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Future Trends of Flexible Circuit Boards
Rigid-Flex PCB Stack-up for Impedance Controlled Designs
Control Impedance Between Rigid PCB and Flex PCB
Flex PCB Reliability and Bendability
Normal Flex PCB Specifications
Flex PCB Polyimide Coverlay and Solder Mask
Flex PCB Boards and Connectors
About RA Copper and ED Copper
Introduction of Flexible PCB
5 Tips For Designing Flexible PCB
Advantages of FPC (Flexible PCB)
Evolution of the Flex Printed Circuit Board
Benefits of Using Flex Circuit Boards
Why Rigid-Flex PCBs are Economical?
Flexible PCB vs Rigid PCB
Development of Flexible printed circuit board (FPC) market
Traditional Manufacture Engineering of FPC Substrate
Development Trend of FPC Board
Flex PCB and the Manufacturing
About Flex PCB design
About Flex PCB and Assembly
How to Ensure Flex PCB Design Success
How to Select the Appropriate FPC Materials?
The Differences In Rigid PCB, Flex PCB and Rigid-Flex PCB
Flex-Rigid PCB Design Guidelines
Beneficials for Polyimide Flex PCB Boards
About Stiffener on Flex PCB FPC circuit Boards
About ENIG and ENEPIG
PCB Surface Finish Comparison
Copper Thickness for FPC Boards
Interconnect Solutions for Flexible Printed Circuits and Etched Foil Heaters
Advantages and Disadvantages of Rigid-Flex PCB
About FPC Plating Process
About EMI shield design for Flex Printed Circuit Board
PCB Assembly Blog
About PCB Assembly
QFP and BGA and the Development Trend in PCB assembly
Why some components need be baked before reflow soldering
About Flex PCB Assembly
Manual Soldering in SMT Assembly Manufacturing Process
BGA Components and BGA Assembly
Quick Understanding for PCB Assembly Process
About SMT Assembly (Surface Mount Technology)
About THT Assembly (Through-Hole Technology)
About Reflow Soldering
About_Wave_Soldering
PCB Assembly Inspections and Tests
Panel Requirements for PCB Assembly
About SMT (Surface Mount Technology)
In recent years, with the rapid development of portable electronic products, flexible substrate to high-density development, flexible circuit board is widely used in IC circuit packaging, but also promote the development of flexible substrate to high-density direction. In the field of packaging, flexible substrates have some incomparable advantages over rigid substrates. For example, it is suitable for the assembly of ultra-thin chips, flexible electronic system packaging, medical implantable organs and other special purposes. The main application forms of flexible substrate in IC package include BGA(Ball Grid Array), CSP (Chip Scale Package), COF (Chip on Film) and MCM (Multiple Chip Module).
 

In recent years, 3D packaging technology has become a research hotspot, which has attracted the attention of many large semiconductor companies, and now there are practical products.

 

 

Traditional Manufacturing Engineering of Flexible Substrate:

 

Traditional manufacturing processes of flexible substrates include “Roll Reduction Method” and “Sheet Metal Processing Method”. Different manufacturing methods have different characteristics, but the most common manufacturing method is “Sheet Metal Processing Method”. Figure 1 shows the manufacturing flow chart of double-sided flexible substrate processing method.

 

 

The blanking process in the manufacturing process of flexible substrates is different from that of rigid substrates. In addition to the blanking of CCL and cover plates, the blanking of cladding and reinforcing panels is also added. Flexible CCL and cladding are rolled, so we need to use automatic blanking machine when using. In the manufacturing process of flexible substrates, the fouling removal technology of drilling holes is also different from that of rigid substrates. According to the provisions of IPC-A-600F, the depth of concave corrosion in the fouling removal process of flexible substrates shall not exceed 50um. However, polyimide is not resistant to strong alkali, and the strong alkaline potassium permanganate solution commonly used in rigid substrates is no longer suitable for the processing of flexible substrates.

 

In order to ensure the decontamination effect and concave corrosion depth meet the requirements, plasma is generally used in the industry for decontamination and concave corrosion. In the PTH process, it usually uses the alkaline solution.Due to the long-term reaction will cause FPC material swelling, which will lead to holes and poor mechanical properties of the coating, such as ductility and adhesion. At this time, the substrate material is very easy to break under thermal shock, so generally after the thickness of the copper layer during the PTH process reaches 0.3 to 0.5um, it is transferred to the electroless plating of the whole board until its thickness reaches to 3 to 4um, in order to ensure the integrity of the copper layer of hole wall in the subsequent processing process.

 

In terms of line production, flexible substrate etching is slightly different from rigid substrate. In general, a large number of parallel wires are designed at the bending part. In order to ensure the consistency of etching, the spraying direction, pressure and transmission direction of etching solution should be appropriately adjusted during etching. Etched circuit boards are treated to increase adhesion prior to aligning the overlay. The cladding layer after drilling and the etched flexible circuit board have different degrees of moisture absorption, so these materials should be dried in the drying oven for 24 hours before lamination, stack height should not exceed 25 um. In the case of the PCB etched by the covering layer, a special alignment fixture or a visual magnifying glass can be used for alignment. After alignment, butanone or hot flame iron can be used for positioning.

 

According to different FPC materials to determine the lamination time, heating rate and pressure and other lamination process parameters, flexible multilayer substrate lamination ratio than rigid substrate complex, the substrate, adhesive and gasket material matching window is relatively narrow, only the correct choice of materials and appropriate processing parameters can achieve the ideal lamination effect. Pictue below shows the FCCL and the FPC stack-up.

 

 

FCCL Material Thickness
Coverlay/Solder Mask 2Layer FCCL
 PI (12.5/25/50/100um, etc.) Copper (6/9/12/18/35/70/100um, etc.)
 AD (15/25/35/50um, etc.) AD (0/12.5/20/25um, etc.)
  PI (12.5/25/50/100um, etc.)
1Layer FCCL AD (0/12.5/20/25um, etc.)
 Copper (6/9/12/18/35/70/100um, etc.) Copper (6/9/12/18/35/70/100um, etc.)
 AD (0/12.5/20/25um, etc.) 
 PI (12.5/25/50/100um, etc.)

 

Flex PCB Stack-up Structure

 

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