What is a flexible PCB?
Answering your questions including ‘what is a flexible pcb’ and explaining their advantages, keep reading to discover the world of flexible PCBS
What is a Flexible PCB?
Flexible printed circuit boards, also called flexible PCBs, are regarded as printed circuit boards which are flexible and can be bent once or multiple times to fit a product’s requirement. Noting this, there are still significant differences between rigid PCBs and flex circuits when considering design, fabrication and functionality.
A flexible printed circuit consists of a metallic layer of traces – which is usually copper – bonded to a dielectric layer – which is usually polyimide.
The thickness of the metal can vary; it can be extremely thin (<.0001″) to extremely thick (> .010″), and the dielectric thickness can vary from .0005″ to .010″.
Often an adhesive is used to bond the metal to the substrate, but other types of bonding such as vapor deposition can also be used to attach the metal.
Copper tends to readily oxidize so normally the exposed surfaces are covered with a protective layer. Gold or solder are the two most common materials because of their conductivity and environmental durability. For non-contact areas, a dielectric material is used to protect the circuitry from oxidation or electrical shorting.
We’re experts in all things related to flexible pcbs, including flexible PCB design. Keep reading to discover the advantages of flexible PCBs and get in touch with us to get your PCB designed by our specialist team.
What are the advantages of flexible PCB?
Now you know the answer to the question ‘what is a flexible pcb?’ you can start looking at some of their advantages.
An advantage of a flexible PCB is that the number of material combinations that could go in a flexible printed circuit are nearly endless; current, capacitance, chemical and mechanical resistance, temperature extremes and type of flexing are just some of the criteria that impacts the material selections that best meet the functional needs.
This makes flexible PCBs a great solution to a product packaging problem.
Further advantages of flexible PCBs are:
- Flexible circuits allow unique designs which solve interconnection problems
- Flexible PCBs require fewer connection and solder joints, which lowers the cost and improves product reliability
- The ability to fold and form flex circuits enables a package size reduction due to a decrease in weight and space. This considerable weight reduction is a benefit over wire harnesses, and rigid assemblies.
- The thinness of the material makes flexible circuitry the best candidate for dynamic flexing applications up to millions of flexures.
- Flexible circuits dissipate heat at a better rate than any other dielectric materials, meaning that flexible PCBs are a great option to be used in high temperature applications.
You might be surprised that today’s flex circuits are so good at getting rid of heat, especially because the boards are getting smaller and smaller. Size is not important. Instead, the real key is surface-to-volume ratio, and flex circuits offer an excellent surface-to-volume ratio.
Additionally, the circuit’s compact design allows for a shorter thermal path, which prevents the build up of heat. Recently consumer electronics giant Samsung’s probe into its Galaxy Note 7 exploding was found to be overheating. This helps you understand how dangerous it can be when a device doesn’t dissipate heat well.
In addition, flex circuits make the installation and repair both practical and cost effective, and with flexible PCBs, replacing the circuit board and wires is relatively straightforward.
Plus crucially, flexible circuits eliminate human error which is common in wire assemblies. This means that using flexible PCBs ensures that the manufacture of your PCB is least likely to go wrong.
Due to their many advantages listed above, we use flexible PCBs in our custom PCB design process for a range of projects.
Although you should now be able to answer the question ‘what is a flexible pcb?’, you may still not know when different types of circuit are needed. Keep reading to find out more.
When is a double-sided flexible circuit needed?
A double-sided flexible circuit is required when the circuit density and layout can’t be routed on a single layer, Maybe Ground or power plane applications being used for shielding applications.
There may be a dense surface mount assembly preventing all routing on one side, however component assembly is available on both sides.
When is a multilayer flexible circuit needed?
As with a double-sided flexible circuit, a multilayer circuit is used instead of a single layer, where the circuit density and layout can’t be routed on a single layer.
The benefits of using a multilayer flexible circuit are:
- A dense surface mount assembly
- Increased circuit density
- Controlled impedance with shielding
- There can be unbonded regions to increase flexibility
- EMI/RFI shielding
- Ground and power plane applications required for shielding applications
When should you use flexible rigid PCBs?
A flexible rigid PCb has an easier assembly in the final product in comparison with other flexible boards. This maximises the space by reducing the need for interconnects between individual rigid boards.
It’s advised to use flexible rigid PCBS when you’re either using high density dual side components or when controlled impedance is required.
Common materials for flexible PCBs
Flexible Base Film
A bendable film that acts as an insulating (dielectric) base material and coated with a dielectric cover layer on the top:
| Polyimide | .0005″ to .005″ (.0127mm – .1270mm) |
| Polyester | .002″ to .015″ (.0508mm – .1270mm) |
| Adhesiveless Materials | Copper thickness .5 oz. to 2 oz. |
| Flame Retardant | Laminates and Cover |
Base Copper Thickness
This is for use as electrical conductors with conductive pathway printed on the base film:
| 1/2 oz. | .0007″ (.018mm) |
| 1 oz. | .0014″ (.036mm) |
| 2 oz. | .0028″ (.071mm) |
| 3 oz. | .0042″ (.107mm) |
| 4 oz. | .0056″ (.142mm) |
| 5 oz. | .0070″ (.178mm) |
| 6 oz. | .0084″ (.213mm) |
| 7 oz. | .0098″ (.249mm) |
Solder Mask
A thin lacquer-like layer of polymer that is applied to the copper traces of a printed circuit board (PCB) for protection against oxidation, and to prevent solder bridges from forming between closely spaced solder pads. Flexible solder mask is essentially the same epoxy-based material as used on rigid PCBs but with the addition of a flex agent:
| Polyimide coverlay | .0005″ to .005″ (.0127mm – .1270mm) |
| Polyester coverlay | .0015″ to .003″ (.0381mm – .0762mm) |
| Photo-Imageable covercoat | Curable liquid for surface mount and dense applications |
Surface Finish
The surface finish forms a critical interface between the component and the PCB. The finish has two essential functions, to protect the exposed copper circuitry and to provide a solderable surface when assembling (soldering) the components to the printed circuit board.
| Hot Air Solder Level (HASL) | Tin Lead |
| Tin Plating* | Immersion Tin |
| Immersion Silver * | |
| Hard Gold over Nickel * | |
| Soft Gold over Nickel* | Electrolytic – typically used for bonding gold wire to the gold layer |
| ENIG (Electroless Nickel Imersion Gold)* | Electroless – typically used for SMT and through hole soldering |
| Organic Coating OSP * |
Stiffeners
When working with flexible PCBs there are times when certain parts of the flexible PCB need to be rigid. We do this by adding mechanical support to the parts of the PCB that require it. This mechanical support is called a PCB Stiffener.
| FR4
Aluminum Polyimide Polyester Stainless Steel |
Contact us
At Quantum CAD, we’re experts in designing a range of flexible PCBs that suit your needs.
We’ve been designing leading PCBs for clients in a wide range of industries for more than 25 years, meaning that we are well equipped to handle your request.
Interested in speaking to a PCB expert to get your project started?
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