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 How To Make a Flexible PCB-FPC Manufacturing process guide

How To Make a Flexible PCB-FPC Manufacturing process guide

Introduction

As a manufacturer with more than 20 years of Rigid flex PCB production experience. GESP would like to share how to make flexible boards. as well as our flexible board production process. so that customers from the selection of FPC raw materials, to the fpc production process. To understand our professionalism in the manufacturing process of FPC. And our scientific production management.

Let us through the FPC material, FPC featuers of the process. That has been a detailed understanding of each step of the production process of flexible board.

Materials of Flexible PCB

Choose the appropriate base material, which is typically a flexible polymer such as polyimide (PI) or polyester (PET). Select the copper foil thickness based on the desired electrical properties.

→ Types Materials of Copper foil divided

Calendered copper (ROLLED ANNEAL Copper Foil) and electrolyzed copper (ELECTRO DEPOSITED Copper Foil). Its folding resistance and elasticity coefficient higher electrolytic copper foil. and it is more suitable for flexible copper cladding boards. Its copper purity (99.9%) is higher than that of electrolytic copper foil (99.8%). It is smoother than electrolytic copper foil on the gross surface. which is conducive to the rapid transmission of electrical signals.
Electrolytic Copper Foil:

Produced on a circular cathode roller by means of a specialized electrolysis machine. This initial product called gross foil. The gross foil is then surface treated.

→ Reinforcement Film 

Mainly used to reinforce the mechanical strength of the FPC. to facilitate the surface mounting operations. Common thickness: 5mil and 9mil.

→ Adhesive

Adhesive thickness generally depends on customer requirements. Generally 0.5mil epoxy resin adhesive.

→ Coverlay

Coverlay is a combination of substrate + adhesive, mainly for surface insulation. Commonly used materials for PI (polyimide), common thickness of 1 mil and 1/2 mil.

→ Printing ink

Printing inks are generally distinguished as solder resist ink (Solder Mask color) text ink (Legen white, Black) Silver Ink (Silver Ink silver). three kinds of ink and ink types divided into UV hardening type (UV) and thermal baking. Cure and thermal baking type (Thermal Post Cure) two kinds.

→ Adhesive backing

Generally, there are Acrylic adhesive and Silicone adhesive backing. The double-sided adhesive idivided into substrate adhesive and non-substrate adhesive.

Features of FPC

Features of FPC
Advantage of FPCDisvantage of FPC
LightLow mechanical strength, easy to crack
Thin about 0.13MMDifficulty in process design
Short Assembly TimeLow possibility of re-processing
Reduce The Product VolumeDifficulty in inspection
  • Light: lighter than the PCB, can reduce the weight of the final product.
  • Thin: Thinness than PCB thin, can improve the flexibility. PCB  1.6MM thick, and FPC about 0.13MM, only PCB 1/10 thickness. 
  • Short: short assembly time.
  • Small: the volume is smaller than PCB. Can effectively reduce the product volume, increase the convenience of carrying.

Process Manufacturing of Flexible PCB/FPC

flex pcb manufacturing process

Step 1-Cutting Flexible Copper Clad Lamination

Cutting the flexible copper clad laminate is a critical step in the production of Flex PCBs. This process involves shaping the flexible material to the desired size and shape. while preserving the integrity of the copper traces. Below is a detailed description of the cutting process :

Cutting Equipment Setup:

Use a cutting machine or equipment designed for Flex PCB cutting. Install the appropriate cutting tool or blade on the machine. The choice of cutting tool depends on the cutting requirements and the material.

Material Alignment:

Position the flexible copper clad laminate material on the cutting machine’s work surface. Ensure that the material aligned and fixed in place. To prevent any shifting during the cutting process.

Programming and Parameters:

Input the cutting parameters into the machine’s control system. These parameters specify the dimensions and shape of the Flex PCBs to cut. Parameters may include the length, width, shape of cutouts. and any routing or contouring requirements. The cutting machine controlled or operated using CNC for precision and automation.

Cutting Process:

Start the cutting process by activating the machine. The cutting tool, which could be a rotating blade, laser cutter or other precision tool. follows the programmed path to accurately cut the flexible copper clad laminate. The cutting tool can make both straight and curved cuts, depending on the design of the Flex PCBs.

Quality Inspection:

After the cutting process, inspect the edges of the flexible copper clad laminate for any irregularities. such as burrs, roughness, or copper damage.

Edge Treatment (Optional):

In some cases, additional processes like edge plating or deburring required. To smoothen the cut edges and improve the PCB’s functionality or aesthetics.

Step 2-CNC Drilling

  • Tool Selection: A suitable CNC drilling machine selected for the job. These machines equipped with high-speed rotating drill bits, often made of tungsten carbide. which can precisely cut through the flexible substrate without damaging it.
  • Loading Material: The FPC material loaded onto the CNC drilling machine. It is essential to ensure that the material is properly secured and aligned. To prevent any misalignment or damage during the drilling process.
  • Machine Setup: The CNC machine programmed with the design specifications. and toolpaths generated from the CAD design. The operator sets parameters such as drill speed, feed rate, and tool change intervals.
  • Drilling Process: The CNC machine starts the drilling process by moving the drill bit to the specified locations . The drill bit penetrates the material. creating precise holes with dimensions matching the design specifications.
  • Tool Changes: Depending on the complexity of the FPC design. multiple drill bits with different sizes and geometries used. The CNC machine can change the drill bits as needed without operator intervention.
  • Quality Control: Throughout the drilling process, quality control measures are in place. To ensure that the holes are being drilled accurately and without any defects. Automated inspection systems or manual checks used to verify hole dimensions and alignment.
  • Deburring: After drilling, there may be burrs or sharp edges around the holes. These are typically removed through a deburring process. to ensure the FPC’s integrity and prevent any damage to components that will mounted.
  • Cleaning and Inspection: Cleaned the FPC to remove any debris or contaminants. A final inspection conducted to verify that all holes are within tolerance.
1.cutting board

Step 3-Chemical Cleaning

Cleaning Chemical Selection:

The appropriate cleaning chemicals selected based on the type. and level of contaminants present on the FPC materials. Common cleaning chemicals used include organic solvents, alkaline cleaners, or specialized cleaning solutions.

Immersion or Spray Cleaning:

Depending on the cleaning chemicals chosen and the specific FPC design. the cleaning process can involve immersion or spray cleaning methods.

  • Immersion Cleaning: The FPC immersed in a chemical bath containing the cleaning solution. This allows the chemicals to dissolve and remove contaminants from the surfaces.
  • Spray Cleaning: The cleaning solution sprayed onto the FPC under pressure. The force of the spray helps dislodge and remove contaminants.

Agitation and Ultrasonic Cleaning (Optional):

In some cases, especially when dealing with stubborn or tightly adhering contaminants. agitation or ultrasonic cleaning used. Ultrasonic cleaning involves the use of high-frequency sound waves. to create tiny cavitation bubbles in the cleaning solution. which agitate and dislodge contaminants from the surfaces.

Rinse and Drying:

After the cleaning process, the FPC materials rinsed thoroughly with deionized water. or a suitable rinse solution to remove any remaining cleaning residues and chemicals.

Drying:

The cleaned FPC materials dried carefully to prevent recontamination. Air drying or low-temperature ovens used, depending on the materials and cleanliness requirements.

Step 4-PTH(Plating Through Hole)

FPC plating through hole, also known as plated through-hole (PTH) technology in the context of flexible printed circuits (FPCs), is a crucial process that involves adding a conductive layer inside drilled holes to establish electrical connections between different layers of the FPC. Plated through holes are essential for creating interconnections between various circuit layers and for attaching components to the FPC. Below is a detailed description of the FPC plating through-hole process:

  • Activation: The cleaned copper surfaces are then activated using a chemical treatment. Activation promotes adhesion the subsequent plating material to adhere to the copper effectively. Activation chemicals often contain palladium or other catalytic agents.
  • Electroless Copper Plating: In the next step, an electroless copper plating process used to deposit a thin layer of copper. evenly on the interior walls of the drilled holes and the exposed copper traces on the FPC’s surface. Electroless plating does not require an external power source. instead, it relies on a chemical reaction to deposit the copper. This process helps create a continuous conductive path from one side of the FPC to the other.
  • Copper Plating Thickness Control: The thickness of the copper plating controlled to meet the design specifications. The plating process repeated multiple times to achieve the desired thickness. and quality control checks performed to ensure uniform plating.
chemical cleaning
PANEL PLATING

Step 5- Dry-film lamination

Dry Film Application:

A roll of dry film, which consists of three layers – a clear polyester backing. a photosensitive layer, and a protective cover sheet,unwound and aligned with the copper-clad laminate.

The dry film is then applied to the substrate using a lamination machine. which typically includes rollers and heated platens to ensure uniform pressure and adhesion.

The protective cover sheet peeled off, exposing the photosensitive layer to the substrate.

After plating the through holes completed a heated roller. utilized to pressurize the cleaned after the through-hole plating completed. the dry film laminated on the cleaned material as etching resist by heating roller and pressurizing.

Operating environment: Because the dry film is sensitive to ultraviolet rays. To avoid the reaction of the dry film before the dry film laminated. the operating environment of the dry film lamination must be in the yellow light area.

Step 6- Exposure

The finished material laminated and transferred. To the dry film using the image transfer method.

The completed working negative exposed to ultraviolet light. and transferred to the dry film. The negative used for exposure. and the part that hollowed out and transparent is the line and copper area.

Operating environment: yellow light area

Operation mode: manual alignment, jig alignment.

Step 7-Developing

After exposure, the dry film in the area exposed to ultraviolet light polymerized and hardened. and the unexposed hardened part of the film washed away. with a specific solution for developing the material, exposing the copper layer of the material. The shape and type of the line to formed seen in the finished material after developing.

Step 8-Pattern Etching

The exposed copper layer that has not protected by the dry film removed. once the material has developed and rinsed with etching solution, leaving the protected line.

The exposed portion of the copper layer that is not protected by the dry film removed. leaving just the protected line. The line we need etched completed material; etching is the main phase of line molding. The etching chemical process (regeneration and reduction reaction) is the operating principle.

  • Cu+CuCl2→Cu2Cl2
  • Cu2Cl2+HCl+H2O2→2CuCl2+H2O
explosure
developing
Flex Circuits Testing and Validation

Step 9-Stripping of Dry Film

There is still a hardened dry layer on the board surface after etching the final material.

The Dry Film Stripping method used to completely detach the dry film from the material while leaving the wires intact.

The stripping procedure separates the dry coating from the material. exposing the circuit and the copper layer to the elements.

Strong alkaline NaOH solution used for stripping.

Method of operation: (2% & 5% each) (25%HCl)

Step 10-Lamination

FPC (Flexible Printed Circuit) lamination is a key step in the manufacturing of flexible circuits. which used in various electronic devices and applications. due to their flexibility and adaptability. The lamination process involves bonding multiple layers of materials. together to create a flexible and functional circuit. Here is a detailed description of the FPC lamination process:

  • Pre-Lamination:To preserve the wiring and meet customer requirements. it covered with a layer of insulating material known as a “coverlayer,” or “coverlay.” The final coverlayer oriented to the position. and then pseudo-adhered to the cleaned copper foil material throughout the operation.
  • Hot Press Lamination: After the pre-lamination, a heat press used to apply high temperatures. and high pressure to melt the protective film adhesive. which serves to fill the gaps between the wires and attach the copper foil material and the protective film. Working methods include traditional pressing and rapid pressing. Note: The use of hot solvent, the effect of pressing method on the product’s dimensional change
  • Cooling: After the pressure application, the stack is gradually cooled. to set the adhesive layers and solidify the bond between the materials.
lamination
coverlay registration
e-test

Step 11-Surface Finish

Following hot lamination, the exposed copper foil plated under the customer’s specifications.

To prevent the exposed areas of the copper foil from oxidization. and to assure compliance with the standards. the exposed parts electroplated or chemically gold plated with tin-lead-nickel-gold. Various metals serve to protect the exposed part from oxidation and to meet performance criteria.

Working conditions: Confirm the working circumstances and tolerance values. and make the appropriate decision based on the product application. Make the appropriate decision based on the product application.

Surface Treatment Options: Various surface treatment options are available for FPCs. depending on the specific requirements of the application. Common surface treatments include:

  1. Solder Plating (Hot Air Solder Leveling – HASL): – In this process, a layer of solder applied to the copper traces. It provides a flat, even surface for component soldering and enhances solderability. HASL is suitable for applications where component placement and soldering are critical.
  2. Immersion Gold (ENIG – Electroless Nickel Immersion Gold): – ENIG involves depositing a thin layer of nickel. and then a layer of gold on the copper traces. This provides excellent solderability, corrosion resistance. and a flat, planar surface for fine-pitch components. ENIG is commonly used for high-reliability and fine-pitch applications.
  3. Immersion Silver: – Like ENIG, immersion silver involves depositing a thin layer of silver on the copper traces. It offers good solderability and is often used for high-frequency applications.
  4. OSP (Organic Solderability Preservative): – OSP is a layer of organic material applied. to protect the copper surfaces from oxidation and contamination. It provides good solderability and is suitable for lead-free soldering processes.
  5. Electroplating: – Electroplating used to deposit additional copper onto the copper traces. which can enhance conductivity, solderability, and durability.

For more information about rigid-flex PCB surface finish, please see the article

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Flexible Circuit , Rigid-flex PCB, FPC Flexible PCB Assembly For One-Stop Request a Quote for Flexible PCB Fabrication

Step 12-Punching Hole

Customized designs molded using a die/knife or laser cutting. The boards separated from unwanted scrap by using a mold/tooling or laser cutting.

Step 13-Electrical Test

  • Probe to test whether there is a break / short-circuit defective phenomenon, to functional
  • test to check the quality of the components to ensure the customer’s use and
  • reliability of customer’s use.
  • Note: The empty board test needs to designed according to the production capacity.
  • Functional testing of soldered parts requires. Understanding of the test conditions and requirements of each part.

For more information about rigid&flex PCB inspection, please read the article:

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Flexible Circuit , Rigid-flex PCB, FPC Flexible PCB Assembly For One-Stop Request a Quote for Flexible PCB Fabrication

Step14- Packing

  • Antistatic Packaging: FPCs are sensitive to static electricity. so it’s crucial to use antistatic packaging materials. Antistatic bags and foam inserts can help prevent electrostatic discharge (ESD) damage.
  • Clean Environment: Ensure that the packaging area is clean and free of dust, debris. and contaminants to prevent contamination of the FPCs during packaging.
  • Proper Handling: Always handle FPCs with clean, lint-free gloves to avoid leaving fingerprints, oils. or other contaminants on the flexible circuit surfaces.
  • Use Protective Film: For FPCs with exposed copper traces or components. consider using protective films or covers. to shield them from physical damage, moisture, and dust.
  • Padding: Use appropriate padding materials. such as foam inserts or bubble wrap to protect FPCs from shock, vibration. and physical impact during transit. Ensure that the padding is snug but not tight to avoid bending the flexible substrate.
  • Labeling: Clearly label the packaging with information. such as the FPC part number, quantity, and handling instructions. This helps prevent confusion and mishandling.
  • Sealing: Seal the packaging securely to prevent dust, moisture, and contaminants from entering. Use sealing methods like zip-lock bags or heat-sealed antistatic bags.
  • Custom Packaging: Consider custom packaging solutions. designed for the size and shape of your FPCs. Custom packaging can provide a more secure fit and better protection.
  • Humidity Control: If the FPCs are sensitive to humidity, include desiccant packs. or humidity-absorbing materials in the packaging to maintain low humidity levels.
  • Static Shielding Bags: If your FPCs are particularly susceptible to ESD. or if they contain sensitive components, use static shielding bags. These bags provide a higher level of protection against static discharge.
  • Documentation: Include any necessary documentation. such as product specifications, quality control reports. or handling guidelines, inside the package for the recipient’s reference.
  • Avoid Overcrowding: Avoid overcrowding the packaging, as this can lead to bending or damage of FPCs. Allow sufficient space between individual circuits.
  • Double Packaging: For added protection, consider double packaging by placing the FPCs in an antistatic bag. and then inside a rigid container or cardboard box with ample padding.
  • Shipping Labels: Clearly mark the packaging with shipping labels. including the recipient’s address, return address, and any special handling instructions.
  • Fragile Stickers: If necessary, apply “Fragile” stickers or labels to state. that the contents are delicate and require careful handling.
  • Documenting Damage: If you receive damaged FPCs, document the damage with photographs. and notify the sender immediately. This will be crucial for insurance claims or replacements.

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