Flex Circuit Specific Terms and Definitions

Recently, I was having a few discussions with someone very new to flex design. I was trying to be helpful and going through several things that are specific to flex and rigid flex design which should be considered as you start your first design. So, this blog is written for those of you who are new to flex and rigid flex, and may be unfamiliar with some of the terms that get volleyed around in conversation.

Flexible Circuit(flex pcb, pcb flex, flexible pcb, flex circuits):  

A pattern of conductive traces bonded on a flexible substrate. There are several different substrates available, the most common being polyimide. Different than rigid materials, these laminates will have rolled annealed (RA) copper for improved flexibility.

flex pcb
flex pcb

Rigid-Flex PCB(rigid flexible pcb):  

This is a hybrid construction, using flex materials in areas that need to bend or flex and rigid materials in areas with dense component areas, surface mount components on both sides of the PCB, and applications with higher layer count, dense routing areas.  Most common rigid materials can be incorporated into a rigid flex construction.

rigid flex pcb
rigid flex pcb

Flex Tails:  

Typically refers to the areas of flex extended out past the rigid portions of the rigid flex. This may be one flex region, or several bands of flex areas that extend in various directions. Rigid flex is often used to solve packaging issues and connect on multiple planes. Flex tails enable this.


Often used to accommodate a shorter flexible area if there is no room for a service loop. Instead of using the full width of the flex region between areas for each inner layer, the area can be divided into smaller bands of equal width for each inner layer, eliminating buckling and stress in that area.                                                                   


This is a protective barrier material often used in rigid-flex fabrication. Often, this is a coverlay material used to protect exposed flexible materials during processing and is removed from the flexible portion of the board before shipment.


A layer of insulating material applied to the flexible circuit to insulate the conductor pattern. Coverlay is typically a layer of polyimide with acrylic adhesive. Film based coverlay is much more flexible than cover-coat materials and highly recommended for dynamically flexing applications or flex that will have a tight bend radius. It is important to be sure to spec enough adhesive to fully encapsulate the copper conductors.

Bend Radius:  

This is the ratio of the bending radius measured to the inside of the bend to the overall thickness in that area.  Typically, recommendations for non-dynamically flexing designs is 10:1 for single and double-sided construction, and 20:1 for multilayer construction. These can be exceeded but should be evaluated carefully. Dynamically flexing applications should be discussed with your fabricator for a recommended stack up.

I always wrote a blog about how to calculate the bending radius of fpc.

Button Plate:  

Fabrication process to selectively electroplate copper to vias and onto the pads capturing the vias. The remaining copper traces do not have electrodeposited copper, increasing the flexibility of the circuit.

I-Beam Effect:  

Stacking conductors on adjacent layers directly on top of one another, increasing the stiffness of the circuit in the bend or fold areas. Staggered conductors are recommended if possible, to retain the maximum flexibility of the circuit.

I hope this helps explain some of the common terminology with flex and rigid-flex materials and design. Please reach out to me with any questions for further information!

Copper Thickness Requirements for Flex Circuits

If an end user will specify the copper thickness of a printed circuit, there must be many reasons. For example, current carrying capacity, but copper thickness also directly impacts thermal performance and impedance. All these are vital properties, which have a great influence on the functionality and reliability of a flexible circuit.

flexible circuits
flexible circuits

At the point, it is important to understand the functional needs driving a copper thickness requirement.

Some of the common functional requirements could be:

1.Minimum thickness in a connector area to assure robust contact.

2.Adequate current carrying capacity directly related to the cross sectional area of the trace.

3.Proper conductivity, a function of cross sectional area and metal type of the trace.

4.Proper impedance in high speed circuits driven by the cross sectional area of the copper, the surrounding dielectric constant, and distance from signal trace to ground plane.

5.Thermal properties directly related to metal type and trace profile.

Copper weight is used in the industry as a “thickness” measurement. Circuit manufacturers commonly purchase copper foil with descriptions of ½ ounce, 1 ounce, 2 ounce and so on. The number is the weight of copper in a square foot of foil. Also, +/- 10% is the industry accepted tolerance for copper foil thickness from the material supplier.

flexible pcb
flexible pcb

Drawing specifications will frequently define a flexible PCB copper thickness using weight. For example “circuit to be 1 ounce copper”. This can lead to some ambiguity, as copper plating on double sided circuits can easily add an ounce of copper to the surface of a trace. So by specifying thickness in this fashion, it is not clear if this is intended as a finished thickness or an original thickness. Additionally, controlled impedance designs work best when copper plating is restricted to the vias with no copper plated on the surface of the traces. This will minimize trace thickness variability and suggests a specific product category requiring a process known as “Pads Only Plating” or “Button Plating”. For controlled impedance designs, one of these terms should be called out in the drawing notes.

What affects final copper thickness is the variety of manufacturing processes that add or subtract copper thickness. Micro-etching is a common “cleaning” process used to prepare a surface for plating or coating. This process removes a small amount of copper. Likewise copper plating will add thickness. The circuit fabricator will directly measure added (or subtracted) thickness in mils (1 mil =.001”) or microns (25 Îźm=.001”).

The most accurate method for determining thickness is to do a micro section. This is a destructive test, so it is common to use coupons located in unused areas of the processing panel. These coupons are located and sized to be “representative” of the circuitry copper thickness. Copper thicknesses will vary slightly across a panel depending on current density from electroplating. Current density can be a function of the copper trace pattern so differences among various part numbers will occur. As a general rule, copper plating thickness will tend to be thinner on the outer edges of the panel and thicker toward the center.

In summary, when defining the specific copper thickness for an application it is highly recommended to start with a discussion of the myriad functional requirements. Also, the manufacturer can help recommend copper thicknesses and tolerances as well as the best methods for measurement.

Best Technology is the professional vendor of flex circuits, from 1 layer to 10 layers, 2 layers rigid-flex circuits to 16 layers rigid-flex circuits, and one-stop service including components purchasing, assembly, IC programming, testing. Choose us, you can always enjoy our best service at a good price.

Do You Know the Materials Used for Constructing 6 Layer Rigid-Flex PCB?

A six layer Rigid-Flex PCBs, as the name implies, is manufactured using six conductive copper layers. However, do you know the materials used for constructing 6 layer rigid-flex PCB? Now let me lift the veil for you.

6 layers rigid flex pcb

6 layers rigid flex pcb

Base Materials – The most commonly used base materials for manufacturing 6 layer rigid-flex PCB is woven fiberglass impregnated with epoxy resin. The use of cured epoxy makes the board more rigid. Manufacturers also prefer Polyimide rather than normal epoxy resin to ensure extreme reliability in complicated applications. Polyimide is chosen due to their extreme flexibility, toughness, and heat resistance properties. Polyester (PET) is also another material of choice for the PCB fabrication. Manufacturers often choose the most suitable materials according to the specific application requirements of customers.

Films – Manufacturers prefer PET films for producing 6 layers rigid-flex PCB due to their corrosion and heat resistance properties. The thickness of PET films used for the PCB manufacturing vary from 1/3 mil to 3 mils.

Conductors – Copper is the most preferred conductor of choice for making 6 layer rigid-flex PCB and various types of copper are used according to the specific application requirements. Annealed copper is used when repeated creasing or movement of the flex circuit is demanded.

6 layers rigid flex PCB

6 layers rigid flex PCB

Adhesives – The use of high quality adhesive is crucial to make a fine bonding between the conductor and films. Acrylic or epoxy based adhesives are the most commonly used adhesives to achieve a firm bonding, while silicones, hot-melt glues, and epoxy resins can also be used for bonding.

Manufacturers who are very familiar with the properties of the aforementioned materials can conduct their mechanical design, evaluation and testing of the PCBs better. Therefore, a manufacturer who fabricates 6 layers rigid-flex PCB for the automobile industry must quite know the moisture, chemical, shock & vibration resistance properties of the materials used in the construction. This contributes to enhancing the durability of PCBs in the case of industrial specific applications.

Best Technology Wishes You Merry Christmas and Happy New Year 2020

Christmas and New Year is just around the corner. Merry Christmas to everyone. As the year 2019 is coming to an end, the team at Best Technology would like to wish you a happy, healthy, and prosperous New Year 2020. We believe that it is time to focus on the year that has gone and say thanks to all valuable people who helped us to become trusted and well-known PCB manufacturers in China.

Best wishes

Best wishes

We acknowledge that the year 2019 has been a surprise for us, wherein we got several opportunities to serve for our customers. The challenges that came along with these opportunities has helped us refine ourselves for better and stand tall among all our competitors. Being an industry-leader in flex, rigid, and rigid-flex PCB manufacturing, we could easily fulfill your orders in the specified time and in compliance with required specifications. Although technology plays a major role, we could never have achieved this success without our employees, who always made the most of their abilities to help meet our commitment of on-time deliveries.

rigid flex pcb

rigid flex pcb

On this holiday season, we would love to send our warmest wishes to the team who stood by us through challenges and our valuable customers for investing their time, money, and faith in our capabilities. This helps to serve you better. We are expecting to achieve more milestones in the upcoming year. This is not possible without your supports and faith. We hope this relationship will grow stronger in the New Year.

The entire team of Best Technology Wishes you a Merry Christmas and Happy New Year 2020 again!!!

For your easy reference, below is our holiday schedule:

Christmas Day: We will be closed on Dec 25, 2019 and resume working on Dec 26, 2019

The New Year: We will be closed from Jan 18, 2020 to Feb 1, 2020 and we will resume working on Feb 2, 2020