Manufacturing Process of Multilayer PCBs

Multilayer PCB manufacturing methods include the plated-through hole (PTH) and high-density interconnect (HDI) methods, both achieved by combining different processes to realize the circuit board structure. Currently, the most widely used method is the PTH method, which has been developed and refined over more than half a century. The PTH method is mature in terms of equipment, materials, and processes, and has established a solid industrial foundation. It can be used to produce both double-sided and multilayer boards, with process and equipment reuse capabilities. In the PTH method, the insulating substrate surface and internal layer conductor patterns are interconnected through plated holes on the inner wall.

Taking the manufacturing process of a PCB factory as an example, the typical process flow for a rigid multilayer board is as follows:

Material Preparation:

PCB factories do not directly manufacture copper-clad laminates, prepreg sheets, or copper foils. Instead, these base materials are purchased from upstream material manufacturers. The base materials are typically standard large sizes, such as 1m x 1m or 1m x 1.2m. Before manufacturing, these materials are cut into sizes that fit the production line specifications.

Inner Layer Circuit Fabrication:

Multilayer boards typically use thin double-sided copper-clad laminates. After forming the inner layer circuits on the surface, the layers are laminated together.

A photosensitive dry film is applied to the double-sided copper-clad laminates for the inner layers. The film with the inner layer circuits is then attached and exposed to light.

After exposure, the boards are developed, and unwanted copper foil is etched away. After etching, the protective film that prevents etching of the circuits is removed.

Inspection and Brown Oxidation:

After the inner layers are formed, they undergo automatic optical inspection (AOI). Before lamination, to improve the adhesion between copper foil and the prepreg, a brown oxidation process is performed. The purposes of this process are:

· To increase the contact area between copper foil and resin, enhancing their bonding strength.

· To improve the wetting properties between the copper and the flowing resin, allowing the resin to fill any gaps and provide strong adhesion after hardening.

· To create a fine passivation layer on the copper surface to prevent the resin from reacting with the copper under high temperature and pressure, which could cause delamination.

Lamination:

The inner layers are stacked according to the designed layer structure, including the prepared inner layers, prepreg sheets, and outer copper foils, and then pressed together under heat to form a unified structure.

Drilling and Plating:

After lamination, interconnections between the layers are not yet formed, necessitating drilling followed by the deposition of a conductive copper layer on the hole walls to establish connections. The plating process includes three main steps: deburring, chemical copper deposition, and copper plating.

Chemical Copper Deposition and Full-Panel Electroplating

Chemical copper deposition involves using chemical methods to deposit a thin layer of copper on the insulated hole walls. Full-panel electroplating, on the other hand, increases the copper thickness on the conductive hole walls after the chemical copper deposition process. The process mainly includes three steps: desmearing, chemical copper deposition, and copper electroplating.

Desmearing clears resin smears in the holes while creating a slightly rough texture on the hole walls to enhance the adhesion between the copper and the resin. Chemical copper deposition forms a thin metallic layer on the hole walls, which serves as a seed layer for electroplating. Electroplating is used to increase the copper thickness on the hole walls. At this stage, the surface does not yet have circuit patterns, so this step is called “full-panel electroplating.”

Outer Layer Circuit Pattern

Objective: This process transfers the circuit pattern onto the copper-clad laminate.

Pattern Electroplating

Objective: Pattern electroplating is the process of electroplating a copper layer of the required thickness and a tin layer on the exposed copper traces or hole walls of the circuit pattern.

Resist Stripping

Objective: Using a NaOH solution to strip away the plating resist, exposing the non-circuit copper areas.

Etching

Objective: Etching involves using a chemical reaction to remove the copper layer from non-circuit areas.

Solder Mask

Comparison and Differences of Three Character Types

	Exposed Ultra-HD Characters	Stamped Characters	Screen-Printed Characters
Actual Effect
Manufacturing Method	Manufactured using exposure machine	Manufactured using character stamping machine	Manufactured using silk-screen printing
Flatness	Very flat and smooth character surface, clear and neat	Character surface is not very flat	Character surface flatness is good
Contrast Sharpness	High accuracy of contrast	Contrast sharpness is moderate	Contrast sharpness has uniformity issues
Character Edges	Character edges are very smooth	Character edges are not smooth	Character edges are uneven and appear rough
Adhesion	Adhesion is relatively strong	Adhesion is moderate	Adhesion is good
Process Complexity	Complex process, higher cost	Relatively simple, higher cost	Traditional method, lower cost, hard to control quality

The primary function of a PCB is to carry and connect electronic components. After pattern electroplating, areas that will not be contacted or soldered need to be protected with material to prevent damage or oxidation. Solder paste is typically used for component assembly, and the coating applied is referred to as the “solder mask.”

Legend

Objective: Legends are markings provided to make identification easier.

Surface Treatment

Unlike the solder mask process, surface treatment protects the areas that require soldering or contact, preventing the exposed copper foil from oxidizing due to air exposure, which could lead to poor soldering or contact issues.

Forming

Objective: Forming shapes the board according to customer specifications using die stamping or CNC routing. Forming methods include machine routing, punching, hand routing, and manual cutting. Note: CNC routing and punching offer high precision, hand routing is moderately precise, and manual cutting is less precise, limited to simple shapes.

Testing

Objective: Through electronic testing, defects such as open circuits or short circuits, which may affect functionality, are detected and identified.

Final Inspection

Objective: The final inspection checks for surface defects, and minor issues are repaired to prevent defective products from being shipped.

Reprinted from: Wu Chuanbin’s blog

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