All About Flex: Flex Circuit Stiffeners

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. 

The following are some common purposes for flexible circuit stiffeners: 

  • Zero Insertion Force (ZIF) connectors: This is probably the most popular connectorization method seen with flexible circuits. The insertion end requires adequate structural support and thickness to fully engage with the mating ZIF connector.
  • SMT or through-hole components: Stiffeners will help prevent mechanical bending or flexing near soldered components. Bending that occurs near a solder joint can create stress concentration at the junction of the trace and the cover insulation. This mechanical force is a risk to fracture a circuit trace, causing an intermittently open circuit.
  • Mechanical strength: Stiffeners can provide mechanical support that protects the thin circuit film and component from damage in applications where certain areas of the circuit assembly are subject to mechanical wear.
  • Selectively rigidizing: Stiffeners can keep certain areas of a flexible circuit flat and stable. For example, a mechanical actuator may need a firm, flat surface to function properly.  
  • Adding thickness: It is often desirable to add to the surface profile in certain areas of the flexible circuit to “match up” with other parts of the electronic assembly.
  • Facilitate handling: Flexible circuits are thin and flimsy and can be difficult to handle. Adding a selective stiffener can make it easier to assemble the circuit into an end state electronic package. This may include integrating the stiffener(s) as part of a carrier used to facilitate SMT assembly. 

A wide variety of adhesives are used to bond stiffeners to the flex circuit substrate. The two main types of adhesive generically fall into the categories of pressure sensitive adhesives (PSA) and thermally bonded adhesives. Varieties of each are available to meet specific performance requirements for thermal exposure, chemical resistance, and adhesion to various materials. Pressure sensitive adhesives are applied with minimal contact pressure and do not require heating to enhance the bond. Circuits are shipped with the PSA attached and protected by a low tack release liner that is removed when the nest level assembly occurs. PSA bonding is often an economical solution but highest bond strength is generally achieved with thermally cured materials. Thermal-bonded adhesives offer a much stronger and more permanent bond, but require a hydraulic press to achieve temperatures and pressures for attachment. Applications requiring high-temperature exposure will most often specify thermally bonded adhesives.

While stiffeners can be discretely applied to a single flexible circuit, it is usually more efficient to bond an oversized routed stiffener array to a flexible circuit panel. The final dimensions of the stiffener location are often defined coincident with the circuit cutline when the circuit is excised from the manufacturing panel. Bonding routed stiffener arrays is frequently adopted when component assembly is done in panel form. The ideal size and configuration for the stiffener array depends on several factors. Nesting density for panel fabrication needs to be balanced against optimum subpanel layouts used for SMT efficiency during component assembly. This tradeoff is done most economically when there is close cooperation between the circuit fabricator and the assembly supplier. 

 

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

 

 

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2017

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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All About Flex: Volume Considerations

02-16-2017

With any new electronic interconnection project, one immediate question the supplier will inquire about is program volume expectations. Customers often ask “What is your capacity?” with low-, medium- and high-volume having different meanings to different people.

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All About Flex: Creating a Flexible Circuit Cutline

02-02-2017

The perimeter dimensions of a flexible circuit are often referred to as the cutline. While rigid printed circuits are often rectangular and generally a less complex outline, the requirement for a flexible circuit to be an integrated part of the product packaging often involves unusual sizes, shapes and features in the circuit perimeter.

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All About Flex: Are Manufacturing Companies Susceptible to Ransomware?

01-12-2017

Every business (and every individual) needs to pay attention to cyber security. There are many sophisticated hackers throughout the world looking for ways to access or corrupt systems. While manufacturing companies have not been a common target, there are certainly risks that need to be considered.

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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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