PCB Inner Layer Production Process: Complete Step-by-Step Guide for Multilayer PCBs

PCB Inner Layer Production Process: Complete Step-by-Step Guide for Multilayer PCBs

The inner layer fabrication process is the core and foundational stage of multilayer PCB manufacturing. Unlike single-sided and double-sided PCBs, multilayer boards require dedicated inner layer circuit production before lamination, and the precision of inner layer circuits directly determines the alignment accuracy, signal integrity and overall yield of finished multilayer PCBs. This guide systematically introduces the classification of PCB processes by layer count, and explains the full inner layer production workflow, working principles, key control parameters and common quality issues in detail.

1. PCB Process Classification by Layer Count

PCB manufacturing processes vary significantly based on the number of conductive layers. Single-sided and double-sided boards do not require an inner layer process, while multilayer PCBs include a complete inner layer fabrication stage before lamination.

1.1 Single-Sided PCB Process Flow

Board cutting & edge grinding → Drilling → Outer layer pattern transfer → (Full board gold plating) → Etching → Inspection → Solder mask screen printing → (Hot air leveling) → Silkscreen character printing → Profile forming → Electrical testing → Final inspection

1.2 Double-Sided PCB Process Flows

Tin-Plated Double-Sided Board

Board cutting & edge grinding → Drilling → Copper thickening → Outer layer pattern transfer → Tin plating → Etching & tin stripping → Secondary drilling → Inspection → Solder mask printing → Gold-plated plugs → Hot air leveling → Silkscreen characters → Profile forming → Testing → Inspection

Nickel-Gold Plated Double-Sided Board

Board cutting & edge grinding → Drilling → Copper thickening → Outer layer pattern transfer → Nickel & gold plating → Film stripping & etching → Secondary drilling → Inspection → Solder mask printing → Silkscreen characters → Profile forming → Testing → Inspection

1.3 Multilayer PCB Process Flows

All multilayer processes include inner layer fabrication as the first core stage.

Tin-Plated Multilayer Board

Board cutting & edge grinding → Tooling hole drilling → Inner layer pattern transfer → Inner layer etching → AOI inspection → Browning/black oxide treatment → Lamination → Drilling → PTH & copper thickening → Outer pattern transfer → Tin plating → Etching & tin stripping → Secondary drilling → Inspection → Solder mask → Gold-plated plugs → HASL → Silkscreen → Forming → Testing → Inspection

Nickel-Gold Plated Multilayer Board

Board cutting & edge grinding → Tooling hole drilling → Inner layer pattern transfer → Inner layer etching → Inspection → Browning → Lamination → Drilling → PTH copper thickening → Outer pattern transfer → Gold plating → Film stripping & etching → Secondary drilling → Inspection → Solder mask → Silkscreen → Forming → Testing → Inspection

ENIG (Electroless Nickel Immersion Gold) Multilayer Board

Board cutting & edge grinding → Tooling hole drilling → Inner layer pattern transfer → Inner layer etching → Inspection → Browning → Lamination → Drilling → PTH copper thickening → Outer pattern transfer → Tin plating → Etching & tin stripping → Secondary drilling → Inspection → Solder mask → Electroless nickel gold → Silkscreen → Forming → Testing → Inspection

2. Detailed PCB Inner Layer Production Process (Pattern Transfer)

Inner layer production is the process of transferring designed circuit patterns onto inner core copper surfaces, consisting of five core stages: board cutting, inner layer pre-treatment, dry film lamination, UV exposure, and the DES (Developing-Etching-Stripping) production line.

2.1 Board Cutting (Material Preparation)

Purpose: Cut large-format raw copper clad laminates into production panel sizes specified by manufacturing instructions (MI), according to pre-designed panelization plans.

Main raw materials: Copper clad laminate core material, saw blades

Core substrates consist of copper foil and insulating laminate, available in standard copper weight grades such as H/H, 1OZ/1OZ and 2OZ/2OZ.

Key operation requirements & control points:

Edge trimming & corner rounding: Remove sharp glass fiber burrs on board edges via mechanical grinding to prevent scratches and quality risks during subsequent production.

Pre-baking: Bake panels before processing to remove moisture and organic volatiles, release internal stress, promote resin cross-linking, and improve dimensional stability, chemical resistance and mechanical strength.

Grain direction consistency: Maintain consistent mechanical grain direction of all panels to ensure uniform expansion and contraction during lamination.

Inspection items: Panel dimensions, board thickness, material grade, copper weight; baking time & temperature, stacking height.

2.2 Inner Layer Copper Surface Pre-Treatment

Purpose: Remove grease, oxides and other impurities from the copper surface, and create controlled micro-roughness to improve dry film adhesion for subsequent lamination.

Common pre-treatment methods:

Sandblasting

Chemical micro-etching treatment

Mechanical brush grinding

Chemical cleaning principle: Alkaline solution first removes oil, fingerprints and organic contaminants; acidic solution then removes oxide layers and protective coatings; final micro-etching creates a uniformly roughened copper surface with optimal dry film adhesion.

Core control parameters:

Grinding speed: 2.5–3.2m/min

Wear scar width: 8–14mm for 500# needle brush; 8–16mm for 800# non-woven brush

Drying temperature: 80–90°C

2.3 Dry Film Lamination

Purpose: Laminate photosensitive dry film onto the pre-treated copper surface via hot pressing.

Working principle: Peel off the polyethylene protective film from the dry film, then bond the photoresist layer onto the copper clad laminate under heat and pressure. The resist layer softens when heated, and adheres firmly to the copper surface via pressure and built-in binders.

Three core parameters of dry film lamination: pressure, temperature, transfer speed

Control points:

Lamination speed: 1.5 ± 0.5m/min

Lamination pressure: 5 ± 1kg/cm²

Lamination temperature: 110 ± 10°C

Panel surface temperature: 40–60°C

Dry Film vs. Wet Film

Dry film: A water-soluble photosensitive resist film available in 1.2mil, 1.5mil and 2mil thicknesses. It consists of three layers: polyester protective film, polyethylene separator and photosensitive resist layer. Unexposed dry film is easily dissolved in sodium carbonate solution.

Wet film: A single-component liquid photosensitive coating composed of high-sensitivity resin, photoinitiator, colorant, filler and solvent. Applied via screen printing or spraying, with production viscosity of 10–15dPa·s, offering good etching and plating resistance.

2.4 UV Exposure

Purpose: Transfer circuit patterns from the photomask film onto the photosensitive dry film via ultraviolet irradiation.

Working principle: Photoinitiators in the exposed resist area absorb UV photons and generate free radicals, which trigger cross-linking reactions to form a three-dimensional polymer structure insoluble in dilute alkali. Unexposed areas remain soluble in alkaline solution.

Inner vs. outer layer film difference:

Inner layers use negative film: transparent (white) areas polymerize and remain, black opaque areas do not react.

Outer layers use positive film, which works in the opposite manner.

Environmental requirements:

Cleanroom class: ≤10,000 particles

Temperature: 22 ± 3°C

Humidity: 55 ± 10%

These conditions prevent film deformation and dust-induced circuit defects.

Control points: Alignment accuracy, exposure energy, exposure step wedge (6–8 steps standard), dwell time

2.5 Developing

Purpose: Wash away unexposed, unpolymerized dry film with dilute alkaline solution, leaving polymerized resist as the protective layer for subsequent etching.

Main chemical: Sodium carbonate (Na₂CO₃) solution

Working principle: Active groups in unexposed photoresist react with dilute alkali to form soluble substances, while fully exposed cross-linked dry film remains undissolved.

Control points:

Developing speed: 1.5–2.2m/min

Developing temperature: 30 ± 2°C

Developing pressure: 1.4–2.0kg/cm²

Developer concentration: 0.85–1.3% Na₂CO₃

2.6 Inner Layer Etching

Purpose: Etch away exposed copper areas not protected by dry film, forming the final inner layer circuit pattern.

Main etchant: Acidic copper chloride (CuCl₂) solution

Working principle: Dry film or ink covers the designed circuit patterns to protect copper from etching. Unprotected bare copper is removed via chemical reaction, leaving precise conductive traces.

Common quality defects: Under-etching, over-etching, line width reduction, open circuits, short circuits

Control points:

Etching speed, temperature (48–52°C), spray pressure (1.2–2.5kg/cm²)

Stripping section: 44–54°C, 8–12% NaOH solution

2.7 Dry Film Stripping

Purpose: Strip the cured resist layer from the copper surface with strong alkaline solution to expose the finished inner layer circuit pattern.

Main chemical: Sodium hydroxide (NaOH) solution

3. Conclusion

Inner layer production is the most critical process that determines multilayer PCB precision and yield. The core quality control logic lies in stable copper surface pre-treatment, precise lamination parameters, accurate exposure alignment and controlled etching line width. Strict control of temperature, pressure, speed and concentration parameters at each stage ensures inner layer circuit accuracy, providing a reliable foundation for subsequent browning, lamination and outer layer processing.

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