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.

Banding:  

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.                                                                   

Pouch:  

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.

Coverlay:  

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!

3 Notices for Layout Double-Side BGA for FPC

Layout double-side BGA for FPC is used in some products, but since the complexity of manufacturing process and maintainability need to be taken into consideration, the designers need to pay attention in followings:

1. Do not overlap when layout double-side BGA and a certain of distance is required left. The reason why to do that as it is difficult to repair if there is poor BGA on one of sides.

2. In order to prevent the BGA on the bottom from melting during the second time reflowing, it is required to decrease the bottom temperature in the second time reflowing. It is better to decrease the bottom temperature lower than to the melting temperature of solder joints to ensure that the solder joints do not melt again. What’s more, you can fix the bottom BGA with jig.

4 ways to increase deformation resistance for flex circuits:

1. The stress resistance of BGA can be strengthened by a supporting iron frame around it.

2. The stress resistance of BGA can be strengthened by gluing around it or on the corresponding back of flexible PCB.

3. If you aim at protecting the BGA, then you are able to add screw to fix the other parts around BGA. In this way, the other parts around BGA won’t deform easily.

4. Strengthening the outer shell of flex circuits to avoid its deformation affecting the inner circuit.

As the premier expert in flexible circuit fabrication, Best Technology has a team which always striding for high quality flex circuits and services. If you would like to know more about Best Technology, please give us a call at 0755-29091601 or view our website: https://www.bestfpc.com/

How to Paste Coverlay on FPC?

If you have ever learned something about flex circuits, you would know there is a layer of coverlay on it. Pasting coverlay on flexible circuit is to prevent it from the damages of dust, moisture and so on. Meanwhile, it also plays an role in insulation. We have learned something about the benefits of pasting coverlay on flex circuits, but how to paste the coverlay? Let’s take a look.
To begin with, we need to prepare the coverlay material, checking whether the soldering iron has power, cleaning the residue of soldering iron and the dust-free paper.

The procedures of pasting the coverlay on FPC:

  1. Checking whether the coverlay and film have problems, including missing punching, missing drilling, too large burr on the coverlay etc.. After that, clean the dust on the surface of coverlay with packaging cleaner.
  2. Cleaning the worktable and putting the coverlay, soldering iron and other tools in the right place.
  3. Checking whether there are oxidization, litter, dust etc. on the flex circuits to be pasted.
  4. Pasting the coverlay should follow the requirements of process and the MI data. It should be strictly follow the ‚ÄúC‚ÄĚmarking line, pad or counterpoint hole when align.
  5. How to peel the coverlay off: putting the covelay on the optical desk, but please remember that the release film is facing up and the coverlay is facing down. After that, holding the coverlay then peeling off the release film slowly.

peeling the coverlay off

peeling the coverlay off

  1. Fixing the coverlay on FPCwith soldering iron after the coverlay is aligned with the ‚ÄúC‚Ä̬†marking line of the flex circuit board¬†and ensure it is correct, so that the coverlay can not deviate or peel off. However, please remember the soldering iron can not burn the products.

pasting coverlay on FPC

pasting coverlay on FPC

  1. Single-side lamination: after pasted the coverlay on FPC, it should be gone through the machine with temperature between 160¬įC-165¬įC. Next, until the board got cold, the release film should be peel off and carry out the single-side lamination and self-checking.

Notices:
1. It is necessary to wear rubber gloves to prevent the flexible printed circuit board from oxidization or dirty.

  1. It must be kept clean during pasting coverlay on FPCto prevent there are dirty residues on flex circuits.
  2. It is required to check whether the coverlay is broken, wrinkled, scratched, etc..
  3. Do not wear accessories to avoid scratching the board.
  4. It is necessary to confirm that the coverlay leave a little bit space for plating if the flex circuitsare tin or gold plating.
  5. The hand could not beyond the handle to avoid burning when holding the soldering iron.

Best Technology is the premier expert in the manufacture of electronic components, brings a unique combination of technology and quality that offers a competitive supply of flex circuits and rigid-flex circuits and other flexible circuit products. With high qualified engineers, we are able to help you find a FPC with high quality. To find out more, contact Best Technology at 0755-29091601 or check the website: https://www.bestfpc.com

The Keys in the Mobile Phone

With the development of smart phone, the mechanical key has been already eliminated from the stage. There is only the HOME key left on the front side of the phone. Even sometimes the HOME key is also out and there are only two keys left–volume key and power key. These two are usually used a FPC¬†to save cost as well as space. The structure showing as below:

volume key & FPC

volume key & FPC

power key & FPC

power key & FPC

For the design of volume key and power key, there are two things required to considerate. On one hand, the position of the flex circuit. Since the flexible circuit is glued on the supported wall, three directions need to be limited if you would like to fix the FPC. As well, the designers still need to consider the installation position of the keys itself, which determines the basic structure of the keys.

On the other hand, the volume key and power key are moving parts. Therefore, taking the stroke of metal domes into account is necessary and as general rule the stroke is 0.3. In addition to hard contact mode, there is also a soft contact mode (P+R), which is to use rubber or TPU (Thermoplastic polyurethanes) for the part where the key is contact with the dome, so that the sound is softer.

key structure

key structure

Best Technology is the premier expert in the manufacture of electronic components, brings a unique combination of technology and quality that offers a competitive supply of flex circuits and rigid-flex circuits and other flexible circuit products. To find out more, contact Best Technology at 0755-29091601 or check the website: https://www.bestfpc.com/