All About Flex: Button Plating on a Flexible Circuit

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. The initial imaging/developing process provides electroplating chemistry access to the copper vias and pads. The second process repeats the imaging to define the copper trace pattern. The term button is descriptive as the selectively plated pads are slightly raised from the surrounding copper regions and create a physical signature that looks like a button.

There are two primary applications where button plating is desirable.

  1. Dynamic flex applications (when continuous or frequent bending of the flex circuit is required) and
  2. Impedance control applications (often a requirement in high signal speed applications)

Dynamic Flex

Some applications for flexible circuits require specific areas of a circuit to be flexed during the operation of the consumer device. This could be thousands (opening and closing of a notebook computer) if not millions (interconnection to the read/write heads on a disc drive) of flex cycles required during the life of the product. Each dynamic flex application has a unique combination of motion, bend radius, cycle acceleration and proximate circuitry features that may affect flex life performance. Avoiding the addition of electroplated copper onto traces by button plating is a common practice for extending flex life. Since electroplated copper has a different grain structure vs. rolled annealed copper, the grain structure is less flexible and prone to fracture with repeated bending.

To further optimize flex life, additional design rules with specific material sets and features will often be adopted and/or recommended by the fabricator:

  • Use of rolled annealed copper which has a grain direction parallel to the roll length
  • Orienting the circuit on the panel to position the grain direction perpendicular to the flex plane
  • Balancing the material thickness composition to locate the copper layer in the neutral axis of the flex circuit sandwich. This prevents the copper from being in either compression or tension when bent
  • Avoiding any unusual shapes of traces or inclusion of features such as vias in the flexing region (parallel traces of equal width are optimum features through a dynamically flexing region)
  • Thinner copper will generally perform better than thicker copper for extended flexing

These design practices, in combination with button plating, will help ensure a circuit design has been defined for optimum flexing. Ideally, a flexible circuit should undergo verification testing in a simulated end-product environment to confirm expected performance with some empirical data.

Impedance Control

Applications using high-speed electronics frequently require impedance control. As with dynamic flex applications, a specific structure and material set is ideal:

  • Homogenous dielectric material (ER)
  • Equivalent dielectric thickness spacing between top and bottom ground planes
  • Consistent copper thickness and copper width
  • Consistent copper spacing

Electroplated copper thicknesses are more variable with electroplated surfaces vs. non-plated copper. It is not uncommon to see a 10% variation in a circuit’s copper plating thickness as current densities cause features to plate at higher or lower rates. This variation can translate to a similar variation in the impedance. Pads only plating of circuitry requiring impedance control will result in less variability within an individual part and between multiple product builds.

Another critical aspect of impedance control is the relative dielectric (ER) constant of the dielectric stack-up. The dielectric stack-up can consist of one or more layers of dielectric film bonded together with adhesives. The adhesive has a different dielectric coefficient than the dielectric film. Using adhesiveless laminates reduces the variation caused by adhesive and will normally result in a more consistent impedance.

References

1. Processing options when plating vias on double-sided flexible circuitry have been discussed in previous columns publishing at flex.iconnect007.com: Plated Through-holes in Flexible Circuits  and Plating Process Options for Flexible Circuits.

2. Impedance calculator, www.ideaconsulting.com.

3. Considerations for Impedance Control in Flexible Circuits, flex.iconnec007.com.

4. Tradeoffs when implementing impedance control, www.allflexinc.com.

5. Impedance control using microstrip and strip line, www.allflexinc.com.

 

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

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2017

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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All About Flex: Trends in the Medical Electronics Industry

07-17-2017

The U.S. medical electronics industry has been one of the fastest growing industries over the past decade. Similarly, to the rest of the electronics world, growth has been accompanied by the adoption of significant new product technology and innovation.

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All About Flex: CAD for Flexible Circuits

07-12-2017

CAD engineers take a CAD file that defines a single part and panelize the data by creating a nested pattern repeated across the panel. Reverse nesting and off-angle part placement may optimize material utilization, which is a constant cost concern. But this optimization needs to be balanced by ease of stiffener placement and component assembly.

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All About Flex: Flexible Circuit Component Assembly…and a Math Lesson

07-06-2017

The market for rigid PCBs is estimated to be about 10X the market size for flexible printed circuits (FPCs). As a result, the equipment infrastructure is driven primarily by the needs of the rigid board market. This is true of both equipment used to fabricate the circuitry (image, etch, copper plate, AOI, etc.) and equipment used for component assembly (wave solder and SMT assembly).

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All About Flex: Etchback on Type 3 and Type 4 Flexible Circuits

06-22-2017

Through-hole etchback is a requirement that is sometimes specified on medical, military and aerospace procurement documents for multilayer flexible circuits and combination multilayer flex/rigid board circuits. It specifically relates to the copper plated through-holes and the relative dimensions between the dielectric layers and copper layers.

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All About Flex: Back-Bared Flexible Circuits

06-01-2017

Back-bared pad flexible circuits are a distinctive type of single-sided flexible circuit providing some advantages over more standard circuits. In the printed circuit industry, back-bared pad circuit designs are also referred to as dual-access or reversed bared.

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All About Flex: Origami Interconnection

05-11-2017

Origami is the art of folding paper; it was believed to have originated in Japan, but historical evidence suggests it existed in several parts of the world during the same period. Origami artistry starts with a flat sheet of paper and by making a series of folds and creases, the result is a three-dimensional figure. Creating even just a simple figure takes imagination and a unique ability to visualize in three dimensions. Three-dimensional folding to fit into a multiplanar shape? Why does this sound familiar?

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All About Flex: FAQs for Extended Length Flexible Circuits

04-27-2017

Extended length flexible circuits are larger than typically offered sizes in the interconnect industry. The length of these oversized circuits can be anywhere from two feet to 10 feet or longer. A long, continuous flexible circuit can offer design advantages over using normal sized circuits where multiple connections and connection assembly steps are required.

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All About Flex: Flex Circuit Stiffeners

04-13-2017

Many flexible circuit designs require selectively bonded stiffeners…they’re just too flexible! Stiffener materials can be any number of materials, but they are usually polyimide films or FR-4 glass/epoxy substrates and are available in a wide variety of thicknesses. Three-dimensional metal stiffeners may also be attached for thermal dissipation properties. The purpose of a stiffener is to rigidize or structurally support discrete areas of the flexible circuit.

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All About Flex: Considering a Flexible Heater?

03-02-2017

Custom flexible heaters are available in an infinite variety of sizes, shapes and materials. The most common flexible materials are polyimide and silicone rubber. While silicone rubber has traditionally been thought of as the higher temperature flexible heater option, recent advances in polyimide-based materials have allowed high performance heater constructions to operate successfully at temperatures that exceed 250°C.

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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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