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PCB Circuit Board Production Process: A Precise Journey from Design to Finished Product

PCB (Printed Circuit Board) is the skeleton and nerve of modern electronic devices, present in almost all electronic equipment. It connects various electronic components to ensure the orderly transmission of signals and power. The birth of a PCB is a complex process that integrates precision mechanics, chemistry, and optics. The following will gradually explain its core production process.

The first stage: Design and Pre-treatment

1. Circuit Design and Output

· Everything begins with electronic engineers using EDA (Electronic Design Automation) software (such as Altium Designer, KiCad, etc.) to design the circuit schematic and PCB layout.

· After the design is completed, a set of crucial production files, usually called Gerber files, will be output. These files contain the precise geometric information of each layer of the circuit, such as traces, pads, vias, as well as drilling files, assembly diagrams, etc.

2. Base Material Preparation

· The base material (also known as copper-clad laminate) is typically composed of an insulating material (such as FR-4, a glass fiber epoxy resin) and copper foil laminated on both sides. The first step in production is to cut large sheets of copper-clad laminate into small panels suitable for the production line based on the order requirements.

The second stage: Inner Layer Line Fabrication (for multi-layer boards)

1. Inner Layer Graphic Transfer

· Cleaning and Coating: First, clean the copper-clad laminate to ensure a clean surface. Then, apply a layer of UV-sensitive photosensitive dry film on the copper surface.

· Exposure: Produce the circuit patterns in the Gerber files into transparent film negatives. Cover the coated board with the negative film and expose it to ultraviolet light. The areas exposed by the negative film will undergo a polymerization reaction and harden, while the areas covered by black will not.

· Development: Use alkaline chemicals to rinse the board. The unhardened photosensitive film will be washed away, revealing the copper beneath; while the hardened photosensitive film will remain, forming the "protective layer" of the circuit.

2. Inner Layer Etching and Strip

· Etching: Place the board in an etching machine and spray it with chemical solution (such as acidic copper chloride). The copper beneath the unprotected areas will be corroded, while the copper protected by the photosensitive film will remain, forming the final copper lines.

· Strip: Use chemicals to remove the already "completed" hardened photosensitive film, revealing the clear inner layer copper lines.

3. Inner Layer AOI Inspection and Oxidation

· AOI (Automatic Optical Inspection): Use high-precision optical cameras to scan the inner layer lines and compare them with the original Gerber data to automatically detect defects such as open circuits, short circuits, and gaps.

· Oxidation Treatment: To make the inner core board and the subsequent laminated pre-cured sheet (PP sheet) bond more firmly, the copper lines will undergo micro-roughening treatment (blackening or brownening).

The third stage: Lamination and Drilling

1. Lamination

· Stack the fabricated inner core board, pre-cured sheet (epoxy resin pre-impregnated material), and the outer copper foil like a "sandwich". In a vacuum press, through high-temperature and high-pressure, the pre-cured sheet melts and solidifies, firmly bonding each layer into a single whole, forming a multi-layer board.

2. Drilling

· Use a precise CNC drilling machine to drill through-holes and component mounting holes according to the drilling files on the board.

· After drilling, the inner layer is covered with insulating base material, which needs subsequent processes to make it conductive.

The fourth stage: Hole Metalization and Outer Layer Lines

1. Hole Metalization (plating, electroplating)

· This is a crucial step, aiming to deposit copper on the drilled holes to achieve electrical connections between layers.

· Chemical copper plating: Firstly, through a series of complex chemical treatments, a thin layer of chemical copper (usually less than 1 micrometer) is deposited on the insulating material of the hole wall, making the hole wall initially conductive.

· Electroplating thickening: Place the entire board in the electroplating tank as the cathode and apply an electric current. Utilizing the principles of electrochemistry, the copper layer is thickened to a sufficient thickness (usually 20-30 micrometers) to ensure good conductivity. At the same time, a thin layer of tin will also be electroplated as a protective layer.

2. Outer layer graphic transfer

· The production process of the outer layer is similar to the inner layer (coating, exposure, development), but the purpose is opposite. The outer layer uses a negative film process, where the lines on the film are transparent and the non-line areas are black. After exposure and development, the copper lines that need to be retained are covered by the photosensitive film, while the blank areas that need to be etched are exposed to the copper surface.

Phase 5: Final etching and surface treatment

1. Outer layer etching and tin removal

· Perform a second etching. At this time, the copper foil (including the surface and the copper inside the holes) that is not protected by the photosensitive film will be etched away. However, due to the tin protective layer in the holes and on the surface of the lines, the copper in these areas is perfectly retained.

· Finally, remove the tin protective layer and expose the complete outer layer lines and conductive vias.

2. Anti-solder mask and screen printing

· Anti-solder mask (green oil): Apply a layer of anti-solder ink on both sides of the board, usually green. Through the exposure and development process, the pads and the areas that need to be soldered are exposed, while the remaining parts are covered by the anti-solder layer. The anti-solder layer can prevent short circuits during soldering and protect the lines.

· Screen printing layer (characters): Use screen printing technology to print white text, symbols (such as component labels, polarity indications, version numbers, etc.) on the board, facilitating the assembly and maintenance of subsequent components.

3. Surface treatment

· To protect the exposed pads (copper surfaces) from oxidation and ensure good solderability, surface treatment is required. Common processes include:

· Spraying tin (HASL): Low cost, good solderability, but the surface is not smooth.

· Gold plating (ENIG): Smooth surface, long lifespan, suitable for fine-pitch components.

· Tin/silver plating: Excellent solderability.

· OSP (organic soldering film): Low cost, environmentally friendly, but has a shorter shelf life.

Phase 6: Forming and testing

1. Forming (Tambourine/V-Cut)

· Divide the entire production panel into individual small boards (keep the spliced boards) through a CNC milling machine (锣板) or V-type cutting machine.

2. Electrical testing

· Flying probe test/fixture test： Use probes or dedicated test fixtures to conduct power-on tests on each PCB to verify whether all lines are connected， whether there are short circuits， and ensure that the electrical performance is consistent with the design.

3. Final inspection and packaging

· Conduct final appearance checks to confirm whether the dimensions， hole positions， and screen printing meet the standards.

· The qualified products， after vacuum packaging， can be shipped to customers for electronic component assembly (SMT).

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