How Do I Order Flex PCB?

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A flex PCB, also known as a flexible circuit, is PCB technology. It has many similarities to traditional PCBs but differs slightly in the manufacturing and assembly process. Like traditional circuit boards, flex PCBs are used to mount electronic components. But unlike traditional PCBs, the electronic components are placed on a flexible substrate rather than a rigid board.

Flex PCBs are rapidly growing in popularity due to their ability to fit and adjust to any shape and space. They are used in the automotive, medical, and consumer industries. If you want to order a flex PCB for your project, you must provide specific and accurate details.

This article will discuss the specifications you need to provide when ordering a flex PCB. Let’s get started!

Flex PCB Board Type

The board type for flex PCB is classified based on the number of layers you want, for example:

  • Single-sided Flex PCB: Consists of a single layer of copper and polyimide. You can utilize the conductive copper layer from only one side of the board.
  • Single-sided Flex PCB with Dual Access: This also consists of a single layer of polyimide and copper, but you can access the conductive layer from both sides of the board.
  • Double-sided Flex PCB: In this board type, the base polyimide layer has a conductive copper layer on both sides, and the layers are connected through a hole.
  • Multilayer Flex PCB: Consists of multiple combinations of single-sided and double-sided flex PCBs connected through different holes in the circuit board. Complex assembly requires accurate details for design and manufacturing.

Flex pcb Board Material

The board material or substrate used for the flex circuit needs to be flexible. The substrate acts as a conductor carrier and insulator between the circuits. The most commonly used board material for flex PCBs are:

Polyimide

Polyimide is a popular choice because it has a high temperature-resistant capacity and thus will not delaminate under the soldering process. It is cost-effective, and humidity absorption has significantly dropped in the upgraded polyimide. It has average electrical characteristics and good physical capabilities.

Polytetrafluoroethylene (PTFE)

It is extremely stable and has a low dissipation rate. It can also withstand high heat and is used for materials requiring high signal transmission, such as in the automotive and defense industry.

Polyester Film (PET)

Polyester films are another reliable board material for flex PCB. They can be used in copper laminates or adhesive overlays. PET can be used in mobile phones, LCD screens, the aerospace industry, and other digital products.

Flex pcb Stack Up Type

The stack-up type also depends on the number of layers. A flex PCB can have as many layers as possible, but increased layers decrease flexibility. The stack-up specification is necessary for the manufacturing and ordering process so that PCB manufacturers can suggest the suitable material and assembly depending on the required layers and thickness.

Single Layer

This is the simplest and most cost-effective stackup type for flex PCBs. Its thickness range is around 0.08mm to 0.16mm.

Polyimide

Coverlay

Adhesive

Copper

Base Material

Adhesive

Polyimide

 

Two Layer

Layer thickness ranges from 0.1mm to 0.29mm.

Polyimide

Coverlay

Adhesive

Copper

Base Material

Adhesive

Polyimide

Adhesive

Copper

Bottom Copper Layer

Adhesive

Bottom Coverlay

Polyimide

 

 

Four Layer

Layer thickness ranges from 0.2mm to 0.26mm.

Polyimide

Coverlay

Adhesive

Copper

Top Copper Layer

Adhesive

Polyimide

Pure gum

 

Copper

2nd Copper Layer

Adhesive

Polyimide

Adhesive

Copper

3rd Copper Layer

Pure gum

Polyimide

Adhesive

Copper

Bottom Copper Layer

Adhesive

Bottom Coverlay

Polyimide

 

Surface Finish

Surface finishes are used to cover up the copper features in flex PCBs. Copper is most widely used as a conductive layer. It has many advantages and disadvantages, but one major con is that it corrodes easily and faster. Corrosion can lead to high resistance in surface contact, which leads to decreased signal integrity. Thus, the need arises to add a surface finish on copper components.

There is a wide range of surface finishes to choose from, but the most commonly used are:

Electroless Nickel Immersion Gold (ENIG)

This is the ideal RoHS-compliant solder paste reflow. It is extremely smooth and easily accepts solder. ENIG accepts direct chip attachment, and you can easily place components on it. The nickel in the surface finish acts as a barrier between the copper and gold components to prevent corrosion.

Organic Solderability Preservative (OSP)

OSP is an eco-friendly option in surface finishes. It is cost-effective and is applied by spraying or dipping. The surface finish is formulated to attach only to copper components. It is a short-term antioxidant and has a limited shelf life.  

Immersion Tin

Immersion tin might not be the ideal surface finish, but it is commonly used. It helps in short-term oxidation prevention, but overtime, the tin can oxidize and cause dendritic growth problems. Immersion tin can is also not used in areas that require a high degree of flexibility.

Flex pcb Stiffeners

Stiffeners are not necessary for your flexible circuit board. They are only added when needed. Stiffeners help provide mechanical support and are used to strengthen specific parts of the circuit board. You can also add stiffeners to increase the thickness of the board or ensure compliance with the ZIP connector.

Depending on the thickness you want, flex PCB stiffeners are available in two types:

  • FR-4 Stiffeners: 0.008” to 0.059” thickness
  • Kapton: 0.002” to 0.010” thickness

Other options for stiffeners include stainless steel and aluminum.

Coverlay

A coverlay, also known as a cover film, is a lamination process used to encapsulate any external circuitry of the flex PCB. It serves the same purpose as a soldermask and can be used in combination with a soldermask. For example, apply soldermask in rigid component areas and coverlay in flexible areas.

The choice of coverlay depends on the thickness required. Typically, a polyimide layer is used, laminated using heat and pressure. Cover lays are preferred over soldermask as they increase the flexibility and reliability of the flex circuit board.

And that’s it. Aside from these, you also need to provide information about controlled impedance, via type, and solder mask color. It’s best to consult a PCB manufacturer with an easy ordering process, such as Hemeixin. You have to provide the specifications in the ready-made form, get an instant quote, add to the basket, and then choose your preferred payment and shipping method.

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