### Manufacturing Method of PCB Copper-Clad Laminate
The manufacturing of PCB copper-clad laminate primarily involves three steps: resin solution preparation, reinforcement dipping, and press molding.
1. **Main Raw Materials for Manufacturing PCB Copper-Clad Laminate**
The primary raw materials for copper-clad laminates are resin, paper, glass cloth, and copper foil.
(1) **Resins**: The resins used in PCB copper-clad laminates include phenolic resin, epoxy resin, polyester, and polyimide, with phenolic and epoxy resins being the most commonly used. Phenolic resin is produced by the condensation polymerization of phenols and aldehydes in an acidic or alkaline medium. Phenolic resin polycondensed with phenol and formaldehyde in an alkaline medium is a key material for paper-based PCB boards. For optimal performance, phenolic resin often requires modification, and careful control of free phenol and volatile matter content is essential to prevent delamination and foaming under thermal shock.
Epoxy resin is the primary material for glass cloth-based PCB boards, known for its excellent bonding, electrical, and physical properties. Common types include E-20, E-44, E-51, as well as flame-retardant E-20 and E-25. For better transparency and defect detection in PCB production, epoxy resin should have a light color.
(2) **Impregnated Paper**: Common types include cotton lint paper, wood pulp paper, and bleached wood pulp paper. Cotton lint paper, made from short cotton fibers, offers superior resin permeability and better blanking and electrical properties. Wood pulp paper, derived from wood fibers, is typically more affordable and provides higher mechanical strength. Bleached wood pulp paper enhances the appearance of the board. To ensure board quality, the impregnated paper must meet specifications for thickness deviation, weight, breaking strength, and water absorption.
(3) **Alkali-Free Glass Cloth**: Alkali-free glass cloth is used as the reinforcing material in glass cloth-based PCB boards. For specialized high-frequency applications, quartz glass cloth may be employed. According to IEC standards, the alkali content in alkali-free glass cloth (expressed as Na2O) should not exceed 1%, with stricter limits of 0.8% in JIS standard R3413-1978, 0.5% in the former Soviet Union standard TOCT5937-68, and 0.5% in the Chinese Ministry of Construction standard JC-170-80.
Various models of glass cloth for PCB boards are available abroad, with thicknesses ranging from 0.025 mm to 0.234 mm. Some specialized glass cloth is post-treated with coupling agents. Non-woven glass fiber (also known as glass felt) has recently been developed to improve machinability and reduce costs in epoxy glass cloth-based PCB boards.
(4) **Copper Foil**: Used for copper foil PCB clad plates, copper foil is preferred over metals like nickel and aluminum due to its superior conductivity, weldability, elongation, adhesion, and cost-effectiveness. Copper foil is categorized into calendered and electrolytic types. Calendered copper foil is mainly used for flexible circuits, while electrolytic copper foil is common in PCB production. According to IEC-249-34 and Chinese standards, copper purity must be at least 99.8%.
In China, the typical thickness of copper foil for printed boards is 35 µm, with 50 µm foil used as a transitional product. High-precision double-sided or multilayer boards may use thinner copper foils, such as 18 µm, 9 µm, or 5 µm, while some multilayer boards use thicker foils, up to 70 µm. To enhance adhesion strength, copper oxide foil (oxidized to form a layer of copper oxide or cuprous oxide) or roughened copper foil (coarsened to increase surface area) is used. Surface treatments, such as TW copper foil with a thin zinc layer or TC type copper foil with a copper-zinc alloy, improve thermal discoloration resistance, oxidation resistance, and cyanide resistance.
The copper foil surface should be smooth, free from wrinkles, oxidation spots, scratches, pits, or stains. For copper foil of 305 g/m² and above, porosity should not exceed 8 penetration points per 300 mm² area; the total pore area in a 0.5 m² region should not exceed a circular area with a diameter of 0.125 mm. For copper foil below 305 g/m², porosity and pore size must be agreed upon by the supplier and buyer. Prior to use, copper foil samples may be tested for peel strength and surface quality.
2. **Manufacturing Process of Copper-Clad Laminate for Multilayer Circuit Boards**
The production process for multilayer circuit board copper-clad laminates involves: resin synthesis and glue preparation, reinforcement dipping and drying, shearing and inspection of dipping material, lamination of dipping material and copper foil, hot pressing, cutting, and final inspection and packaging.
Resin synthesis and preparation are conducted in a reactor. Phenolic resin for paper-based PCB boards is primarily synthesized by multilayer circuit board manufacturers. For glass cloth-based multilayer circuit boards, epoxy resin and curing agents from the raw material factory are mixed, dissolved in acetone, dimethylformamide, and ethylene glycol methyl ether, and stirred to form a uniform resin solution, which is then cured for 8 to 24 hours before use.
Dipping is performed using either horizontal or vertical dipping machines. Horizontal machines are used for paper, while vertical machines are used for high-strength glass cloth. After drying in the drying channel, the impregnated paper or glass cloth is cut to size and inspected before being set aside for use.
