All About Flex: Design and Construction Features that Can Cause Trace Fracturing in a Flexible Circuit

Flexible circuits are used in applications requiring millions of flex cycles. But this does not suggest they never experience failures due to flex life. In fact, occurrences of performance issues as the result of fractured traces have been experienced in a variety of applications. Solutions are often the result of design, construction, and feature location modifications. But a much better solution is to avoid the problem in the first place!

A flexible circuit’s ability to be bent, twisted, folded and continuously flexed is a significant advantage offered as a designer considers this interconnect method in a new product. The design options are myriad with “origami” packaging being an accurate description of how the product is often used. But it is true that improper designs and/or specifications have resulted in “bad experience” anecdotes about trace fracturing problems.

The following are some pitfalls to avoid and minimize the chance a design will experience premature failure.

High Stress Points  

There are several ways a designer might inadvertently lay out traces and create a higher level of mechanical stress during bending. Designs with traces on opposite sides of the dielectric laying directly over each other will create an “I-beam effect” (Figure 1).

BeckerFig1.jpg

Figure 1

With traces mapped directly over each other, added stress occurs on the traces located on the outside of a bend radius. The traces on the outside of a bend radius are in tension mode while the layers on the inside are in compression. Traces in tension tend to develop micro-fractures if they are flexed repeatedly. During failure analysis these can be particularly difficult to diagnose as the open circuit often occurs intermittently.

Another mechanical stress point occurs at the end of a stiffener, which will tend to focus stress at the stiffener termination line. A simple design feature solution is often to overlap the coverlay termination on the opposite side (Figure 2).

BeckerFig2.jpg

Figure 2

Solder joints/vias too close to bend points

While copper foil is fairly ductile, the alloy resulting on a solder joint becomes brittle. These regions are particularly susceptible to trace fractures as the region defined by the junction of coverfilm and solder pad cannot take significant bending. Figure 3 shows a safe distance between the bend location and a solder joint or plated through-hole.

BeckerFig3.jpg 

Figure 3

Unbalanced Dielectric Thickness

This problem is avoided by locating the copper traces in the “neutral axis” of the circuit (i.e., by having equivalent material thicknesses above and below copper in regions to be repeatedly flexed). Polyimide film has been proven as a robust material for dynamic flex applications and is used almost exclusively in these applications. Although screen-printed or photoimaged solder mask are sometimes used as dielectrics, it is rare to find these alternative materials used in applications requiring high flex life. They tend to be brittle and are likely to craze or crack when folded or repeatedly bent.

In some applications it is necessary to use photoimaged soldermask for its high-resolution capability for component openings. But if these parts are also to be dynamically flexed, polyimide is again used in the flexing region of the circuit, creating a circuit with dual insulation materials and processing.

Copper Selection

There are two basic types of copper foil: electrodeposited (ED) and rolled annealed (RA). They have significantly different grain structures and bending properties. Using ED copper for high flex applications can result in infant mortality. RA copper offers significantly better flex life. Also important is the copper grain direction on RA copper as flex life is much higher when the fold or bend line is perpendicular to the grain. Keeping the circuit composite thin perpetuates flex life. A copper thickness of one ounce and thinner is a good design practice.

Copper selection errors can also occur with two+ layer circuits when creating plated through-holes. Avoiding the addition of electroplated copper in a flex region is possible by masking during copper plating. This restricts the addition of electroplated copper to the copper pads and the vias. This fabrication method avoids the addition of electroplated copper in a flex zone and keeps copper thicknesses to a minimum. This is a pretty common process known as “pads only plating” or “button plating.” Both features can be key to a robust design and are often called out specifically as requirements on a customer’s drawing.

There are many industry guidelines a designer can reference to avoid these and other issues when designing/laying out a flexible circuit. I always encourage customers to specifically mention dynamic flexing and flex location requirements on their product documentation. This helps the supplier make recommendations regarding circuit layout and construction options.

Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.

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2018

All About Flex: Design and Construction Features that Can Cause Trace Fracturing in a Flexible Circuit

02-22-2018

Flexible circuits are used in applications requiring millions of flex cycles. But this does not suggest they never experience failures due to flex life. In fact, occurrences of performance issues as the result of fractured traces have been experienced in a variety of applications. Solutions are often the result of design, construction, and feature location modifications.

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Polyimide vs. Silicone for Flexible Heaters

02-08-2018

Flexible heaters are sub-divided into two primary technology platforms: etched-foil and wire-based technology. Wire-based (wire strands woven together or single strand wire) is the more common technology, with multiple customers offering a wide range of products.

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All About Flex: Copper Thickness Requirements for Flex Circuits

02-05-2018

An end user will specify the copper thickness of a printed circuit for different reasons. The most obvious reason would be for current-carrying capacity, but copper thickness also directly impacts thermal performance and impedance. All these are vital properties impacting the functionality and reliability of a flexible circuit.

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All About Flex: Copper Grain Direction

01-04-2018

Many materials have different characteristics depending upon the orientation of the material. For example, woven textile materials have a warp and a weft direction. The warp direction is the longitude direction and the weft is the transverse.

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2017

All About Flex: Options for Purchasing Flexible Heaters

11-15-2017

Flexible heaters represent a product technology generically described as thin and flexible heating elements used as attachments to various heat sinks to provide freeze protection, consistent temperatures, and thermal control from -80°C to +230°C and higher.

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All About Flex: More on UAVs and Flexible Circuits

10-30-2017

The use of drones or unmanned aircraft vehicles (UAVs) is growing at a nearly exponential rate. This includes drones used by the government, private companies and hobbyists. If you have $100, you can find a long list of drones for purchase.

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All About Flex: Flexible Circuit Failure Analysis

10-19-2017

Design reviews and early involvement by a circuit board fabrication house can minimize the possibility of field problems, but despite best efforts, there remain occasions when diagnosis of a poorly performing design is required.

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All About Flex: Flexible Circuits for Reusable Medical Products

09-21-2017

Flexible circuits have been used in medical devices and medical instrumentation for several decades. Flex circuits and flexible heaters can be ideal solutions in applications requiring lightweight, continuous movement, and highly reliable interconnections that are biocompatible.

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All About Flex: Flexible Circuits and Man-Made Satellites

09-12-2017

The first satellite was launched by the USSR in 1957. The U.S. successfully launched its first satellite, Explorer 1, in 1958 while announcing the intention to “win” the race to outer space. Today satellites serve many vital purposes.

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All About Flex: Using ZIF Connectors with Flexible Circuits

09-07-2017

Zero insertion force (ZIF) connectors are probably the most popular flexible circuitry connector because they allow the circuit to be inserted and removed multiple times with very little mechanical wear on the copper traces.

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All About Flex: The Anatomy of a Flex Circuit Cutline

08-22-2017

Numerous methods are used to create the cutline of a flexible circuit. With the various tooling options, the methods, process steps, tooling and technology are different. These differences affect the actual physics of cutting, and create slight variations on the circuit material.

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All About Flex: ITAR Registration

08-15-2017

Vendors building product for the defense industry often stipulate a supplier needs to be ITAR registered. ITAR stands for International Traffic in Arms Regulations and is a program run by the U.S. government to control the export of defense-related technology to foreign countries.

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Successful Flex Circuit Assembly

08-02-2017

Many contract manufacturers are reluctant to mix rigid and flexible circuits on the same assembly line as the handling and fixturing requirements can be quite different. Characteristics allowing a flexible circuit to be flexible can often present learning curve challenges when component or mechanical assembly is required. This article details some of the common issues experienced when assembling flexible circuits, and strategies to ensure reliable assembly.

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All About Flex: Button Plating on a Flexible Circuit

07-20-2017

Button plating describes a fabrication process widely used in the flex circuit industry to selectively electroplate copper to the vias and onto the pads capturing the vias. The rest of the copper traces do not have plating. Another industry term used to describe this feature is pads only plating. Producing a circuit with this processing methodology requires two photolithography steps.

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2016

All About Flex: Customer Acquisition

12-22-2016

The “Customer Acquisition” process can be thought of as consisting of three major segments: collection, selection and execution. While these sub-divisions should be considered as intimately interrelated, examining them as separate disciplines can be enlightening.

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All About Flex: Disruption in the Supply Chain

12-08-2016

Manufacturers need a highly dependable supply chain to successfully support their products. This is especially true of custom designed and built components, as many times, only one supplier is available for a component since tooling and development costs discourage dual sourcing.

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All About Flex: Packaging Flexible Circuits and Assemblies

12-01-2016

Many facets are involved in delivering a flexible circuit. During the quote and design phase, requirements are reviewed. So assuming the relevant product documentation was gathered, the salesperson turned around the quote, and the customer placed an order and parts were built, it’s all over, right? Not quite. One critical aspect that does not get much discussion is packaging and shipping.

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All About Flex: Non-Copper Flexible Circuit Applications

11-22-2016

While pure copper is the most common choice for flexible circuit fabrication, there are times a different metal is more suitable for an application. Copper is well known for its excellent electrical and thermal conductivity, but there are applications where the best thermal or electrical conductivity can be a disadvantage.

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All About Flex: Flex Circuit Specifications for Commercial and Military Applications

10-27-2016

Applications across the various markets for printed circuit boards can have significantly different specifications and performance requirements. Circuits for toys and games logically have lower performance requirements than those used in medical devices. IPC-6013 is an industry-driven specification that defines the performance requirements and acceptance features for flexible printed circuit boards.

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All About Flex: Five Characteristics of a Reliable Flexible Circuit Supplier

10-27-2016

Due diligence when selecting a source for a custom electronic product can be a critical sourcing procedure. Chains are only as good as the weakest link, and the electronic components assembled to create a marketable product need to combine into a robust solution.

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All About Flex: Flexible Circuit Prototypes

10-13-2016

Most electronic projects begin with at least one build of prototype parts before moving into volume manufacturing. But the definition of a flex circuit prototype can vary considerably from one project to another. In many cases, a prototype build is only a few parts used to verify form, fit and function, with engineering trying to determine if something actually works.

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Flex Circuit Specifications for Commercial and Military Applications

09-30-2016

Applications across the various markets for printed circuit boards can have significantly different specifications and performance requirements. Circuits for toys and games logically have lower performance requirements than those used in medical devices. IPC 6013 is an industry-driven specification that defines the performance requirements and acceptance features for flexible printed circuit boards.

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All About Flex: Lead-Free Soldering Flexible Circuits

09-23-2016

Ever since the European community adopted the RoHS directive in 2006, the U.S. electronics industry has been steadily increasing its use of lead-free solder. Medical was the first U.S. industry to go totally lead-free. Today, a significant percentage of electronics soldering is done with lead-free solder.

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All About Flex: FAQs on RoHS for Flex Circuits

09-02-2016

In 2003, the European Union (EU) adopted a standard called the Restriction of Hazardous Substances (RoHS), which restricts the use of certain materials in electronic products and electronic equipment. The intent is to reduce the environmental impact of known hazardous materials and has driven changes in manufacturing processes and materials used to manufacture a wide array of electronic products.

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2015

All About Flex: Embracing the Mess

12-03-2015

Marketing in the world of printed circuits is an important discipline, but I have learned it is better to be prepared with a nimble reaction than to expect the marketing department to consistently be successful in predicting the future. The path to the goal is often achieved much more quickly by making an early decision followed by a course correction rather than waiting for all the information.

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All About Flex: Flexible PCB: What’s in a Name?

11-12-2015

Flexible PCB is a common term that is synonymous with flexible circuits. While the term “PCB” is generally used to describe rigid printed circuitry, “flexible PCB” is a little contradictory because “boards” aren’t really flexible. Some companies, like All Flex, design and manufactures flexible PCBs, but not rigid PCBs. There are many similarities between the two, but also significant differences.

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Plated Through-holes in Flexible Circuits

10-29-2015

There is probably no more important feature than the plated through-hole (also called via or via hole) with regard to the reliability and integrity of a flexible circuit. The through-hole provides electrical connection between insulated layers and enables electrical functionality on double-sided and multilayer flexible circuits.

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Testing Flexible Circuits, Part 3: The Completed Flex Circuit

10-15-2015

Most flex houses perform a variety of tests on completed flexible circuits. The type, frequency, and complexity of these tests vary with customer and application. Test requirements are generally defined by the customer, but input is often solicited from the supplier during the quote process.

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Testing Flexible Circuits, Part I: Requirements and Procedures

09-17-2015

In this first of a three part series regarding tests for flexible circuits, I will examine overall requirements and procedures; the second installment will focus on raw materials, and the third and final part will focus on testing for bare flexible circuit and circuit assemblies.

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Catching Up to Yesterday

09-02-2015

Recently, KPMG, an international consultancy that operates as a network of member firms offering audit, tax and advisory services, came out with their 6th annual survey of manufacturing executives focusing on global manufacturing trends.

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The Butterfly Effect

08-20-2015

If a random initial disturbance from the wings of a butterfly can have a dramatic effect, just think what can be accomplished with intentional acts aimed at making sure our customers are receiving proactive attention.

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Is Wearable Technology Just a Fad?

08-13-2015

Wearable technology is in its infancy. The industry needs to mature and go back to basic marketing—finding a need and filling it. Flexible circuits have been around since the mid-1960s and have been successfully filling needs. Flexible circuits are ideal for wearable technology because they are thin and lightweight. As the marketing matures, the applications will come and flexible circuits will be there to fill the technical needs.

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Unique Single-Sided Flexible Circuits

08-06-2015

The number of iterations, sequences and combinations possible when manufacturing a flexible circuit can create unique product features to reduce hand assembly of wires, create switch contacts, or eliminate connectors. With minor alterations in basic processing steps, a flex circuit applications engineer can often imagine and configure a dramatically different flexible circuit.

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Agricultural Drones and Flexible Circuits

07-28-2015

According to MIT Technology Review, one of theTop 10 breakthrough technologies last year was the agricultural drone. I focused on drones in one of my recent columns, Flexible Circuits and UAV Applications, which briefly mentioned agriculture as one of the uses for drones.

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