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Flexible Thinking: The Key to a Successful Flex Circuit Design Transfer

This month, I will discuss the most common design errors that fabricators see, typical areas of miscommunication between design and fabrication, and what designers can do to avoid putting their jobs on hold.

This is no simple task, given the many things that can go wrong in flex circuit manufacturing, but many of these issues originate in the design process. I have written or mentioned previously that designers must educate themselves about the commonly employed flex manufacturing processes. However, an alternative is to “design with manufacturing” by tapping into the manufacturer’s engineering team’s experiences to avoid problems before launching a product specification for bid. I also recommend visiting a flex manufacturing facility, if possible. As the saying goes, “A picture is worth a thousand words,” and it will offer an appreciation of the processing and its complexities. The fact that something can be drawn on paper doesn’t mean that someone can easily produce it in three dimensions (think of M.C. Escher’s out-of-the-box drawings and sketches).

The following is a prescription for a manufacturing document package for a flex circuit. The freedom that flexible circuits give designers prevents this prescription from ever being comprehensive. With the rising use of flex circuits in such diverse sectors as military, aerospace, automotive, medical devices, wearables, and consumer goods, imaginations are often unrestrained. The boon in these flex circuits is thanks to their lightweight, compact nature, and ability to bend repeatedly and be shaped to conform to tight spaces.

A flex circuit manufacturing specification is a crucial document that defines the technical/performance requirements and standards necessary for the design, fabrication, and assembly of flexible printed circuits. Done correctly, the documentation ensures the circuits meet the necessary performance, reliability, and quality standards. Below are some of the important roles of a flex circuit manufacturing specification.

Communication Is Key
The primary role of any manufacturing specification is to communicate, where designers and manufacturers meet and transfer expectations and knowledge to ensure the customer receives accurate and consistent products. Flex circuits can be simple or complex. They can have anything from one to many metal layers and tentacles that branch in many directions, often with different layer counts. Feature sizes are from macro to micro and often intricate. The capabilities of different manufacturers can vary widely. That’s why the product’s requirements and the manufacturer’s capabilities must match. Candor is vital at every stage but particularly at the outset to avoid costly misunderstandings.

The documentation provides guidelines for and expectations of the circuit features employed, such as minimum trace and space widths, acceptable limits for via sizes, pad dimensions, and breakout limits, as well as stackup configurations to ensure the finished product meets both performance and manufacturability standards. Without clear and comprehensive documentation, the production process can deliver inconsistencies, defects, or products that fail to meet design specifications, leading to rejection at inspection or potential malfunctions in the end product in the field. Following are a few examples.

Material Requirements
A variety of flexible substrates (predominately polyimide and polyester), copper traces, and protective coatings comprise flex circuits. Materials play a significant role in determining the performance (electrical, environmental, physical, mechanical, and thermal), assembly method, and reliability of the flex circuit in the application. There should be discussions with the manufacturing engineering team to determine the exact materials for each element of the circuit, ensuring that the final product meets the functional, mechanical, and environmental requirements. Here is a short checklist of material structure/construction requirements: 

• Substrate type, thickness, and desired electrical properties: For example, engineers may specify polyimide for its thermal stability and flexibility, while they might use polyester for lower-cost applications and PTFE for high-frequency applications.
• Copper thickness: The type and thickness of the copper traces affect the circuit’s electrical/electronic properties, flexural endurance or formability, conductivity, and heat dissipation properties.
• Coverlayer or protective coatings: Define the flexible circuit coatings being used, such as coverlayer or cover coatings, including flexible photoimageable materials for environmental protection against moisture, chemicals, foreign debris, or shorting. The choice will also affect the circuit’s flexibility endurance, and the bonding materials between circuit layers are critical to the circuit’s overall strength and durability.
• Manufacturing processes and capabilities: A well-drafted flex circuit product specification provides a guide to the flex circuit manufacturer, outlining the product requirements. However, the manufacturing processes and in-house capabilities of the flex manufacturer will determine what they can make. Moreover, their control of those processes is critical. It’s essential to know the capabilities of the chosen supplier, which can be done by looking at product samples and asking for customer references. This may seem tedious, but the product must perform to expectations.
• There isn’t enough space here to go into all the details when looking to place a design with a manufacturer. Ultimately, it’s important to develop a working relationship with your suppliers that allows you to tap into their experiences, expertise, and advice to steer clear of the landmines that can blow up a product because of oversights, omissions, and miscommunications. Spend the time getting it right. As the saying goes, “It can take many years to build a reputation but only one screwup to lose it.” To learn more on this subject, flex circuit suppliers have in-house publications for their customers, and authors have written several instructive books over the past 40 to 50 years, including my book, Flexible Circuit Technology. As the quote commonly attributed to former Havard University President Derek Bok notes, “If you think education is expensive, try ignorance.” It still holds true.

This column originally appeared in the March 2025 issue of Design007 Magazine.