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.

How is etchback created?

Etchback is affected by two common flexible circuit fabrication processes:  micro-etch and plasma etch.

To improve plating adhesion, the substrate or base laminate is often submerged or sprayed to remove a slight amount of copper in a micro-etch process. After the various circuit layers are laminated together the through-holes are drilled. This is then followed by a plasma etch step to remove organic resins such as smeared adhesives and dielectric debris. This sequence of processing insures the copper surface in the hole is free from contaminants. This is often referred to as desmearing.

The plasma etch process is controlled to remove a specified amount of dielectric. Etching or removing the dielectric to expose the copper layers as shown in Figure 1 is called etchback or positive etchback.  Etchback exposes more of the copper so a larger area can be plated. Essentially three sides of the copper layer can now be plated. This physical feature is used to create a more reliable plated through-hole. 

Etchback1.jpg 

Figure 1: Diagram showing positive etchback.

Etchback is often a requirement for high-reliability applications found in medical, military and aerospace electronics for Type 3 and Type 4 circuits.

A Type 3 circuit is a multilayer flexible circuit consisting of three or more layers with plated through-holes. A Type 4 circuit is a multilayer flexible circuit and rigid board combination consisting of three or more layers with plated through-holes.

IPC-6013 states that etchback shall expose at least 0.003 mm (0.001”) and a maximum of 0.08 mm (0.003”) of copper, when etchback is specified on the procurement documentation. In other words, IPC-6013 does not require etchback on a Type 3 or 4 Class 3 circuit. It is up to the procurement organization to specify that requirement.

Negative etchback is when the copper layer is etched back from the through-hole barrel as shown in Figure 2. IPC-6013 specifies a minimum negative etchback for Class 1, 2 and 3 circuits.

Etchback2.jpg 

Figure 2: Diagram showing negative etchback.

Maximum allowed negative etchback per IPC6013:

  • Class 1: 25 µm (0.001”)
  • Class 2: 25 µm (0.001”)
  • Class 3: 13 µm(0.0005”)

While postive etchback is usually achieved by a plasma etch process that attacks the dielectric but leaves the metal intact, negative etchback is caused by a copper etchant used in a micro-etch process. Micro-etch is a common step that is performed immediately before the plating step. The micro-etch process normally removes up to two microns of copper, but long dwell times in the etchant or inadequate rinsing may cause negative etchback. IPC-6013 specifically states that no negative etchback is allowed if the procurement document specifies positive etchback.  

Etchback has been a requirement for military and aerospace applications for many years. The DoD determined through a series of tests that postive etchback improves the reliabilty of the through-hole. This requirement was developed in the late ‘70s when plating technogies, chemistries and materials were  quite different vs. today. Considering the many changes and improvments in plating, there is some controversy about whether etchback is really needed today .  

It is doubtful this legacy requirement will go away any time soon as the defense industry is adverse to changes with out compelling motivations. Since most fabricators use plasma etch for desmearing through-holes, achieving the etchback requirement is not a significant added cost. Through-hole performance with etchback has decades of historical performance data to support the requirement. With reliability established over years of performance, decision-making leans toward a conservative approach. As such, the old saying “if it ain’t broke, don’t fix it” will probably rule the day. 

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

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2017

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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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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