Will All Thin Materials Flex?
Telling “war stories” about flexible circuit designs is something that can provide endless hours of entertainment when you get a bunch of “geeky” PCB people together. In fact, it seems that the best of these stories have all caused countless hours of frustration while in the midst of them, but once that frustration eases, we seem to be able to find both a little (or a lot of) humor in the situation and clarity on the lesson learned. Today, I am going to share a war story that was recently shared with me and the lesson learned about flexible material selection.
Why isn’t this flexible circuit working properly?
Starting with a little background, the OEM, a medical startup company, was developing their first “wearable” application. The product wasn’t a smartwatch, or fit bit type application, but rather something that was going to be attached to patient's clothing and worn while monitoring a specific condition and transmitting data back to the medical facility. The development team made the decision to design a flexible circuit for this application to take advantage of its thin and lightweight characteristics. This was going to be the very first flexible circuit design for this development group, and there was nobody on the team who had experience with flex design or materials. In fact, the team had very little PCB design experience in general.
Here is where the “war story”, or maybe more accurately put, series of missteps begins. Rather than work with the traditional polyimide-based flexible materials, the decision was made to use thin core rigid materials instead. The logic being that FR4 materials were available with .002”/ .003” thin core, FR4 is commercially available, eliminating the worry of lead-time or high minimum purchase requirements, the material is rugged and as importantly, these were the materials that they were most familiar with.
The first version of this design was built using .003” rigid FR4 material with flexible solder mask. In the field there were performance issues with intermittent failures. A slight redesign was done, again using these same materials. Unfortunately, with similar result. As problem solving progressed, a cross-section of the PCB was done and showed cracking in the rigid core materials. I imagine you are reading this, thinking “of course” the rigid material was cracking. But that is the funny thing about these stories, hindsight is always so clear.
The next version of the design was produced using Kapton based coverlay to mitigate this problem. As I am sure that you are predicting, the circuit again was not performing and again had cracking of the core material. I wish I could say that was the final version on rigid core materials, but there was one more version built with this construction.
Finally, the circuit was redesigned with flexible base materials and coverlay and performed as intended. Success! I imagine it took a while for the story to become comical to that team. A lot of time, money and resources were spent to get to that point.
How could this have been prevented?
I think there were some unusual factors that contributed to this “flex that didn’t flex story.” First, as I mentioned, this team not only did not have experience with flexible circuit design and materials, they did not have a lot of PCB experience at all. For previous rigid designs, they were working with a machine shop that offered to help with the PCB along with other work they were doing. The foundry also did not have a lot of experience with PCBs. Collectively, there was not a lot of knowledge about materials and material usage. Had the design with thin-core rigid materials and flexible solder mask been reviewed by a PCB fabricator, red-flags would have been thrown. Had the design with rigid materials and Kapton coverlay been reviewed by a PCB fabricator, ALL the bells and whistles would have sounded, prompting a lot of questions and detailed discussion.
There was also the assumption that rigid materials would be more readily available and come at a lower cost than flexible materials. Once the decision was made to change to flexible materials and the design was quoted by a fabricator specializing in flex, they found it to be a little less expensive than the price they were paying the machine shop for the rigid PCB design. Flex materials are more expensive than rigid materials, but in this case, the flex fabricator purchased enough material they received a volume discount and the flex option was less expensive. Lesson Learned.
What advice can be shared from this war story?
One piece of advice we hear repeatedly from flex and rigid flex fabricators is to work with your fabricators early in the design for stack up creation, for review and advice on how to “flexize” your design. In this case, they were working with their fabricator, prompting the caveat to be sure that the fabricator you have selected is familiar with flex and rigid flex materials and how to process them. The material set is different; the processing and handling of the material requires specialized knowledge, and designing for flexible materials is more complex than what we are accustomed to with rigid materials.
So where is the humor in the story? As it was being shared with me, the largest chuckle was “of course rigid materials would crack, just because they are thin doesn’t mean they are flexible”. Well……yes, of course. It all seems so clear in hindsight.