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

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? Let’s look at a few success stories.

Gold Conductors
Flexible circuits are commonly fabricated on polyimide dielectrics with copper conductors. Fabricators purchase the raw material as laminate with rolled annealed copper bonded to the base dielectric. Interestingly, I have been seeing an increase in demand for gold conductors on flex materials rather than copper conductors. This combination of materials provides a much more biocompatible solution for medical applications. Exciting, right? But this significantly changes the manufacturing process and moves this design into something “out of the ordinary,” which needs to be accounted for and understood in the early design phases.

To meet this requirement, the fabricator pivoted from subtractive etch processing to additive PCB fabrication technology. With subtractive etch, as the name implies, the unneeded copper is being etched away from the panel, creating the circuit pattern. While this additive technology is more commonly used for copper conductors with very fine feature sizes (30 microns or below), the additive process first removes all the copper from the panel and then adds back the conductive metal to form traces. Consequently, that conductive metal is no longer limited to copper, opening the opportunity to meet this requirement of gold conductors on polyimide. Although the additive process is most often used for fine feature sizes, it can also be applied to larger traces. In this case, the minimum feature size was a 75-micron trace and space.

The challenge with this new process is just that—it is new. There are not decades of experience and quality specifications to rely on, at least not yet. I think designers and fabricators are very good at communicating design requirements via the fab drawing for standard technology. But with something new, in my opinion, the way to be successful is to communicate not only through the fab drawing but with actual conversation. In the recent applications with gold conductors, not only were there discussions before finalizing the design, but there were also conversations throughout the fabrication cycle, bringing all parties together to discuss the progress and results and talk through any recommendations for process and design improvement for future applications.

Complex Rigid-Flex
Another story I would like to share involved a complex rigid-flex design. To some, complex rigid-flex designs are something they are well versed in, but to others, this is a new technology. To provide some context, in this particular application, there was the need for mixed materials, unbalanced copper weights, and no less than 10 different flex areas, with only selective layers going into those different areas.

As is often the case, the team working on this had competing needs that were driving this complexity. Thankfully, understanding that this was quickly becoming a more complicated design than had been done previously, this team reached out to the fabricator for advice and review before jumping in and working on the PCB layout. Again, communication was key to the success of this highly complex design.

There were many conference calls pulling in the full group and the fabricator until the material configuration and layout concept were adjusted to something that was both manufacturable and met the overall design objectives. It would have been a costly mistake to have completed the design and sent it for fabrication without fabricator input. Fabricators are a significant source of knowledge and are happy to share that knowledge early in the design phase.

The Power of Communication
I share these two success stories to illustrate the power of communication, particularly conversations between design and fabrication. During the recent “Just Ask Tara Dunn” series, one of the questions that I was asked was “Why don’t flex and rigid fabricators provide more feedback to designers, especially if it’s not good design and engineering work?” That is a powerful question that shines a light on the opportunity to improve communication to the benefit of both fabricators and designers.

I cannot imagine a scenario where a designer would not want feedback that would improve the overall design and improve manufacturability. I also cannot imagine a scenario where a fabricator would not be happy to provide that feedback. What I can easily imagine is the impact this improved communication could have on circuit performance, yields, manufacturability, and time to market.

This is an exciting time in the industry. Semi-additive and modified semi-additive PCB processes are being implemented, opening fabrication capabilities that were not previously available, with a significant impact on PCB design. New materials are being released at a rapid pace. The advancements all provide benefits but also come with a learning curve.

My recommendation for successfully navigating that learning curve is to start a dialog with your fabricator as early in the design process as possible. Reach out to learn more about the process or materials that are not familiar. What is the best way to add those requirements to a fabrication drawing? What are the potential challenges to manufacturing with these new processes or materials? How can the design maximize these benefits?

Learning, understanding, and working together will save time, money, and likely plenty of headaches with technology that is new and “outside of the box.”

This column originally appeared in the December 2020 issue of Design007 Magazine.