What is Flex PCB? — An Overview of Flex and Rigid-Flex PCB

The most common and used type of Printed Circuit Boards is the rigid type, but the flex one is another type that you may think of using it after getting yourself familiar with, especially that it is now more available than before as more manufacturers are providing this type.

In this article we will see what flex PCB is, where flex PCB can be used, why it is more costly, and finally how to order a flex PCB from a manufacturer like PCBway.

For the first impression, readers may think that PCBs may come in 2 types either rigid type or flex type only, while there is another third hybrid type called rigid-flex PCB.  Rigid-flex PCB is a circuit board that has both rigid circuit section and flex circuit section and they are laminated together. Flex PCB are also known as Flex Circuit Board or FCB.

The below figures are showing example of flex and rigid-flex PCBs.

Image Courtesy to PCBWay. On the right, an assembled flex board. On the left, a panel of rigid-flex board

Flex boards can be used as connectors, and as full circuits assembled with components. And yes it can hold the soldering heat moreover it can stand way more than rigid ones with high temperature based on the used film material type.

Rigid-flex PCB from a systematic point of view may not have the flex part laminated permanently, it could be soldered or connected with it through a connector like the 3 different types of rigid-flex PCB image below shows.

Image courtesy to Würth Elektronik Flex-Rigid Design Guide. In the left figure, the flex is soldered in or plugged into the rigid section. In the middle figure, the 2 boards have the flex part as part of their design stack-up. In the right figure, one board has the flex part in its stack-up and connected  to a rigid PCB through a connector.

However, if you choose the rigid-flex option, then It is up to you to select which layer to be the flex one in your design layer stack-up. The below image shows a flex-rigid board with flex as bottom layer.

Image Courtesy of EEVblog #1262

Keeping in mind that it is advised to have the flex layer in the middle, when that is applicable.

Image courtesy of epec Engineering Technology, different configurations of flex layer in design stack-up.

The reason behind the freedom of choosing the flex layer position is that the manufacturing process will be similar. The multi-layer will be built layer by layer.

Another first impressions about flex PCBs may be thinking that it’s single or double side only, but actually flex PCB can be multi-layer just like the usual rigid PCB.

Image Courtesy to PCBWay. Showing single-layer , double-layer and multi-layer flex PCB

Flex PCB is More Costly than Rigid One, Why ?

It is about complexity and the number of material and process involved to make your flex PCB done. Flex PCB manufacturing has the same rigid pcb steps in addition to other ones.

The following video clip from Youtube shows the process in details, and according to this video it’s clearly that flex pcb has more complexity and human interaction than the usual rigid boards and as a result of this: more cost!

Keeping in mind that customers may have other special requirements that would increase the cost even more. Example of these special requests is to add what is called Stiffener, which is a rigid material (can be FR4) added to specified areas of flex PCB  to increase overall thickness.

Image courtesy of PCBWay, showing a stiffener (dark brown)

Moreover, a webinar from Epec Engineered Technologies on Youtube showed a roughly comparison between the rigid vs flex PCB manufacturing from a number of processes and material perspective. Rigid PCB has 8 steps of manufacturing compared to 17 in flex PCB with more materials involved.

Image Courtesy of  Epec Engineered Technologies webinar, showing difference between manufacturing rigid PCB and flex PCB.

Flex PCB Use Cases

Actually the examples of using flex PCB is unlimited, but showing some could clarify the importance of this technology rather than thinking that it is only good for connectors and screen panels.

Regards using flex PCB as connectors, it could be a cost effective approach compared to rigid ones connected with wires. This is because connecting 2 PCBs together may require in some cases designing a special harness cable or connector which will add additional steps in assembly and introduce more cost for testing and assembly.

Another case of using flex PCB is when doing a replacement of a part with a different package without changing the PCB design. This is what Dave from evvblog did to replace stm32f070f6p6 / TSSOP package with stm32f070CB / LQFP package.

Image courtesy of EEVblog #1262. A sketch of flex-pcb outline that will be a TSSOP to LQFP replacement.

Dave went through a lot of details while designing the PCB and finally showing how to order from a manufacturer, PCBway.

Image courtesy of EEVblog #1262. The flex PCB design in 3D.

Roles of Thumb in Flex PCB Design

This paragraph will not state all roles of thumb in FPC design, rather it will mention few in order to give the reader an idea of what things to take into consideration while using flexible boards. Some of them are shared even with rigid PCB.

Wide to Thin Copper Traces

If you want to have more flexibility, then you would use thinner copper traces to have less resistance to bending. In the same time you may use wide endings at the edge to allow soldering the flex cable somewhere else. If the transaction is done in a sharp way then this will be considered as a weak point when soldering. So it is better to do a gradient change in width between thin and wide traces in the mentioned case.

Image Courtesy of EE Journal. Ben Jordan from the video presented this figure, showing the desired change in width in the right figure, while the left one is not desired.

Adding Offset between Top and Bottom Traces

Another tip is making an offset between the traces on top and bottom and avoid making them on top of each other, this will give better mechanical performance to avoid cracking traces.

Image Courtesy of EE Journal. Double sided flex-pcb. An offset is desired to be added between top and bottom traces.

Pads in Flex-PCB / Hold-down tabs

In order to make pads stronger in flex-PCB especially for single side and non-plated through holes, parts of Pad should be covered with coverlay/solder mask, like the following configurations in the figures below.

Image Courtesy of Minco flex-circuit design guide

Another tip is to add extra copper in the shape of a tab! The image below shows how this is done.

Image Courtesy of Minco flex-circuit design guide

No Solid Polygons

When it is applicable, solid polygons should be avoided as solid copper will make the PCB less flexible. Instead, a hatched polygon could be used, like the image below.

Original Image Courtesy to PCBWay

Rounded Corners

It is advised to use rounded edges in the areas of expected/designed to be bended, this will greater the tear resistance if compared to 90 degree sharp edges.

Image Courtesy of Minco flex-circuit design guide

Walk Through of a Flex PCBWay Quotation Calculator

PCBway has a detailed calculator to find how much your design will cost you. As flex PCB is more costly than rigid one, then it is more important to check your manufacturer calculator to make sure that your design complies to your manufacturer rules and more importantly to check how you can decrease/optimize the manufacturing cost.

PCBWay Calculator

The first part of information to define is PCB type: flex or rigid-flex. Next, the number of layers, and if you need your quantity to be in a panel or cutted into single pieces. Panel is important if you want to assemble your board with a PnP (Pick and Place) machine.

An important note about size and quantity should drag your attention, as changing quantity may not increase/decrease the price for you. Here is an example:

The next part of the calculator, is to select the flex substrate/film material type, currently only Polyimide Flex is provided. Knowing that Polyamide is the most common material and that’s what Wikipedia stated: “There are a number of different materials used as base films including: polyester (PET), polyimide (PI), polyethylene naphthalate (PEN), polyetherimide (PEI), along with various fluropolymers (FEP) and copolymers. Polyimide films are most prevalent owing to their blend of advantageous electrical, mechanical, chemical and thermal properties. “.

The other thing to select is the film thickness of FPC. This is similar to FR4 thickness selection in rigid PCB. Some thickness options are related to the number of layers. For example, in the image below options for single layer, upto 0.15 mm, while 2-layers is up to 0.26mm . Increasing the thickness may increase the total cost. If thickness is a key thing for your design, then you should check their thickness tables for flex PCB stack-up.

Next, is the turn of selecting some colors of flex PCB: solder mask/coverlay , silkscreen and if needed plating some edge terminals with gold. These options seem to have no effect on cost.

Another important part a flex PCB design may need is stiffeners which are ,as we said before , selected areas in the PCB need to have some rigid material in order to give it additional mechanical stability.

The rigid part will be laminated to flex. The material type provided in PCBWay is FR4 and Metal (not mentioned what type of metal but stainless steel or aluminum are usually used). The image below shows an example of using stiffeners for areas like the edge connector and mounting holes. This option affects the manufacturing cost.

Original Image Courtesy to PCBWay

Also customers need to choose the type of surface finish, which is a process to add extra surface (coating) on top of copper layer to help solderability. The surface finish has 4 common types: Immersion Gold (ENIG), Organic Solderability Preservatives (OSP), Immersion Silver (Ag) and Immersion Tin. All of these to preserve copper from oxidation and to achieve better properties for components assembly. However, each type has its advantages/disadvantages.

It seems only increasing the thickness of immersion gold will increase the cost.

Options are not finished yet, as you need to choose the copper layer thickness between few available options. Some of the thickness is related to the number of layers and base thickness. Check the clip below:

This option ‘may’ increase your manufacturing cost. Now, we reach the last part of the calculator.

PCBWay may do some electrical test for your PCB to make sure nothing went wrong during manufacturing, .i.e. short circuit or a cut in a trace … etc. This will increase the cost.

The next option is to add adhesive tape to selected areas of your flex PCB, this could be needed to mount the PCB to a surface or stick it to something. This option ‘may’ increase the cost a little.

Finally, if you concern about Electromagnetic interference (EMI) then you can add a shielding film. This is similar to the metal house added to some part of rigid PCB as shown in the figure below.

The shield is the black area in the left image. The right Image shows the metal shield in rigid PCBs.

After all, it is time to upload your design files and submit your order. If you like to see a next article about flex PCB design, how to generate design files and putting an order, then state that in the comments section.

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