Common PCB Working Layers:  

- **Routing Layer**: Located on the top layer, bottom layer, and inner layers of multi-layer boards. Used to draw copper traces that carry current, such as pads, traces, and copper pours.  

- **Silkscreen Layer**: Also known as the legend layer. Used to mark component annotations and outline component shapes, facilitating component placement and maintenance.  

- **Solder Mask Layer**: Used to apply solder resist to the circuit board. Areas that need to be exposed (e.g., for soldering) are designed using this layer.  

- **Paste Mask Layer**: Used to create stencils. Openings are made in this layer for all SMD pads to facilitate solder paste application.  

- **Board Outline Layer**: Used to define the shape of the circuit board and mounting holes.  

- **3D Model Layer**: Used to preview the circuit board's appearance and identify potential issues during the design phase.  

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**PCB Base Materials**:  

**Classification by Flexibility**:  

- **Flexible PCBs** (e.g., FPC cables)  

- **Rigid PCBs** (e.g., fiberglass, aluminum substrates)  

**Classification by Reinforcement Material**:  

- Paper-based substrates  

- Fiberglass-based substrates  

- Composite substrates  

- Build-up multilayer boards  

- Special substrates such as ceramic and metal-core boards  

**Resin Adhesives**: Commonly used resins include phenolic resin, epoxy resin, and polyester resin.  

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**Manufacturing of Double-Layer and Multi-Layer Boards (Using FR-4 as an Example)**:  

Currently, the most common double-layer boards are made from FR-4 material. This material uses fiberglass cloth as the reinforcement and epoxy resin as the adhesive. It is then coated with copper on both sides and laminated under heat and pressure.  

**Note**: FR-4 is not a model of the material but a grade of flame-retardant material, indicating that the material can self-extinguish after burning.  

**Multi-Layer Board Manufacturing (Using FR-4 as an Example)**:  

Multi-layer boards are made by using FR-4 as the core, stacking prepreg (PP) and copper foil, and then bonding them together through hot and cold pressing. Prepreg primarily consists of fiberglass and epoxy resin.  

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**PCB Pads**:  

Almost all components are attached to pads, and almost all traces start and end at pads. When the circuit board is operational, current flows from one pad to another.  

**Classification**:  

- **Through-Hole Pads**: Have holes and are used to connect circuits on the top and bottom layers.  

  - The pins of through-hole electronic components are typically cylindrical, rectangular, or flat. Cylindrical pins should have a diameter slightly smaller than the hole diameter to ensure smooth insertion. Rectangular pins should have a diagonal length slightly smaller than the hole diameter. Flat pins often require oval holes.  

- **Surface-Mount Pads**: Located only on the top or bottom layer, commonly rectangular, circular, or oval. Traces for surface-mount pads require vias to connect the top and bottom layers.  

**Surface Finishes**:  

- **ENIG (Electroless Nickel Immersion Gold)**: Provides a smoother surface.  

- **HASL (Hot Air Solder Leveling)**: Another common finish.  

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**PCB Vias**:  

**Purpose**: Vias connect traces between different layers, establishing electrical connections.  

**Classification**:  

- **Through-Hole Vias**: The simplest to manufacture, these vias penetrate all layers and are visible to the naked eye.  

- **Blind Vias**: Used only in multi-layer boards, these vias connect the top layer to inner layers and are visible on one side only.  

- **Buried Vias**: Used only in multi-layer boards, these vias are entirely within inner layers and are not visible externally.  

**Design Considerations**: Blind and buried vias are more complex to manufacture, so through-hole vias are preferred whenever possible.  

**Via Characteristics**:  

- Vias have parasitic capacitance and inductance. Smaller vias are theoretically better, but manufacturing limitations impose constraints on inner and outer diameters.  

- The inner diameter of a via must always be slightly smaller than the outer diameter to avoid open circuits.  

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**Plated and Non-Plated Holes**:  

PCB holes are generally classified as **PTH (Plated Through-Hole)** and **NPTH (Non-Plated Through-Hole)**.  

- **PTH (Plated Holes)**: These holes are coated with copper and provide electrical connections, such as through-hole pads and vias.  

- **NPTH (Non-Plated Holes)**: These holes lack copper and are used for mechanical purposes, such as mounting components or securing the board to an enclosure.  

**Tolerance**: The dimensional tolerance for NPTH holes is negligible.  

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**Trace Width and Spacing**:  

The default trace width is typically **10 mil (0.254 mm)**. Traces narrower than this may risk breakage.