Rigid-Flex Prototyping Tips
Question: What are a few key tips for flex and rigid-flex prototyping?
My answer to this question is simple: OVER COMMUNICATE with your fabricator! Typically, prototypes are needed quickly, yet approximately 75%-80% of new part numbers go on hold for engineering questions and clarifications. This can have a significant impact on scheduling. Before you can work on ways to communicate clearly with your fabricator, you need to select the right fabricator for your project, and we will go into more detail on that topic in a future blog. For today, we will assume that you are working with the right fabricator and have set up a strong communication loop.
What are key things to be sure to communicate clearly?
There are two common material related issues that can cause delays with flexible circuit materials. One, flex materials are not always clearly called out in the fab notes. Two, material that is called out is not in stock and the order can not be released to fabrication until material arrives. This could be a delay of a couple days or even sometimes a couple weeks. One way to mitigate this risk is to work with your fabricator early in the design cycle to develop a stack-up for your application using common materials that they will typically have in stock and then include that stack-up with your data package. If you are not able to use common materials, work with your fabricator to pre-order the material so it is available when you are ready to release the design.
Does your application require flame retardant materials to be used? This should be captured in both the fabrication notes and the stack-up that was developed. Being UL certified for specific materials is less common in flex fabrication than rigid board fabrication, likely because of the exceedingly high number of material combinations. If your fabricator is not able to stamp their UL marking on the flex itself, it is often acceptable to use UL materials and have the fabricator certify to that on the C of C.
How is the circuit going to flex in end use?
So, this one won’t actually cause engineering questions or delays, but it is a good practice that can help ensure your flex design performs as expected. Fabricators work with two-dimensional data, build the flexible circuits in two-dimensional panels yet this process is creating a three-dimensional product. Adding a reference image to explain how the product will be used AND specifically asking your fabricator to review your design for flexibility can save time and headache. They may recommend different materials, cross hatch shielding, un-bonded layers or button plating. The raw material itself has a grain direction and the orientation of the parts on the production panel to align with the grain direction in your bending or flexing area can also help improve the flexibility. This review is something that should be specifically asked for. It may not be common practice at the fabricator.
Run a DFM early in the process:
I know this one seems like common sense, but you might be surprised at how many times it is a step that is missed. Most fabricators have software and processes in place to run a quick DFM against their manufacturing design rules. This is high-level review and doesn’t guarantee that there won’t be issues found once a more thorough review is done when tooling the design, but it will catch the bigger issues, and anytime something can be caught and corrected before the official release, you will be saving time. If time is the most critical aspect of the project and you are concerned with the complexity of a design, most fabricators will also run the more detailed DFM for you if asked. Some will run this at no cost, and some will charge a fee for the engineering time to review. But depending on the project and time pressure, this cost is beneficial to ensure there are no unexpected project delays.
Are all processes done in house?
Flex and rigid flex fabricators, as well as rigid board fabricators, routinely outsource certain services. That may be special test services or more often final surface finishes that are not in-house. Knowing what those processes are can help reduce the lead-time for your prototype. For example, if you are calling out lead-free HASL as a final finish and that needs to be outsourced, you are likely adding 3-5 days - occasionally even more - to your lead time. For prototyping, when time is critical, could you use ENIG or another RoHS compliant surface finish that is in house? There may be reasons why you can’t, but it is something worth thinking about!
In summary, when prototyping a new design time is often a critical component of the project. The more you can communicate, or as I stated above, over-communicate with your fabricator, the more likely the project will be completed in the time frame required and perform as intended.