Guide to the Manufacturing Methods of Printed Circuit Boards

1. Overview

This article discusses the methods for surface treating copper foil, carrier-attached copper foil, and their use in manufacturing copper-clad laminates and printed circuit boards. The patent primarily focuses on how to optimize the adhesion of plated circuits, etchability of electroless copper, and dry film resolution in the semi-additive process (SAP) of printed circuit board manufacturing by controlling specific surface profile parameters of the surface-treated copper foil.

2. Core Concepts and Principles

1. Surface-Treated Copper Foil:

• Key Characteristics: At least one side has a treated surface, and the arithmetic mean curvature Spc value of the treated surface and its transfer to the resin film, as measured under ISO25178 standards, should be above 55 mm^-1. Additionally, the peak density Spd of the treated surface should be between 5000 mm^-2 and 20000 mm^-2.
• Spc (Arithmetic Mean Curvature of Peaks): A parameter that measures the sharpness of the surface protrusions according to ISO25178 standards. A higher Spc value indicates sharper protrusions.
• Spd (Peak Density): A parameter that measures the number of peak points per unit area according to ISO25178 standards. A higher Spd value indicates more contact points.
• Smr1 (Load Area Ratio of Separated Protruding Peak Part and Central Part): A parameter that measures the proportion of the protruding peak part according to ISO25178 standards. A higher Smr1 value indicates a larger proportion of protruding peaks.
• Optimization Objective: To provide a surface profile that balances the adhesion of plated circuits, etchability of electroless copper, and dry film resolution in the SAP process.

1. Semi-Additive Process (SAP):

• A manufacturing process suitable for fine circuit printing of printed circuit boards.
• Main Steps (Figures 1 and 2):

1. Pressing and bonding ultra-thin copper foil onto an insulating resin substrate (Step (a)).
2. Peeling off the carrier (if using carrier-attached copper foil), and possibly forming conductive holes through laser perforation (Step (b)).
3. Etching to remove the ultra-thin copper foil, exposing the roughened surface profile of the primer layer (Step (c)).
4. Applying electroless copper plating to the roughened surface (Step (d)).
5. Pattern masking through dry film exposure and development (Step (e)).
6. Applying electroplated copper (Step (f)).
7. Removing the dry film to form the wiring part (Step (g)).
8. Etching to remove unwanted electroless copper, forming the wiring (Step (h)).

• Difference from MSAP Process: In the SAP process, after removing the dry film, only the electroless copper layer needs to be etched, requiring less etching, which is more conducive to forming fine circuits.

1. Carrier-Attached Copper Foil:

• Composition: A structure consisting of a carrier, release layer, and surface-treated copper foil with the treated surface on the outside.
• Carrier: Supports the copper foil, enhancing processability, typically a copper or aluminum foil with a thickness of less than 200 μm.
• Release Layer: Reduces the peel strength between the carrier and copper foil, ensuring stability and suppressing high-temperature diffusion. It can be an organic release layer such as nitrogen/sulfur compounds or carboxylic acids, or an inorganic release layer such as Ni, Mo, or Cr.
• Ultra-Thin Copper Foil: The surface-treated copper foil of the carrier-attached copper foil is preferably ultra-thin, with a thickness of 0.1 μm to 7 μm.

1. Manufacturing Method:

• Roughening Treatment: Conducted through two-stage electroplating, with the first stage having a copper concentration of 8-12 g/L and sulfuric acid concentration of 200-280 g/L; the second stage has a copper concentration of 65-80 g/L and sulfuric acid concentration of 200-280 g/L. The key is to control the ratio of the first stage charge Q1 to the second stage charge Q2 to be less than 1.0, to achieve a roughened surface profile that does not become finer in the intermediate stage.
• Rust Prevention Treatment: Optional, usually includes galvanization (such as zinc-nickel alloy treatment) and chromate treatment to enhance rust resistance.
• Silane Coupling Agent Treatment: Optional, forms a silane coupling agent layer to improve moisture resistance, chemical resistance, and adhesion to adhesives.

3. Key Learning Points

• Understand the physical significance of various surface parameters (Spc, Spd, Smr1) of surface-treated copper foil and their roles in printed circuit board manufacturing.
• Master the specific process flow of the SAP method and be able to compare it with the MSAP method, understanding the advantages of the SAP method in forming fine circuits.
• Understand the structure of carrier-attached copper foil and the functions of its components.
• Understand how the two-stage electroplating process in the roughening treatment affects the surface profile, particularly the optimization of Spc and Smr1 parameters.
• Analyze the experimental data provided in the patent, understanding the impact of different process parameters on the final product performance (peel strength, etchability, dry film resolution).
• Identify the existing technical problems addressed by this invention (the difficulty of balancing the adhesion of plated circuits and the etchability of electroless copper) and its solutions.

4. Exercises

1. Short Answer Questions (2-3 sentences)

1. Please briefly describe the core features of the surface-treated copper foil of this invention, which surface parameters primarily define it?
2. Why does the Spc value of the transferred surface need to be maintained above 55 mm^-1 after transferring the surface shape to the resin film?
3. Explain why it is necessary to etch away the ultra-thin copper foil in step (c) of the SAP process?
4. What positive impact does the “intermediate non-fine uneven shape” mentioned in the patent have on the etchability of electroless copper?
5. What is the purpose of setting the charge ratio Q1/Q2 to be less than 1.0 in the two-stage electroplating of the roughening treatment?
6. Besides roughening treatment, what other optional surface treatment steps may be involved in the manufacturing process of the surface-treated copper foil? What are their respective functions?
7. What is the role of the “release layer” in the carrier-attached copper foil? List two possible types of release layers.
8. What does the Spc value represent according to ISO25178 standards? How is it related to the “roundness” or “sharpness” of the surface protrusions?
9. Please explain the physical significance of Smr1 (Load Area Ratio of Separated Protruding Peak Part and Central Part), and what does the preferred range of this value reflect about the surface characteristics in this invention?
10. What type of products is the surface-treated copper foil proposed in this invention primarily used for? What are the characteristics of this application?

2. Short Answer Question Answers

1. The core feature of the surface-treated copper foil of this invention lies in its treated surface and the surface transferred to the resin film, with the arithmetic mean curvature Spc measured according to ISO25178 needing to be above 55 mm^-1. Additionally, the peak density Spd of the treated surface must be between 5000 mm^-2 and 20000 mm^-2.
2. Maintaining the Spc value of the transferred surface above 55 mm^-1 ensures that the transferred uneven shape still has sharp protrusions, thus forming an “intermediate non-fine uneven shape”. This shape helps ensure good adhesion of the plated circuits while optimizing the etch performance of electroless copper.
3. In the SAP process, etching away the ultra-thin copper foil in step (c) is to expose the roughened surface profile of the underlying insulating resin (such as the primer layer). This transferred uneven shape will be used for the subsequent adhesion of the plated circuits and ensures that the etching amount is less than that of the MSAP method.
4. The “intermediate non-fine uneven shape” allows the electroless copper layer to faithfully follow the uneven shape of the treated surface, without being easily buried or sealed in the recesses by the copper plating. This reduces the difficulty and amount of etching required to remove the electroless copper, thus improving etchability.
5. Setting the charge ratio Q1/Q2 of the two-stage electroplating to be less than 1.0 aims to achieve a roughened treatment surface with an “intermediate non-fine” uneven shape. This control helps optimize the adhesion of the plated circuits, etchability, and dry film resolution of the final product.
6. In addition to roughening treatment, rust prevention treatment and silane coupling agent treatment may also be involved. Rust prevention treatment (such as zinc-nickel alloy coating and chromate treatment) aims to enhance the rust resistance of the copper foil. Silane coupling agent treatment is used to improve the moisture resistance, chemical resistance, and adhesion to adhesives of the copper foil.
7. The release layer in the carrier-attached copper foil is designed to weaken the peel strength between the carrier and the ultra-thin copper foil, ensuring the stability of the peeling and suppressing mutual diffusion during high-temperature pressing. Two possible types of release layers include organic release layers (such as nitrogen-containing organic compounds) and inorganic release layers (such as Ni, Cr).
8. The Spc value, measured according to ISO25178 standards, represents the arithmetic mean curvature of the surface peak points. A higher value indicates sharper contact points with other objects; conversely, a lower value indicates rounder contact points. In short, it quantifies the sharpness of the protruding parts.
9. Smr1 (Load Area Ratio of Separated Protruding Peak Part and Central Part) indicates the ratio of the load area at the intersection of the height of the central part to the load curve, i.e., the proportion of the protruding peak part. A higher Smr1 value indicates a larger proportion of protruding peaks, reflecting that the surface has more significant, unflattened protrusions, which is beneficial for forming the desired “intermediate non-fine” uneven shape.
10. The surface-treated copper foil proposed in this invention is primarily used in the manufacturing of printed circuit boards, especially in the production of printed circuit boards based on the semi-additive process (SAP). This application enables the formation of extremely fine circuits, as it provides excellent adhesion of plated circuits, good etchability of electroless copper, and high dry film resolution.

5. Thesis Topics

1. In-depth exploration of how the arithmetic mean curvature of peaks (Spc) and peak density (Spd) of surface-treated copper foil work together to achieve the optimal balance of adhesion of plated circuits and etchability of electroless copper in the semi-additive process (SAP).
2. Comparison of the advantages and disadvantages of the semi-additive process (SAP) and the modified semi-additive process (MSAP) in forming fine circuits, and analysis of how the surface-treated copper foil in this invention further enhances the circuit miniaturization capability in the SAP process.
3. Detailed explanation of the mechanism of the two-stage electroplating process in the roughening treatment of surface-treated copper foil, particularly how the charge ratio (Q1/Q2) affects the Spc and Smr1 parameters of the copper foil surface profile, thereby influencing the performance of the final printed circuit board.
4. Analysis of the differences between the roughening particle shape control mentioned in this invention and traditional roughening treatments, and discussion of the specific impacts of this shape on dry film resolution, adhesion of plated circuits, and etchability.
5. Prospects for the application of surface-treated copper foil in the future development of high-density, high-performance printed circuit boards, and discussion of potential technical challenges and solutions.

6. Glossary of Keywords

• Surface-Treated Copper Foil: Copper foil with a specially treated surface on at least one side, aimed at improving its adhesion, etchability, and other properties with resin.
• Carrier-Attached Copper Foil: A copper foil structure that includes a carrier, release layer, and thin copper foil layer, where the carrier supports the thin copper foil layer for processing.
• Copper Clad Laminate (CCL): A board material that includes a copper foil layer and an insulating resin layer, serving as the basic material for manufacturing printed circuit boards.
• Printed Circuit Board (PCB): A board that connects electronic components by forming conductive patterns (wiring) on insulating materials.
• Semi-Additive Process (SAP): A printed circuit board manufacturing process that involves first forming a thin conductive layer on an insulating layer, followed by patterning, electroplating, and etching to create circuits.
• Arithmetic Mean Curvature of Peaks (Spc): A parameter measured according to ISO25178 standards, representing the sharpness of surface protrusions, in units of mm^-1.
• Peak Density (Spd): A parameter measured according to ISO25178 standards, representing the number of peak points per unit area, in units of mm^-2.
• Load Area Ratio of Separated Protruding Peak Part and Central Part (Smr1): A parameter measured according to ISO25178 standards, representing the proportion of the protruding peak part, in units of %.
• Roughening Treatment: A process that forms micro-rough structures on the surface of copper foil through electroplating and other methods to increase surface area and mechanical anchoring effects.
• Electroless Copper Plating: A method of depositing copper film on non-conductive substrates or conductive thin layers through chemical reactions without relying on external power sources.
• Dry Film: A photosensitive polymer film used as a photomask in the manufacturing of printed circuit boards.
• Etchability: The ease with which a material can be corroded or removed by etching solution, specifically referring to the difficulty of removing the electroless copper layer in this patent.
• Adhesion of Plated Circuit: The bonding strength between the electroplated circuit layer and the substrate or insulating layer.
• Dry Film Resolution: The ability of the dry film photolithography process to clearly reproduce the minimum line width/spacing (L/S), reflecting the degree of circuit miniaturization.
• Prepreg: Reinforced materials (such as glass fiber cloth) impregnated with resin, which are flexible or semi-cured before heating and curing.
• Carrier: A thicker metal foil or resin film used to support the ultra-thin copper foil in carrier-attached copper foil.
• Release Layer: A layer set between the carrier and the ultra-thin copper foil, allowing the ultra-thin copper foil to be peeled off from the carrier after specific processes.
• Silane Coupling Agent: An organic silicon compound commonly used to improve the interfacial bonding performance between inorganic materials and organic polymers.