1. **PCB Processing Flow [Inner Circuit]**
The copper foil substrate is initially cut to a size suitable for processing and production. Before laminating the substrate, the copper foil surface is typically roughened through brushing, microetching, etc., and then the dry film photoresist is adhered tightly to it under appropriate temperature and pressure. The substrate with the dry film photoresist is then exposed to UV light in a UV exposure machine. The photoresist polymerizes under ultraviolet rays in the light-transmitting areas, transferring the circuit image from the film to the dry film photoresist on the board. After removing the protective film from the surface, sodium carbonate aqueous solution is used to develop and remove the unexposed areas, followed by hydrogen peroxide mixed solution to etch away the exposed copper foil, forming the circuit. Finally, the functional dry film photoresist is washed away with a lightly oxidized sodium water solution.
2. **Pressing**
The inner circuit board must be bonded to the outer circuit’s copper foil using glass fiber resin film. Prior to pressing, the inner layer board needs to be blackened (oxidized) to passivate the copper surface, enhancing insulation; the copper surface is also roughened to ensure good adhesion to the film. For stacking, the inner circuit boards (six layers or more) are first riveted in pairs using a riveting machine. They are then neatly stacked between mirror steel plates using a tray and sent to a vacuum laminator to harden and bond the film under appropriate temperature and pressure. After pressing, the target holes are drilled by an X-ray automatic positioning target drilling machine for alignment of the inner and outer layers, and the board edges are fine-cut to facilitate subsequent processing.
3. **Drilling**
The circuit board is drilled with a CNC drilling machine to create conduction channels for the interlayer circuits and fixing holes for the welding parts. During drilling, the circuit board is fixed on the drilling machine table using pins through the previously drilled target holes. Additionally, a flat bottom plate (phenolic resin board or wood pulp board) and an upper cover plate (aluminum plate) are used simultaneously to minimize drilling burrs.
4. **Plating Through Holes**
After forming the interlayer vias, a metal copper layer must be deposited to complete the interlayer circuit conduction. Initially, heavy brushing and high-pressure washing are used to clean any debris and dust from the hole. The cleaned hole wall is then coated with tin.
1. **One-time copper:** A palladium colloidal layer is applied first and then reduced to metallic palladium. The circuit board is then immersed in a chemical copper solution, where copper ions are reduced and deposited on the hole walls through the catalytic action of the palladium to form a through-hole circuit. Next, the copper layer in the via holes is thickened through copper sulfate bath electroplating to a sufficient thickness to withstand the impact of subsequent processing and environmental conditions.
2. **Outer circuit secondary copper:** The circuit image transfer process for the outer circuit is similar to that of the inner circuit, but circuit etching is carried out using two methods: positive film and negative film. For the negative film, the process is identical to that of the inner layer circuit; after development, the copper is etched directly and the film is removed. For the positive film, copper and tin-lead are applied twice after development (the tin-lead in this area acts as an etching resist in the subsequent copper etching step). After film removal, an alkaline solution of ammonia and copper chloride is used to etch and remove the exposed copper foil to form the circuit. Finally, a tin-lead stripping solution is used to remove the tin-lead layer that has served its purpose (previously, this layer was retained to protect the circuit, but this practice is now largely obsolete).
3. **PCB circuit board processing flow [Solder Resist Ink Text Printing]:** Earlier, green solder resist paint was hardened by direct thermal drying or ultraviolet irradiation after screen printing. This method often caused the paint to penetrate the copper surface of the circuit terminals, leading to issues with soldering and component use. Nowadays, for most applications, photosensitive green paint is used in production to avoid these problems.
The copper foil substrate is initially cut to a size suitable for processing and production. Before laminating the substrate, the copper foil surface is typically roughened through brushing, microetching, etc., and then the dry film photoresist is adhered tightly to it under appropriate temperature and pressure. The substrate with the dry film photoresist is then exposed to UV light in a UV exposure machine. The photoresist polymerizes under ultraviolet rays in the light-transmitting areas, transferring the circuit image from the film to the dry film photoresist on the board. After removing the protective film from the surface, sodium carbonate aqueous solution is used to develop and remove the unexposed areas, followed by hydrogen peroxide mixed solution to etch away the exposed copper foil, forming the circuit. Finally, the functional dry film photoresist is washed away with a lightly oxidized sodium water solution.
2. **Pressing**
The inner circuit board must be bonded to the outer circuit’s copper foil using glass fiber resin film. Prior to pressing, the inner layer board needs to be blackened (oxidized) to passivate the copper surface, enhancing insulation; the copper surface is also roughened to ensure good adhesion to the film. For stacking, the inner circuit boards (six layers or more) are first riveted in pairs using a riveting machine. They are then neatly stacked between mirror steel plates using a tray and sent to a vacuum laminator to harden and bond the film under appropriate temperature and pressure. After pressing, the target holes are drilled by an X-ray automatic positioning target drilling machine for alignment of the inner and outer layers, and the board edges are fine-cut to facilitate subsequent processing.
3. **Drilling**
The circuit board is drilled with a CNC drilling machine to create conduction channels for the interlayer circuits and fixing holes for the welding parts. During drilling, the circuit board is fixed on the drilling machine table using pins through the previously drilled target holes. Additionally, a flat bottom plate (phenolic resin board or wood pulp board) and an upper cover plate (aluminum plate) are used simultaneously to minimize drilling burrs.
4. **Plating Through Holes**
After forming the interlayer vias, a metal copper layer must be deposited to complete the interlayer circuit conduction. Initially, heavy brushing and high-pressure washing are used to clean any debris and dust from the hole. The cleaned hole wall is then coated with tin.
1. **One-time copper:** A palladium colloidal layer is applied first and then reduced to metallic palladium. The circuit board is then immersed in a chemical copper solution, where copper ions are reduced and deposited on the hole walls through the catalytic action of the palladium to form a through-hole circuit. Next, the copper layer in the via holes is thickened through copper sulfate bath electroplating to a sufficient thickness to withstand the impact of subsequent processing and environmental conditions.
2. **Outer circuit secondary copper:** The circuit image transfer process for the outer circuit is similar to that of the inner circuit, but circuit etching is carried out using two methods: positive film and negative film. For the negative film, the process is identical to that of the inner layer circuit; after development, the copper is etched directly and the film is removed. For the positive film, copper and tin-lead are applied twice after development (the tin-lead in this area acts as an etching resist in the subsequent copper etching step). After film removal, an alkaline solution of ammonia and copper chloride is used to etch and remove the exposed copper foil to form the circuit. Finally, a tin-lead stripping solution is used to remove the tin-lead layer that has served its purpose (previously, this layer was retained to protect the circuit, but this practice is now largely obsolete).
3. **PCB circuit board processing flow [Solder Resist Ink Text Printing]:** Earlier, green solder resist paint was hardened by direct thermal drying or ultraviolet irradiation after screen printing. This method often caused the paint to penetrate the copper surface of the circuit terminals, leading to issues with soldering and component use. Nowadays, for most applications, photosensitive green paint is used in production to avoid these problems.
