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🔌 1. Definition of a Multilayer PCB

The core advantage of multilayer PCBs lies in their ability to implement complex wiring within limited space, meeting the miniaturization and high-performance requirements of electronic products. They are widely used in smartphones, servers, medical devices, and other fields.

• Conductive layers (copper foil) transmit signals and power
• Insulating layers are mostly prepregs (e.g., epoxy resin)
• Solder mask layers protect circuits from short circuits
• Surface finishing prevents copper foil oxidation

The diagram below shows the stackup structure of a 6-layer 1-step HDI (High-Density Interconnect) multilayer PCB, illustrating its layer stacking and via design:

⚙ 2. Core Design Challenges and Solutions

🔧 3. Key Considerations for Multilayer PCB Fabrication

The manufacturing process of multilayer PCBs is complex. Parameter deviations or improper operations in any link—from early material selection to final shaping and testing—may affect the quality of finished products. Below are the critical considerations for each core manufacturing stage:

• 1 Pre-Manufacturing: Material Selection and Inner Layer Fabrication
  Material Matching

  • Choose substrates based on application scenarios: Cost-effective FR-4 substrates are suitable for general consumer electronics, while Rogers materials (low dielectric loss) or polyimide materials (high temperature resistance) are ideal for high-frequency communication devices or aerospace equipment.
  • Select copper foil thickness according to current requirements: Thicker copper foil is recommended for high-current power circuits to reduce heat generation.
  • Control the resin content of prepregs to avoid impacting interlayer adhesion and signal transmission.

  Circuit Patterning and Surface Treatment

  • After transferring circuit patterns to copper-clad laminates via exposure and development, precisely control etching parameters to prevent line width deviations or residual circuits.
  • Conduct black oxidation or brown oxidation treatment on the board surface after etching to increase copper foil roughness, enhance interlayer bonding strength during subsequent lamination, and avoid delamination.
• 2 Lamination Process
  • This is a critical step to stack and bond inner layers, prepregs, and outer copper foils.
  • Stack materials in strict accordance with the designed sequence to avoid interlayer misalignment.
  • Strictly control lamination parameters: Temperature is typically maintained at 180–220℃, pressure at 3–5MPa, and lamination time must be precisely managed. Imbalanced parameters may cause defects such as bubbles or delamination in the board.
  • Prioritize even-layer stacking with a symmetrical structure to minimize board warpage after lamination.
• 3 Drilling and Hole Metallization
  Precision Drilling

  • Use CNC drilling machines to ensure hole position accuracy: The diameter of standard through-holes should be controlled above 0.2mm, with a tolerance of ±0.05mm or less.
  • Distinguish application scenarios for through-holes, blind vias, and buried vias to avoid unnecessary complexity. For example, buried vias can be used for BGA packages but require proper hole plugging.

  Hole Metallization Quality

  • Form conductive hole walls through electroless copper plating and electrolytic copper plating; control plating thickness and uniformity. Insufficient or uneven plating may lead to voids or fractures in hole walls, affecting interlayer connectivity.
  • Remove residual impurities from hole walls to prevent plating peeling.
• 4 Outer Layer Fabrication and Surface Finish
  Outer Circuit Processing

  • The process is similar to inner layer fabrication, but higher precision is required as outer circuits directly affect component soldering and signal transmission. Ensure line width/spacing meets design requirements during photolithography, and avoid defects such as circuit burrs or short circuits.

  Appropriate Surface Finish Selection

  • HASL (Hot Air Solder Leveling): Cost-effective with good solderability, suitable for consumer electronics.
  • Immersion Gold: Flat surface and stable performance, ideal for high-end precision equipment.
  • OSP (Organic Solderability Preservative): Low-cost for mild operating environments, but note its poor protection under high temperature and humidity.
• 5 Shaping and Testing / Inspection
  Precision Shaping

  • Use CNC routing machines to process the board outline, with cutting precision controlled within ±0.2mm. For special structures such as V-cuts, control the cutting depth to avoid board fracture.
  • Reserve 3–5mm wide process edges to facilitate subsequent SMT assembly and testing.

  Comprehensive Inspection

  • Conduct visual inspection with AOI (Automated Optical Inspection) equipment to detect defects such as open circuits or uneven solder mask.
  • Perform electrical performance tests (e.g., flying probe test, ICT test) to ensure impedance deviation is within ±10%.
  • Use X-ray inspection for hidden multilayer circuits to verify via quality and interlayer alignment, eliminating potential defects.

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