printers. I started in the industry learning about flex fabrication and design before I was introduced to rigid board technology and was fortunate to have the opportunity to get some hands-on experience with the manufacturing of flexible circuits as I was just getting started in the industry. I would recommend that anybody designing flex take the opportunity to visit their fabricator and really understand the differences in and challenges of fabrication with flexible materials. I also think it is important, even for experienced flex designers, to attend industry conferences, such as the AltiumLive event, IPC events, SMTA events, etc. Technology is developing at a rapid pace and the more we know about flexible materials, the more opportunity there is to incorporate them into your designs.

Warner: When looking at a potential fabricator for flex, what types of things help identify a capable supplier?

Dunn: At the surface level, many of these things will be the same types of things as a rigid board supplier.  It is important to look at the quality system in place, equipment set and capabilities, and certifications.  Specific to flex and rigid-flex, I think one important factor is to look at the percentage of flex and rigid-flex that the supplier is doing.  Manufacturing flexible materials requires specialized processing and handling. This expertise takes time to develop and should be part of standard processing.  It is also important to evaluate the level of technology that the supplier can offer and make sure that is a match with the technology you expect to use. For example, there are fabricators that specialize in single and double-sided flex and fabricators that specialize in blind, buried and microvia technology with flex and rigid-flex.  Make sure you are looking for the best match. One last key item I would recommend to identify a capable supplier would be to take a look at their applications engineering experience and response time. There are often questions during the design phase and it is important to have knowledgeable and speedy responses.

Warner: Do you have any comparable advice to identify capable assembly partners?

Dunn: I think the same things are important when identifying a capable assembly partner as a capable fabricator.   Not that many years ago, it was difficult to find an assembly partner that would offer flex and rigid-flex assembly.  It is more common today and understanding and evaluating the experience level of your assembly partner is highly recommended.  

Warner: What is you favorite and least favorite aspect of working with flex and rigid-flex boards?

Dunn: Someone once told me that flex designs are really only limited by our imagination. That has always stuck with me. One of my favorite things about working with flex and rigid-flex is when I am sitting down with a designer going through flex samples and you can see that light bulb moment where something clicks and the ideas for solving their packaging issue start flowing.

I am not sure there is anything that I don't like about working with flex and rigid-flex, but if I have to choose something, it would be that because the materials are less predictable than we are used to with rigid boards, it is more difficult to predict the performance of the design until the first parts have been tested.  A design may be more difficult to fabricate than expected or there is an unknown stress on the flex which introduces cracking. The development cycle can take a bit longer than with a rigid board. Even through that process, there is always something to learn as we are problem-solving and that knowledge can be applied to all future designs.

Warner: If you could only give one key piece of advice to PCB designers relative to flex design, what would it be?

Dunn: That is easy: work with your fabricator as early in the design process as possible. As an example, each fabricator is going to have preferred materials that you will want to incorporate into your stackup to help keep cost down and lead time short.  Beyond that, there is a wide range of fabrication capabilities throughout the industry. Understanding your fabricators capabilities and working with them on your design will ensure the highest yielding design possible. Fabricators work with flex day in and day out; learn from that knowledge!

Warner: Thank you so much, Tara! I learn something every time we talk and I trust our readers will really appreciate your insight.

Dunn: My pleasure, Judy! Thanks for your time and interest.