Manufacturing method of PCB copper-clad laminate
PCB copper-clad laminate is the substrate material for making printed circuit boards. In addition to supporting various components, it can also achieve electrical connection or electrical insulation between them.
The manufacturing process of PCB copper-clad laminate is to impregnate glass fiber cloth, glass fiber felt, paper and other reinforcing materials with adhesives such as epoxy resin and phenolic resin, dry them to the B stage at an appropriate temperature to obtain pre-impregnated materials (referred to as impregnated materials), and then laminate them with copper foil according to process requirements, and heat and pressurize them on a laminator to obtain the required PCB copper-clad laminate.
- Classification of PCB copper-clad laminates PCB copper-clad laminates are composed of three parts: copper foil, reinforcing materials, and adhesives. Sheets are usually classified according to the type of reinforcing materials and adhesives or the characteristics of the sheets.
- Classification by reinforcement materials The most commonly used reinforcement materials for PCB copper-clad laminates are alkali-free (alkali metal oxide content does not exceed 0.5%) glass fiber products (such as glass cloth, glass felt) or paper (such as wood pulp paper, bleached wood pulp paper, cotton linter paper), etc. Therefore, PCB copper-clad laminates can be divided into two categories: glass cloth-based and paper-based.
- Classification by adhesive type The adhesives used for PCB foil-clad laminates are mainly phenolic, epoxy, polyester, polyimide, polytetrafluoroethylene resin, etc. Therefore, PCB foil-clad laminates are also divided into phenolic, epoxy, polyester, polyimide, and polytetrafluoroethylene PCB foil-clad laminates.
- Classification by substrate characteristics and uses According to the degree of combustion of the substrate in the flame and after leaving the fire source, it can be divided into general type and self-extinguishing type; according to the degree of bending of the substrate, it can be divided into rigid and flexible PCB foil-clad laminates; according to the working temperature and working environment conditions of the substrate, it can be divided into heat-resistant type, radiation-resistant type, high-frequency PCB foil-clad laminates, etc. In addition, there are PCB foil clad boards used in special occasions, such as prefabricated inner layer foil clad boards, metal-based foil clad boards, and foil clad boards that can be divided into copper foil, nickel foil, silver foil, aluminum foil, copper foil, and beryllium copper foil according to the type of foil.
- Common PCB foil clad board models According to GB4721-1984, PCB copper foil laminates are generally represented by a combination of five English letters: the first letter C represents the copper foil, and the second and third letters represent the adhesive resin selected for the substrate. For example: PE represents phenolic; EP represents epoxy; uP represents unsaturated polyester; SI represents silicone; TF represents polytetrafluoroethylene; PI represents polyimide. The fourth and fifth letters represent the reinforcing materials selected for the substrate. For example: CP represents cellulose fiber paper; GC represents alkali-free glass fiber cloth; GM represents alkali-free glass fiber felt.
If the core of the base material of the PCB clad laminate is made of fiber paper or cellulose as the reinforcing material, and alkali-free glass cloth is attached to both sides, G can be added after CP. The two digits to the right of the horizontal line in the model number represent the product number of the same type but different performance. For example, the copper-clad phenolic paper laminate is numbered O1~20, the copper-clad epoxy paper laminate is numbered 21~30; the copper-clad epoxy glass cloth laminate is numbered 31~40. If the letter F is added after the product number, it means that the PCB clad laminate is self-extinguishing.
II. Manufacturing method of PCB copper-clad laminate The manufacturing of PCB copper-clad laminate mainly includes three steps: resin solution preparation, reinforcing material dipping and pressing.
- Main raw materials for manufacturing PCB copper-clad laminate The main raw materials for manufacturing copper-clad laminate are resin, paper, glass cloth and copper foil.
(1) Resin The resins used for PCB copper-clad laminates include phenolic, epoxy, polyester, polyimide, etc. Among them, phenolic resin and epoxy resin are used in the largest amount.
Phenolic resin is a type of resin formed by the condensation of phenols and aldehydes in an acidic or alkaline medium. Among them, the resin formed by the condensation of phenol and formaldehyde in an alkaline medium is the main raw material for paper-based PCB foil-clad boards. In the manufacture of paper-based PCB foil-clad boards, in order to obtain various boards with excellent performance, it is often necessary to modify the phenolic resin in various ways and strictly control the free phenol and volatile content of the resin to ensure that the board does not delaminate or bubble under thermal shock.
Epoxy resin is the main raw material for glass cloth-based PCB foil-clad boards. It has excellent bonding properties and electrical and physical properties. The more commonly used ones are E-20, E-44, E-51 and self-extinguishing E-20 and E-25. In order to improve the transparency of the PCB foil-clad board substrate and to check for graphic defects in printed board production, the epoxy resin is required to have a lighter color.
(2) Impregnated paper Commonly used impregnated papers include cotton linter paper, wood pulp paper and bleached wood pulp paper.
Cotton lint paper is made of cotton fibers with shorter fibers. It is characterized by good resin permeability, and the resulting board has good punching and electrical properties. Wood pulp paper is mainly made of wood fibers. It is generally cheaper than cotton lint paper, but has higher mechanical strength. The use of bleached wood pulp paper can improve the appearance of the board.
In order to improve the performance of the board, the thickness deviation, standard weight, breaking strength and water absorption of the impregnated paper need to be guaranteed.
(3) Alkali-free glass cloth Alkali-free glass cloth is a reinforcing material for glass cloth-based PCB foil-clad boards. For special high-frequency applications, quartz glass cloth can be used.
For the alkali content of alkali-free glass cloth (expressed as Na20), the IEC standard stipulates that it shall not exceed 1%, the JIS standard R3413-1978 stipulates that it shall not exceed 0.8%, the former Soviet Union TOCT5937-68 standard stipulates that it shall not exceed 0.5%, and the Ministry of Construction of my country standard JC-170-80 stipulates that it shall not exceed 0.5%.
In order to meet the needs of general-purpose, thin and multi-layer printed circuit boards, the glass cloth models used in foreign PCB foil boards have been serialized. The thickness range is 0.025~0.234mm. The glass cloth required specifically is post-processed by coupling. In order to improve the mechanical processing performance of epoxy glass cloth-based PCB foil boards and reduce the cost of boards, non-woven glass fibers (also known as glass felt) have been developed in recent years.
(4) Copper foil PCB foil boards can be made of copper, nickel, aluminum and other metal foils. However, based on the conductivity, solderability, elongation, adhesion to the substrate and price of the metal foil, copper foil is the most suitable except for special uses.
Copper foil can be divided into rolled copper foil and electrolytic copper foil. Rolled copper foil is mainly used in flexible printed circuits and some other special uses. In the production of PCB foil boards, electrolytic copper foil is widely used. For the purity of copper, IEC-249-34 and my country’s standards both stipulate that it must not be less than 99.8%.
At present, the thickness of copper foil used in domestic printed circuit boards is mostly 35um, and 50um copper foil is used as a transitional product. In the manufacture of high-precision hole metallized double-sided or multi-layer boards, it is hoped to use copper foil thinner than 35um, such as 18um, 9um and 5um. Some multi-layer boards use thicker copper foils for the inner layer of PCB cladding boards, such as 70um. In order to improve the bonding strength of copper foil to the substrate, oxidized copper foil (that is, after oxidation treatment, a layer of copper oxide or cuprous oxide is generated on the surface of the copper foil, and the bonding strength between the copper foil and the substrate is improved due to the polarity effect) or roughened copper foil (an electrochemical method is used to generate a roughening layer on the surface of the copper foil, which increases the surface area of the copper foil, and the anchoring effect of the roughening layer on the substrate improves the bonding strength between the copper foil and the substrate).
In order to avoid the copper oxide powder from falling off and moving to the substrate, the surface treatment method of the copper foil is also constantly improved. For example, TW type copper foil is a thin layer of zinc plated on the roughened surface of the copper foil, and the surface of the copper foil is gray; TC type copper foil is a thin layer of copper-zinc alloy plated on the roughened surface of the copper foil, and the surface of the copper foil is gold. After special treatment, the copper foil’s resistance to heat discoloration, oxidation resistance and cyanide resistance in printed circuit board manufacturing are correspondingly improved.
The surface of the copper foil should be smooth and free of obvious wrinkles, oxidation spots, scratches, pits, pits and stains. The porosity of copper foil of 305g/m2 and above requires no more than 8 penetration points within an area of 300ram×300mm; the total pore area of the copper foil on an area of 0.5m2 does not exceed the area of a circle with a diameter of 0.125mm. The porosity and pore size of copper foil below 305g/m2 are agreed upon by the supply and demand parties.
Before the copper foil is put into use, samples should be taken for compression test if necessary. The compression test can show its peel strength and general surface quality.
- PCB copper-clad laminate manufacturing process The production process of PCB copper-clad laminate is as follows: resin synthesis and glue solution preparation – reinforcing material dipping and drying – dipping material shearing and inspection – dipping material and copper foil lamination – hot pressing molding – cutting – inspection and packaging.
The synthesis and preparation of resin solution are carried out in the reactor. The phenolic resin used for paper-based PCB foil-clad boards is mostly synthesized by PCB foil-clad board factories.
The production of glass cloth-based PCB foil-clad boards is to mix and dissolve the epoxy resin and curing agent provided by the raw material factory in acetone or dimethylformamide, ethylene glycol methyl ether, and stir to make it a uniform resin solution. The resin solution can be used for dipping after aging for 8 to 24 hours.
The dipping is carried out on a dipping machine. Dipping machines are divided into horizontal and vertical types. Horizontal dipping is mainly used for dipping paper, and vertical dipping machines are mainly used for dipping glass cloth with higher dipping strength. The paper or glass cloth impregnated with resin liquid passes through the rubber squeezing roller and enters the drying tunnel for drying. It is then cut into a certain size and is ready for use after passing the inspection.
