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Flex Talk: The Black Magic in the Business

“For rigid boards, I really don’t need a drawing and can get the fabrication vendor where they’re going based on material and a starting point. But flex is different, often called ‘black magic’ in the business.” 

Before giving a presentation on flex and rigid-flex a couple of years ago, I reached out to several designers who I knew were experienced in flex design and asked for their best tips I could pass along to those brand new to flex and rigid-flex design. Apparently, I have some generous friends in the industry because I received excellent advice to pass along. The introductory quote above is one that I always remember and often use when speaking about flex. On the surface, it makes me laugh, but I understand what they were saying. When you work with flex or rigid-flex, the communication between designer and fabricator needs to be impeccable, and the primary method of transferring information is through the fabrication notes.

There are many resources available that provide a checklist of items these fab notes must include, including materials, IPC quality standards, tolerances, critical dimensions, UL requirements, via plugging requirements, and special quality requirements, just to name a few. Today, I will give a recommendation that, although not strictly required by the fabricator to build the product, will certainly benefit the end-user. 

When creating the fab notes, include a picture of how the flex is going to be bent, folded, or flexed during end-use. This information, if included in the data package, is more likely to be included in the assembly drawings, but those drawings rarely get passed along to the bare board fabricator. Most often, although the flexible circuit is intended for three dimensions, the data files and information the fabricator receives is two-dimensional. Including the end-use requirements in the data package, and then taking the extra step of asking your fabricator to review the design for any recommendations to improve flexibility and reliability, is a simple combination that can yield great benefits. Who better than the fabricator, building flexible materials day in and out, to provide recommendations on product performance?

There are many subtle ways to improve flexibility and reliability. From a fabrication standpoint, the way parts are oriented on the production panel can improve flexibility. The rolled annealed copper on flexible laminates has a grain direction; orienting the circuit so that the bend areas are aligned with the grain direction can impact flexibility. Another fabrication tool used to improve flexibility is the decision to button or pattern plate. Some fabricators elect to use a panel plating process as a default, while others choose button plating as their standard. When panel plating, the entire panel is plating with electrodeposited copper. This electrodeposited copper is not as flexible as rolled annealed copper on the flexible laminates. Button plating, on the other hand, plates only the pads and through-holes, leaving the circuitry to be formed on the rolled annealed copper. If this is a concern, adding a note to the fab drawing will ensure the button plating process is used.

It is common practice to include the materials stackup in the fabrication notes, and we also encourage designers to use the IPC materials slash sheets when calling out materials. The overall thickness is important in rigid board design but takes on even more significance with flex and rigid-flex design. IPC-2223 provides guidelines for minimum bend radius. Again, your fabricator is another excellent resource when there are concerns about flexibility. Once your fabricator understands how the circuit is designed for end-use, they can offer insights based on their years of experience. One tip to improve flexibility is to use “un-bonded” layers to reduce the overall thickness. This simple adjustment significantly improves flexibility and prevents the dreaded “flex that doesn’t flex” experience.

Another recommendation may be using crosshatch shielding rather than solid copper shielding. This is often a balance of electrical and mechanical requirements, and the crosshatch pattern is highly customizable. For example, if solid copper shielding is required, but there are concerns in the bend area, the crosshatch pattern can be created in just the bend area, leaving areas that are not flexing as a solid copper shield. Removing more or less copper in those regions or layers has a significant impact on flexibility.

These are just a few of the ways that your flexible circuit fabricator can provide guidance and suggestions to improve the flexibility of the circuit in end-use. Communicating those requirements in the data package and working with your fabricator during the design process is simple and can have a significant impact on the successful outcome of the design. While I don’t really think that flexible circuit design is black magic, I do think there are tips and tricks that, learned over time, can greatly affect the performance of flexible circuits in end-use.

This column originally appeared in the August 2020 issue of Design007 Magazine in the FLEX007 section.