Multilayer PCBs are made by laminating multiple layers of thin, single- or double-sided PCBs together with insulating material (prepreg) under high temperature and pressure, creating a single, robust board with complex interconnections.
Here's a detailed breakdown of the manufacturing process:
1. Core Production:
- Thin, double-sided PCB cores are fabricated. These cores consist of a laminate material (e.g., FR-4) with copper cladding on both sides. The desired circuit patterns are etched onto these copper layers using photolithography and chemical etching processes, similar to single- or double-sided PCB manufacturing.
2. Layer Preparation:
- Inner Layer Etching: The inner layers (cores) are etched with the desired circuit patterns. This process involves applying a photoresist, exposing it to UV light through a photomask, developing the resist, and then etching away the unwanted copper.
- AOI Inspection: Automated Optical Inspection (AOI) systems meticulously examine the etched inner layers for defects such as opens, shorts, and copper voids.
3. Prepreg Application:
- Prepreg, which is a partially cured fiberglass cloth impregnated with resin, is applied as an insulating layer between the cores. The prepreg softens and bonds the layers together during the lamination process.
4. Stacking and Lamination:
- The layers are carefully stacked in the correct order, ensuring proper alignment. This typically involves alternating prepreg layers with the etched core layers and copper foil for the outer layers.
- The stack is then placed in a lamination press. High temperature and pressure are applied, melting the prepreg resin and bonding the layers together to form a solid, multilayer PCB.
5. Drilling:
- After lamination, holes are drilled through the board. These holes are used for component mounting and for creating electrical connections between different layers (vias).
6. Plating:
- The drilled holes are chemically cleaned and then plated with copper. This creates a conductive path through the board, connecting different layers together via the drilled holes (vias).
7. Outer Layer Imaging and Etching:
- The outer layers are imaged and etched, defining the final circuit patterns on the top and bottom of the board. The process is similar to the inner layer etching, using photolithography and chemical etching.
8. Solder Mask Application:
- A solder mask is applied to the board to protect the copper traces from oxidation and to prevent solder bridges during component soldering. Solder mask is typically a polymer coating applied using screen printing or spray coating.
9. Surface Finish:
- A surface finish is applied to the exposed copper pads to improve solderability and protect against corrosion. Common surface finishes include HASL (Hot Air Solder Leveling), ENIG (Electroless Nickel Immersion Gold), and OSP (Organic Solderability Preservative).
10. Silkscreen and Final Inspection:
- Silkscreen printing is applied to add component designators, logos, and other information to the board.
- A final inspection is conducted to ensure the board meets all quality standards and specifications.
11. Testing:
- Electrical testing is performed to verify the continuity and isolation of the circuits. This may involve using flying probe testers or bed-of-nails testers.
Flexible Multilayer PCBs:
As noted in the reference, flexible multilayer PCBs differ in that they are manufactured on flexible insulating substrates like polyimide film. Lamination of multiple layers may reduce the inherent flexibility depending on the materials and processes used.
In summary, creating a multilayer PCB is a complex and precise process that involves layering and bonding together multiple patterned copper layers with insulating materials to achieve intricate circuit designs.
