All About Flex: The Anatomy of a Flex Circuit Cutline

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.

In most cases, the material being cut is polyimide laminate consisting of multiple layers of monolithic polyimide film, adhesive, and copper. In cases where edge insulation (distance between traces and circuit outline) is required, no copper remains in the cutline area leaving only plastic film and adhesive on the edge of the circuit.  

The following are the common methods of creating a circuit outline (i.e., cutline):

  • Male/female hard tool die
  • Steel rule die
  • NC routing
  • Laser ablation
  • Combination methods

Each has different capabilities with respect to tolerances and through-put, and each method has a different level of tooling investment. The mechanics or physics of the cutting action is different among these methods. A cut edge examined under high-level magnification reveals the physical differences. These differences usually do not affect the form, fit and function of the part, although tolerances and specialty performance applications may encourage one method over another.

The following are the differences in the cutting action.

Male/Female Hard Tool

This cutting action is created by two metal surfaces meshing together to create a shearing action. These tools are often called “zero clearance” as the punch is constructed with a harder steel and slightly shears the cavity metal during the initial punching operation. A properly sharpened hard tool will provide an excellent cut surface over hundreds of thousands of die hits. The side view of the cutline would look very straight and uniform. Circuits requiring dynamic flexing over hundreds of thousands of cycles often adopt hard tools for cutline formation. Tolerances of

Steel Rule Die 

Rule dies are analogous to a cookie cutter with one edge of the steel blade beveled. The cutting action is created by a high level of force concentrated in a very narrow area and causing a bursting action. The steel rule does not need to fully penetrate the material to effectively cut it. Under high magnification the side surface does not look as uniform as a hard tool cut; the side wall is not completely vertical and the exit surface may have a lip. Rule dies are also more likely to create polyimide slivers as a consequence of the bursting cutting action. Tolerances with rule dies are generally limited to about 0.005” or more.

NC Routing

The best way to describe the cutting action of routing is grinding. Routing bits are rapidly spinning and shredding the material in their path. An NC-routed cutline will look ragged when viewed under magnification. The degree of raggedness depends on the material stack-up, router speed and bit sharpness, and overall material thickness. Tolerances at 0.005” are possible with NC routing.

Laser Ablation

The laser focuses a high level of energy on a very small area causing rapid heating which partially burns and vaporizes the material. Under high magnification the side wall looks smooth as if it was slightly melted. Lasers have been increasingly adopted as they can cut sizes and shapes beyond the capability of the other methods. Lasers too may have limits in some applications, as it is quite frequently desirable to simultaneously cut the circuit and an applied pressure sensitive adhesive (PSA). While the laser will cut through the PSA, over time adhesive flow will cause re-attachment across the cut region. Lasers have uncanny accuracy, with 0.001” tolerances possible.

In some cases, the cutting action of rule dies and hard tool dies can create slight micro tears in the circuit material. These may propagate as the material is flexed and stressed. The best way to mitigate this possibility is through one of the design features listed here and shown in Figure 1:

  • Replace sharp inside corners with rounded corners
  • Insert tear stops (a drilled hole) at the end of all slits

Flex_Fig1.jpg 

Figure 1: Flex circuit cutline features.

The top of Figure 1 shows a circuit that has a right angle. The junction of the polyimide cutline should be defined by a rounded corner. If the corner is sharp, then a micro-tear could readily propagate when stress is exerted on the angled region.

The bottom of Figure 1 shows a slit that allows the fold or bend line to be inside the body of the part. In this case a hole is used to create a tear stop. Without this hole, the slit could propagate further when the tail is stressed. Either of these defensive design features significantly reduces the likelihood of tear propagation.

In most cases, the differences in the cutting action will have little impact on the performance of a part and physical differences are unlikely to be noticed under low magnification. In some specialty applications, the differences among the various cuts could impact the function of the part. An experienced application engineer can help determine the best cutting method to avoid any functional or performance problems.

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

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2017

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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07-20-2017

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07-12-2017

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07-06-2017

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

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All About Flex: Back-Bared Flexible Circuits

06-01-2017

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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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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

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All About Flex: Packaging Flexible Circuits and Assemblies

12-01-2016

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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

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Flex Circuit Specifications for Commercial and Military Applications

09-30-2016

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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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09-17-2015

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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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