Background
In 2023, a new customer came to us with a problem. They had a critical rigid-flex circuit in an important new product, but their current flex circuit vendor had issues delivering usable parts. They struggled to hold the impedance requirements within tolerance from part to part, and batch to batch.
Reviewing the Design and Identifying Issues
The customer’s design and stackup had passed their impedance modeling. And since some of the parts that they received did work, it didn’t seem like just a design issue. Wanting to better understand what had happened, and how to move forward, the customer asked us to review the project.
First, we reviewed the customer’s design. It turned out that their design had several complicated features that we could tell right away were difficult, but not impossible, to manufacture. We noted these features as likely sources of failure.
Next, we carefully examined some of the parts to better understand why—and where—they were failing. Our suspicions were correct: the source of the failures were those difficult-to-manufacture features. Specifically, we noticed that certain traces had a considerable amount of variation from one part to the next. This variation came from manufacturing techniques, not the design itself. In this case, an imprecise and inconsistent etching process caused the quality issues.
Impact of Inconsistent Etching on Impedance and Part Failures
The customer had modeled the part’s impedance based on these etched features. The inconsistent etching process led to the inconsistent traces, ultimately affecting impedance and causing part failures. So while the design was correct, and had passed DFM review, the design relied on consistent, reliable execution of the etching, which the customer wasn’t getting from their current vendor.
Proposing a Solution and Initial Prototyping
The customer asked us if we could manufacture the part for them, hopeful that we could deliver satisfactory yields without having to completely redesign the part. We told them that our impedance calculations are different, and we may need to adjust the trace and space based on our impedance calculations.
Our Manufacturing Capabilities
We have a solid understanding of our manufacturing capabilities. Like any good manufacturer, we measure our outputs. We’ve modeled our calculations on a proprietary data set, based on our specific machinery and its output over the course of thousands of jobs.
Understanding Customer’s Project
Additionally, we took time to understand the customer’s project requirements. We wanted to ensure any changes we made to the design would still result in satisfactory parts that would operate as needed in the field.
Updated Design and Prototype Agreement
To prove that our impedance calculations are correct, we took a cross-section of the other vendor’s part and showed the customer how their etching looked compared to ours. Once the customer plugged in our trace geometry numbers, their impedance calculator gave the same value as ours. We had an updated design, and alignment with the customer.
We also discussed the complexity of their design, and some of the inherent manufacturing challenges that went along with it. At our suggestion, the customer agreed to starting with a small prototype order, so we could further dial in the manufacturing process.
Initial Yield Challenges and Incremental Improvements
With our first batch, our yield was around 60%. Though this was much higher than the previous manufacturer, it still wasn’t where the customer needed it to be. We had our work cut out for us.
We tightened up some of our manufacturing processes and tried again. This time, our yields were just over 78%. Though we had made significant improvements, we still weren’t in the high 90s where the customer needed us to be. We needed a different approach.
The quality issues were still coming from etching, so our team worked backward and took a look at our imaging process. The imaging process is crucial for what follows during etching. In imaging, photo-sensitive film covers the panel. We selectively expose the film, setting the foundation for the etching process.
To bring the yields up further, we worked directly with the customer, our materials suppliers, and our imaging equipment suppliers. Through collaboration, our production manager and process engineers came up with an innovative new way to reliably produce rigid-flex circuits with similar trace topography in outer layers. Though it required us to rethink our approach to imaging and etching, we were confident that we could achieve the results that we needed for our customer. We all decided to move forward with this new approach.
Achieving High Yields and Implementing Improved Processes
Ultimately, we achieved yields over 96%, and earned the production order from the customer. Our updated approach to imaging and etching was a resounding success.
We then spent six months doing our own internal validation testing before implementing these revised procedures for all our rigid-flex production. These updates to our processes allowed us to eliminate three separate chemical processes while delivering more precise, reliable results in etching.
This is how failure analysis on a circuit that a customer brought to us led us to rethink traditional imaging and etching procedures, ultimately applying new techniques that offer better results for all of our customers.
Success Through Collaboration and Innovation
These are the types of solution-focused projects that our team gets excited about at Flex Interconnect Technologies. It was an opportunity for us to partner with the customer to understand their project requirements. With the talent of our engineers, and the cooperation of our entire production team, we were able to come up with meaningful solutions for both the customer, and our own manufacturing processes.