PCB Technical - PCB process circuit board printing production process

PCB processcircuit boardprinting production process

In order to better allow customers to have a deeper understanding of the circuit board production process, the circuit board production process is now explained as follows: In electronic assembly, printed circuit boards(Printed Circuit Boards) are a key part. It is equipped with other electronic components and connected to the circuit to provide a stable circuit working environment. For example, the circuit configuration can be divided into three categories:

[Single Panel] Arrange the metal lines that provide the connection of the parts on an insulating substrate material, which is also a support carrier for installing the parts.

[Double-sided board] When the single-sided circuit is not enough to provide the connection requirements of electronic parts, the circuit can be arranged on both sides of the substrate, and through-hole circuits can be deployed on the board to connect the circuits on both sides of the board.

[Multilayer board] For more complex application requirements, the circuit can be arranged in a multi-layer structure and pressed together, and through-hole circuits are arranged between the layers to connect the circuits of each layer.

Inner circuit (only for multilayer circuit boards) The copper foil substrate is first cut into a size suitable for processing and production. Before laminating the substrate, it is usually necessary to properly roughen the copper foil on the surface of the board by brushing, microetching, etc., and then attach the dry film photoresist tightly to it at an appropriate temperature and pressure. Send the dry film photoresist substrate to the UV exposure machine for exposure. The photoresist will undergo polymerization in the light-transmitting area of the negative film after being irradiated by ultraviolet light (the dry film in this area will be affected by the later development and copper etching steps. Keep it as an etching resist), and transfer the circuit image on the negative to the dry film photoresist on the board. After tearing off the protective film on the film surface, first use sodium carbonate aqueous solution to develop and remove the unlit area on the film surface, and then use a mixed solution of hydrochloric acid and hydrogen peroxide to corrode and remove the exposed copper foil to form a circuit. Finally, the dry film photoresist that has worked well is washed away with sodium hydroxide aqueous solution. For inner circuit boards with more than six layers (inclusive), an automatic positioning punching machine is used to punch out the riveting reference holes for the alignment of the interlayer circuits.

Pressing (only applicable to multi-layer boards)
The finished inner circuit board must be bonded with the outer circuit copper foil with glass fiber resin film. Before pressing, the inner layer board needs to be blackened (oxidized) to make the copper surface passivated to increase insulation; and the copper surface of the inner layer circuit is roughened to produce good adhesion to the film. When laminating, first riveting the inner circuit boards with six layers [including] or more with a riveting machine in pairs. Then use a tray to neatly stack it between the mirror steel plates, and send it to a vacuum laminator to harden and bond the film with proper temperature and pressure. After pressing the circuit board, the target hole is drilled by the X-ray automatic positioning target drilling machine as the reference hole for the alignment of the inner and outer layers. And make appropriate fine cutting of the edge of the board to facilitate subsequent processing.

Drilling
The circuit board is drilled with a CNC drilling machine to drill through holes for interlayer circuits and fixing holes for welding parts. When drilling, use the pin to fix the circuit board on the drilling machine table through the previously drilled target hole, and add a flat bottom plate (phenolic resin board or wood pulp board) and upper cover plate (aluminum plate) at the same time In order to reduce the occurrence of drilling hair.

One-time copper plated through holes After forming the interlayer vias, a metal copper layer needs to be laid on it to complete the interlayer circuit conduction. First, use heavy brushing and high-pressure washing to clean the hair on the hole and the dust in the hole, and then remove the scum on the copper surface of the hole wall with potassium permanganate solution. A tin-palladium colloidal layer is soaked and attached to the cleaned hole wall, and then it is reduced to metallic palladium. The circuit board is immersed in a chemical copper solution, and the copper ions in the solution are reduced and deposited on the wall of the hole by the catalytic action of palladium metal to form a through-hole circuit. Then, the copper layer in the via hole is thickened by copper sulfate bath electroplating to a thickness sufficient to resist the impact of subsequent processing and use environment.

Outer circuit secondary copper
The production of line image transfer is the same as the inner line, but in line etching, it is divided into two production methods, positive film and negative film. The production method of the negative film is the same as the production of the inner layer circuit. After the development, the copper is directly etched and the film is removed. The production method of positive film is to add copper and tin lead twice after development (the tin and lead in this area will be retained as an etching resist in the later copper etching step), and after removing the film, use alkaline The mixed solution of ammonia water and copper chloride corrodes and removes the exposed copper foil to form a circuit. Finally, the tin-lead stripping solution is used to strip the tin-lead layer that has worked out (in the early days, the tin-lead layer was retained and used to cover the circuit as a protective layer after re-rotation, but it is mostly not used).

Solder mask green paint
After the outer circuit is completed, an insulating resin layer needs to be covered to protect the circuit from oxidation and solder short circuit. Before painting, it is usually necessary to roughen and clean the copper surface of the circuit board by brushing, micro-etching and other methods. Then, the liquid photosensitive green paint is coated on the board surface by screen printing, curtain coating, electrostatic spraying, etc., and then pre-baked and dried (the dry film photosensitive green paint is pressed and coated with a vacuum laminator On the board). After it cools, it is sent to an ultraviolet exposure machine for exposure. The green paint will polymerize after being irradiated by ultraviolet rays in the light-transmitting area of the film (the green paint in this area will be retained in the later development step), and the sodium carbonate aqueous solution Develop and remove areas on the coating film that are not exposed to light. Finally, it is baked at a high temperature to completely harden the resin in the green paint. The earlier green paint was produced by direct thermal drying (or ultraviolet irradiation) after screen printing to harden the paint film. However, because it often causes green paint to penetrate the copper surface of the circuit terminal contact during the printing and hardening process, which causes problems in welding and use of parts, now in addition to the use of simple and rough circuit boards, photosensitive green paint is often used. in production.

Text printing
Print the text, trademark or part number required by the customer on the surface of the board by screen printing, and then heat the text (or ultraviolet radiation) to harden the text lacquer ink.

Contact processing
The solder resist green paint covers most of the copper surface of the circuit, only exposing the terminal contacts for part welding, electrical testing and circuit board insertion. This terminal needs to be added with a proper protective layer to avoid oxides on the terminal connected to the anode (+) during long-term use, which will affect the stability of the circuit and cause safety concerns.

【Gold Plating】Plating a high-hardness and wear-resistant nickel layer and a highly chemically passivated gold layer on the socket terminals of the circuit board (commonly known as gold fingers) to protect the terminals and provide good connection performance.
[Snipping] The soldering end of the circuit board is covered with a layer of tin-lead alloy in a hot air leveling manner to protect the end of the circuit board and provide good soldering performance.
[Pre-solder] The soldering end of the circuit board is covered with a layer of anti-oxidation pre-soldering film by dip dyeing to temporarily protect the soldering end and provide a relatively flat soldering surface before welding, so that it has good soldering performance.
[Carbon ink] A layer of carbon ink is printed on the contact terminals of the circuit board by screen printing to protect the terminals and provide good connection performance.
Form cutting
The circuit board is cut into the external size required by the customer with a CNC molding machine (or die punch). When cutting, use the pins to fix the circuit board on the bed (or mold) through the previously drilled positioning holes. After cutting, the golden finger parts are then processed to beveled to facilitate the use of the circuit board. For multi-piece molded circuit boards, X-shaped break lines are often required to facilitate customers' splitting and dismantling after plug-in. Finally, clean the dust on the circuit board and the ionic contaminants on the surface.
Final inspection packaging
Before packaging, perform final electrical conduction, impedance test, solderability, and thermal shock resistance tests on the circuit board. And use moderate baking to eliminate the moisture absorbed by the circuit board during the manufacturing process and the accumulated thermal stress, and finally package it in a vacuum bag for shipment.