PCB Technical - Fabrication of copper clad laminate for multilayer circuit board

Multilayer circuit board copper-clad laminate is the substrate material for making a printed circuit board. It can not only support various components but also realize the electrical connection or electrical insulation between them.

The manufacturing process of multilayer circuit board foil clad board is to impregnate glass fiber cloth, glass fiber felt, paper and other reinforcement materials with epoxy resin, phenolic resin and other adhesives, and dry them to phase B at an appropriate temperature, The prepreg materials (hereinafter referred to as dipping materials) are obtained, and then they are laminated with copper foil according to the process requirements, and the required multilayer circuit board copper foil laminate is obtained by heating and pressurizing on the laminate.

Classification of copper-clad laminates for multilayer circuit boards

The copper-clad laminate of multilayer circuit board is composed of copper foil, reinforcing material and adhesive. Plates are usually classified by reinforcement category and adhesive category or plate characteristics.

1. According to the classification of reinforcement materials, the most commonly used reinforcement materials for multilayer circuit board copper-clad laminates are alkali free (alkali metal oxide content does not exceed 0.5%) glass fiber products (such as glass cloth and glass felt) or paper (such as wood pulp paper, bleached wood pulp paper and cotton wool paper). Therefore, multilayer circuit board copper-clad laminates can be divided into glass cloth based and paper-based.

2. According to the type of adhesive, the adhesives used for multilayer circuit board foil are mainly phenolic, epoxy, polyester, polyimide, PTFE resin, etc. Therefore, PCB foil is also divided into phenolic, epoxy, polyester, polyimide and PTFE multilayer circuit board foil.

3. According to the characteristics and application of the base material, it can be divided into general type and self extinguishing type according to the combustion degree of the base material in the flame and after leaving the fire source; According to the bending degree of the substrate, it can be divided into rigid and flexible PCB foil clad plates; According to the working temperature and working environment conditions of the substrate, it can be divided into heat-resistant type, radiation-resistant type, PCB foil coated board for high frequency, etc. In addition, there are PCB foil clad plates used in special occasions, such as prefabricated inner foil clad plates, metal based foil clad plates, and can be divided into copper foil, nickel foil, silver foil, aluminum foil, Kang copper foil and beryllium copper foil clad plates according to the type of foil.

4. The models of commonly used PCB clad laminates are in accordance with gb4721-1984. PCB clad laminates are generally represented by a combination of five English letters: the first letter C represents the clad copper foil, and the second and third letters represent the adhesive resin selected for the substrate. For example, PE represents phenolic; EP stands for epoxy; Up represents unsaturated polyester; Si represents silicone; TF means polytetrafluoroethylene; PI stands for polyimide. The fourth and fifth letters indicate the reinforcement selected for the substrate. For example, CP stands for cellulose fiber paper; GC indicates alkali free glass fiber cloth; GM stands for alkali free glass fiber felt.

For example, if the inner core of the substrate of the PCB foil clad board is reinforced with fiber paper and cellulose and alkali free glass cloth is attached on both sides, the two digits on the right of the horizontal line in the g. model can be added after CP to represent the product number of the same type and different performance. For example, the number of copper-clad phenolic paper laminate is O1 ~ 20, and the number of copper-clad epoxy paper laminate is 21 ~ 30; Copper clad epoxy glass cloth laminate No. 31 ~ 40 If the letter F is added after the product number, it indicates that the PCB foil clad plate is self extinguishing.

Manufacturing method of PCB copper clad laminate

The manufacturing of PCB copper-clad laminate mainly includes three steps: resin solution preparation, reinforcement dipping and press molding.

1. Main raw materials for manufacturing PCB copper clad laminate

The main raw materials for making copper clad laminates are resin, paper, glass cloth and copper foil.

(1) The resins used for resin PCB copper-clad laminate include phenolic resin, epoxy resin, polyester, polyimide, etc. among them, phenolic resin and epoxy resin are the most used.

Phenolic resin is a kind of resin formed by condensation polymerization of phenols and aldehydes in acidic or alkaline medium. The resin polycondensated with phenol and formaldehyde in alkaline medium is the main raw material of paper-based PCB foil board. In the manufacturing of paper-based PCB foil clad plate, in order to obtain various plates with excellent performance, it is often necessary to modify the phenolic resin, and strictly control the content of free phenol and volatile matter of the resin, so as to ensure that the plate will not be layered and foamed under thermal shock.

Epoxy resin is the main raw material of glass cloth Based PCB foil board. It has excellent bonding properties, electrical and physical properties. E-20, E-44, E-51 and self extinguishing E-20 and E-25 are commonly used. In order to improve the transparency of PCB foil clad board substrate and check graphic defects in PCB production, epoxy resin is required to have light color.

(2) Impregnated paper commonly used impregnated paper includes cotton lint paper, wood pulp paper and bleached wood pulp paper. Cotton lint paper is made of cotton fiber with short fiber, which is characterized by better resin permeability and better blanking and electrical properties. Wood pulp paper is mainly made of wood fiber, which is generally cheaper than cotton lint paper and has higher mechanical strength. The use of bleached wood pulp paper can improve the appearance of the board.

In order to improve the properties 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 the reinforcing material of glass cloth Based PCB foil clad plate. For special high-frequency applications, quartz glass cloth can be used.

The alkali content of alkali free glass cloth (expressed in Na20) shall not exceed 1% in IEC standard, 0.8% in JIS standard r3413-1978, 0.5% in former Soviet Union toct5937-68 standard and 0.5% in Chinese Ministry of construction standard jc-170-80.

In order to meet the needs of general-purpose, thin and multilayer printed boards, the models of glass cloth for PCB foil clad boards abroad have been serialized. The thickness range is 0.025 ~ 0.234mm The specially needed glass cloth is post treated with coupling. In order to improve the machinability of epoxy glass cloth Based PCB foil coated board and reduce the board cost, non-woven glass fiber (also known as glass felt) has been developed in recent years.

(4) Copper foil can be used as the foil material of copper foil PCB clad plate, such as copper, nickel, aluminum and other metal foils. However, considering the conductivity, weldability, elongation, adhesion to substrate and price of metal foil, copper foil is the most suitable except for special purposes.

Copper foil can be divided into calendered copper foil and electrolytic copper foil. Calendered copper foil is mainly used in flexible printed circuit and other special purposes. Electrolytic copper foil is widely used in the production of PCB clad plate. The purity of copper shall not be lower than 99.8% according to iec-249-34 and Chinese standards.

At present, the thickness of copper foil for printed boards in China is mostly 35um, and 50um copper foil is used as the transition product. In the manufacturing of high-precision hole metallized double-sided or multilayer boards, it is hoped to use copper foil thinner than 35um, such as 18um, 9um and 5um Some multilayer boards use thicker copper foil, such as 70UM In order to improve the adhesion strength of copper foil to substrate, Generally, copper oxide foil (that is, after oxidation treatment, a layer of copper oxide or cuprous oxide is formed on the surface of copper foil, which improves the bonding strength between copper foil and substrate due to polarity) or roughened copper foil is used (a coarsening layer is formed on the surface of copper foil by electrochemical method, which increases the surface area of copper foil, and improves the bonding strength between copper foil and substrate due to the anchoring effect of coarsening layer on substrate).

In order to avoid copper oxide powder falling off and moving to the substrate, the surface treatment method of copper foil is also continuously improved. For example, TW copper foil is plated with a thin layer of zinc on the coarsened surface of copper foil, and the surface of copper foil is gray at this time; TC type copper foil is plated with a thin layer of copper zinc alloy on the coarsened surface of copper foil. At this time, the surface of copper foil is gold $ After special treatment, the thermal discoloration resistance, oxidation resistance and cyanide resistance of copper foil in printed circuit board manufacturing are improved accordingly.

The surface of copper foil shall be smooth without obvious wrinkles, oxidation spots, scratches, pits, pits and stains. The porosity of copper foil of 305g / m2 and above shall be 300ram * No more than 8 penetration points in 300mm area; The total pore area of copper foil on an area of 0.5m2 shall not exceed the circular area with a diameter of 0.125mm. The porosity and pore size of copper foil below 305g / m2 shall be agreed by the supplier and the buyer.

Before the copper foil is put into use, samples shall be taken for pressing test if necessary. The pressing test can show its peel strength and general surface quality.

2. Manufacturing process of copper-clad laminate for multilayer circuit board

The production process of multilayer circuit board copper-clad laminate is as follows: resin synthesis and glue preparation - reinforcement dipping and drying - dipping material shearing and inspection - dipping material and copper foil lamination - hot pressing forming - cutting - Inspection and packaging.

The synthesis and preparation of resin solution are carried out in the reactor. Phenolic resin for paper-based PCB foil board is mostly synthesized by multilayer circuit board foil board factory.

The production of glass cloth based multilayer circuit board foil clad board is to mix the epoxy resin and curing agent provided by the raw material factory, dissolve them in acetone, dimethylformamide and ethylene glycol methyl ether, and stir them to form a uniform resin solution. The resin solution can be used for dipping after curing for 8 ~ 24h.

Dipping is carried out on the dipping machine. The dipping machine is divided into horizontal and vertical types. The horizontal dipping machine is mainly used for dipping paper, and the vertical dipping machine is mainly used for dipping glass cloth with high strength. The main of paper or glass cloth impregnated with resin liquid shall be cut into a certain size after being dried in the drying channel through the rubber extrusion roller, and shall be used for standby after passing the inspection.