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


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.



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.

View Story

All About Flex: ITAR Registration


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.

View Story

Successful Flex Circuit Assembly


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.

View Story

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.

View Story

All About Flex: Trends in the Medical Electronics Industry


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.

View Story

All About Flex: CAD for Flexible Circuits


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.

View Story

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


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).

View Story

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


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.

View Story

All About Flex: Back-Bared Flexible Circuits


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.

View Story

All About Flex: Origami Interconnection


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?

View Story


All About Flex: Customer Acquisition


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.

View Story

All About Flex: Disruption in the Supply Chain


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.

View Story

All About Flex: Packaging Flexible Circuits and Assemblies


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.

View Story

All About Flex: Non-Copper Flexible Circuit Applications


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.

View Story

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


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.

View Story

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


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.

View Story

All About Flex: Flexible Circuit Prototypes


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.

View Story

Flex Circuit Specifications for Commercial and Military Applications


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.

View Story

All About Flex: Lead-Free Soldering Flexible Circuits


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.

View Story

All About Flex: FAQs on RoHS for Flex Circuits


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.

View Story


All About Flex: Embracing the Mess


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.

View Story

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


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.

View Story

Plated Through-holes in Flexible Circuits


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.

View Story

Testing Flexible Circuits, Part 3: The Completed Flex Circuit


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.

View Story

Testing Flexible Circuits, Part I: Requirements and Procedures


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.

View Story

Catching Up to Yesterday


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.

View Story

The Butterfly Effect


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.

View Story

Is Wearable Technology Just a Fad?


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.

View Story

Unique Single-Sided Flexible Circuits


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.

View Story

Agricultural Drones and Flexible Circuits


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.

View Story
Copyright © 2017 I-Connect007. All rights reserved.