Multi-layer PCBs work by layering multiple single or double-sided circuit boards together, bonding them with heat and adhesive, and then creating electrical connections between the layers.
Understanding Multi-Layer PCBs
Multi-layer PCBs (Printed Circuit Boards) are essentially a sandwich of multiple circuit boards stacked on top of each other. These boards are carefully bonded together using a combination of high temperature and pressure, along with an adhesive material, typically epoxy resin with a glass-woven reinforcement called prepreg. The advantage of this construction is it significantly increases the circuit density and functionality that can be packed into a single board.
Lamination and Layer Construction
The core of a multi-layer PCB lies in the lamination process.
- Prepreg Layers: These are sheets of woven fiberglass cloth impregnated with a resin that is partially cured. When heated under pressure, the resin melts and bonds the layers together.
- Core Layers: These are typically rigid boards made of similar materials as the prepreg, but the resin is fully cured. Copper traces and pads are already etched onto the core layers.
- Copper Layers: These are thin layers of copper foil used for signal traces, power planes, and ground planes.
The stack-up is carefully arranged, with prepreg layers between core layers and copper layers. This entire stack is then placed in a lamination press, where high temperature and pressure are applied to melt the prepreg resin and create a strong bond between all the layers.
Electrical Connections Between Layers (Vias)
Once laminated, the different layers need to be electrically connected. This is achieved primarily through vias:
- Through-hole Vias: These are the most common type. A hole is drilled through the entire board, and then the inside of the hole is plated with copper. This creates a conductive path from the top layer to the bottom layer, and all layers in between.
- Blind Vias: These vias connect an outer layer to one or more inner layers, but do not go all the way through the board.
- Buried Vias: These vias connect only inner layers, and are not visible from the outside of the board.
Blind and buried vias are more complex and expensive to manufacture, but they allow for higher density designs by freeing up space on the outer layers. The fabrication of these vias requires controlled depth drilling and plating before the final lamination process.
Heat Protection Insulation
The reference mentions "layers of heat protection insulation between them". While PCB materials themselves are chosen for their dielectric and thermal properties, the statement might be interpreted as referencing the role of the prepreg and core materials in managing heat.
- Dielectric Properties: The materials used have specific dielectric constants, influencing signal propagation.
- Thermal Management: While not strictly insulation against heat, the materials chosen contribute to the overall thermal conductivity of the PCB, allowing heat to dissipate. Some designs incorporate copper planes or other heat-spreading techniques to improve thermal performance, particularly in high-power applications.
Summarizing the Process
Here’s a table summarizing the key steps in multi-layer PCB manufacturing:
| Step | Description |
|---|---|
| 1. Layer Preparation | Core layers are etched with desired copper patterns; Prepreg layers are cut to size. |
| 2. Stack-up | Layers are arranged in the desired order, typically alternating between core and prepreg layers, with copper foil added for external layers. |
| 3. Lamination | The stack-up is placed in a lamination press and subjected to high temperature and pressure, bonding the layers together. |
| 4. Drilling | Holes for vias and component mounting are drilled through the board. |
| 5. Plating | The inside of the drilled holes is plated with copper to create conductive vias. |
| 6. Outer Layer Etching | The outer copper layers are etched to create the desired circuit patterns. |
| 7. Solder Mask Application | A solder mask is applied to protect the copper traces from oxidation and to prevent solder bridges during component soldering. |
| 8. Silkscreen Printing | Component reference designators and other information are printed onto the board using silkscreen printing. |
| 9. Surface Finish | A surface finish (e.g., ENIG, HASL) is applied to the exposed copper pads to improve solderability and corrosion resistance. |
| 10. Testing and Inspection | The finished board is tested to ensure that it meets all specifications and requirements. |
In conclusion, multi-layer PCBs achieve their increased density and functionality through a carefully controlled process of lamination, via creation, and circuit etching on multiple layers, effectively creating a three-dimensional circuit within a single board.
