PCB Manufacturing Process Flow

In today’s world, where electronic devices are ubiquitous, PCBs (Printed Circuit Boards) serve as the “skeleton” and “nervous system” of electronic products, with their manufacturing processes directly impacting product performance and reliability. Whether you’re an electronics engineer, procurement specialist, or simply interested in PCB manufacturing, understanding the complete PCB fabrication workflow is essential. This article will take you through each critical step of PCB production from raw materials to finished product, while addressing the most common manufacturing challenges.

Detailed Breakdown of Core PCB Manufacturing Processes

1. Panel Cutting (CUT): The Precision Starting Point

Panel cutting marks the first step in PCB manufacturing and forms the foundation for subsequent processes. While seemingly simple, it involves several technical considerations:

• Material Selection: Common copper-clad laminate materials include FR-4 (fiberglass epoxy), aluminum substrates, and high-frequency materials (e.g., Rogers), each requiring different cutting parameters
• Dimensional Control: Precise cutting according to design specifications for UNIT (individual circuit), SET (panelized array), and PANEL (production panel) dimensions
• Accuracy Requirements: Modern PCB manufacturing typically demands cutting tolerances within ±0.10mm
• Edge Treatment: Cut edges require deburring to prevent rough edges from affecting subsequent processes

Key Considerations:

• Verify material type, thickness, and copper weight before cutting
• Account for material expansion/contraction in subsequent processes when determining panel size
• Maintain a clean working environment to prevent surface contamination
• Store different materials separately to prevent mixing

2. Inner Layer Dry Film Imaging: Creating Precise Circuit Patterns

The inner layer dry film process is crucial for accurately transferring design patterns onto PCB substrates, consisting of several sub-processes:

Surface Preparation (Panel Scrubbing)

• Combines chemical cleaning with mechanical abrasion
• Removes oxidation and creates micro-roughness for better dry film adhesion
• Typical parameters: 5-10mm scrub marks, Ra 0.3-0.5μm roughness

Dry Film Lamination

• Thermally bonds photosensitive dry film to the copper surface
• Temperature control: Typically 100-120°C
• Pressure control: Approximately 0.4-0.6MPa
• Speed control: 1.0-1.5m/min

Exposure

• Uses UV light (365nm wavelength) to selectively cure dry film through phototool
• Energy control: 5-10mJ/cm²
• Registration accuracy: Within ±25μm

Development

• Uses 1% sodium carbonate solution to dissolve uncured dry film
• Temperature control: 28-32°C
• Spray pressure: 1.5- 2.5 bar

Etching

• Uses acidic copper chloride solution (CuCl2+HCl) to dissolve exposed copper
• Etch factor (side etch control) >3.0
• Copper thickness uniformity within ±10%

Strip

• Uses 3-5% sodium hydroxide solution to remove protective dry film
• Temperature control: 45-55°C
• Time control: 60-90 seconds

Design Recommendations:

• Minimum inner layer trace/space ≥ 3 mil (0.075mm)
• Avoid isolated copper features to prevent over-etching
• Distribute copper evenly to prevent lamination warpage
• Add design margin for critical signal traces

3. Brown Oxide Treatment: Enhancing Interlayer Bonding

Brown oxide treatment is critical for multilayer PCB manufacturing, primarily improving adhesion between inner layer copper and prepreg (PP):

• Chemical Reaction: Forms a micro-rough organic-metallic complex layer on the copper surface
• Process Control:
• Temperature: 30-40°C
• Time: 1.5-3 minutes
• Copper thickness increase: 0.3-0.8μm
• Quality Verification:
• Color uniformity
• Water contact angle test (should be ≥30°)
• Peel strength test (≥1.0N/mm)

Common Issues:

• Insufficient treatment can cause delamination after lamination
• Over-treatment creates excessive roughness, affecting signal integrity
• Processed panels should be laminated within 8 hours

4. Lamination: Forming Multilayer Structures

Lamination bonds multiple inner layer cores with prepreg (PP) under heat and pressure to create multilayer structures:

• Material Preparation:
• Copper foil (typically 1/3oz or 1/2oz)
• Prepreg (e.g., 1080, 2116, 7628 grades)
• Stainless steel plates, kraft paper, and other auxiliary materials
• Process Parameters:
• Temperature: 170-190°C
• Pressure: 15-25kg/cm²
• Time: 90-180 minutes (depending on board thickness and structure)
• Critical Controls:
• Heating rate: 2-3°C/min
• Cooling rate: 1-2°C/min
• Vacuum level: ≤100mbar

Design Considerations:

• Maintain symmetrical stackup (e.g., 8-layer board: 1-2-3-4-4-3-2-1)
• Orient adjacent layer traces perpendicularly (e.g., horizontal on one layer, vertical on the adjacent)
• Use high-resin-content PP for heavy copper boards
• Consider material flow during lamination for blind/buried via designs

5. Drilling: Creating Precision Interconnects

Drilling creates vertical interconnections between PCB layers, with modern technology achieving exceptional accuracy:

• Drill Types:
• Mechanical drilling (for holes ≥0.15mm)
• Laser drilling (for microvias and blind vias)
• Typical Parameters:
• Spindle speed: 80,000-150,000 RPM
• Feed rate: 1.5-4.0m/min
• Retract rate: 10-20m/min
• Quality Standards:
• Hole wall roughness ≤25μm
• Hole position accuracy ±0.05mm
• No nail heading or burrs

Troubleshooting Common Issues:

• Rough hole walls: Optimize drill parameters, use proper entry/backup materials
• Clogged holes: Improve chip evacuation, adjust drilling sequence
• Broken drills: Verify drill quality, optimize feed rates

6. Electroless Copper Deposition (PTH): Critical Hole Metallization

Electroless copper deposition creates conductive layers on non-conductive hole walls, crucial for PCB reliability:

PTH Process Flow

1. Desmear: Removes resin residue from drilling
2. Electroless Copper:

• An alkaline solution using formaldehyde as a reducing agent
• Temperature: 25-32°C
• Time: 15-25 minutes
• Copper thickness: 0.3-0.8μm

1. Panel Plating:

• Acid copper sulfate solution
• Current density: 1.5- 2.5ASD
• Time: 30-45 minutes
• Copper thickness: 5-8μm

Quality Requirements:

• Backlight test ≥9 level (≥90% hole wall coverage)
• Thermal stress test (288°C, 10 seconds) with no delamination or blistering
• Hole resistance ≤300μΩ/cm

7. Outer Layer Pattern Transfer

Similar to inner layer imaging but with additional plating steps:

1. Surface Prep: Cleaning, micro-etch (removes 0.5-1μm copper)
2. Dry Film Lamination: Uses plating-resistant dry film
3. Exposure: Uses LDI (Laser Direct Imaging) or traditional phototool
4. Development: Creates a plating pattern
5. Pattern Plating:

• Copper thickness: 20-25μm (total)
• Tin thickness: 3-5μm (as etch resist)

1. Strip: Removes plating resist
2. Etching: Removes unwanted copper

Technical Highlights:

• Trace width compensation: Adjust design width based on copper thickness (typically add 10-20%)
• Plating uniformity: Use high throwing power solution and a proper anode configuration
• Side etch control: Optimize etch parameters to maintain trace width accuracy

8. Solder Mask: Circuit Protection Layer

Solder mask protects circuits and affects solder quality and appearance:

• Application Methods:
• Screen printing: For low-precision requirements
• Spray coating: For irregular board shapes
• Curtain coating: High efficiency, excellent uniformity
• Process Flow:

1. Surface prep (cleaning, roughening)
2. Solder mask application
3. Pre-bake (75°C, 20-30 minutes)
4. Exposure (300-500mJ/cm²)
5. Development (1% sodium carbonate solution)
6. Final cure (150°C, 30-60 minutes)

• Quality Standards:
• Hardness ≥6H (pencil hardness)
• Adhesion: 100% pass with 3M tape test
• Solder resistance: 288°C, 10 seconds, 3 cycles with no defects

Design Guidelines:

• Minimum solder mask bridge ≥0.1mm
• BGA area openings: 0.05mm larger than pads per side
• Gold fingers require solder mask coverage

9. Surface Finish: Balancing Solderability and Durability

Different finishes suit different applications:

Selection Guide:

• Standard consumer electronics: HASL or OSP
• High-reliability products: ENIG
• High-speed circuits: Imm Ag or OSP
• Edge connectors: Hard gold plating (1-3μm)

10. Routing: Precision Outline Fabrication

PCB outline processing primarily uses three methods:

• CNC Routing:

• Accuracy: ±0.10mm
• Minimum slot width: 1.0mm
• Corner radius: ≥0.5mm

• V-Scoring:

• Angle: 30° or 45°
• Remaining thickness: 1/3 of board thickness (typically 0.3-0.5mm)
• Position accuracy: ±0.10mm

• Laser Cutting:

• Accuracy: ±0.05mm
• Minimum kerf: 0.2mm
• No mechanical stress

Design Rules:

• Maintain ≥0.3mm clearance between the board edge and circuits
• Include breakaway tabs or mouse bites for panelized designs
• Provide precise DXF files for irregular outlines
• Bevel edges (typically 20-45°) for gold finger boards

11. Electrical Testing: Final Quality Gate

PCB testing ensures functional reliability:

• Test Methods:
• Flying probe: Suitable for low-volume, high-mix production
• Fixture testing: For high-volume production
• AOI (Automated Optical Inspection): Supplemental inspection
• Test Coverage:
• 100% net continuity
• Insulation testing (typically 500V DC)
• Impedance testing (for controlled impedance boards)

Common Issue Resolution:

• Opens: Verify false opens (poor test probe contact)
• Shorts: Analyze short location, check design issues
• Impedance deviation: Verify material parameters and trace width control

12. Final Inspection & Packaging

The last quality verification step:

• Inspection Items:
• Visual: Scratches, stains, solder mask defects
• Dimensional: Thickness, outline, hole sizes
• Marking: Legend clarity and position accuracy
• Functional: Gold finger plating quality, impedance tests
• Packaging Methods:
• Vacuum packaging (anti-oxidation)
• Anti-static packaging (for sensitive components)
• Interleaved paper (prevents surface scratches)
• Custom trays (for high-precision boards)

Shipping Standards:

• IPC-A-600G Class 2 (commercial)
• IPC-A-600G Class 3 (high reliability)
• Customer-specific requirements

PCB Manufacturing FAQ (Q&A)

Q1: Why does my PCB experience copper peeling after soldering?

Root Causes:

1. Poor copper-to-substrate adhesion (material issue)
2. Excessive soldering temperature or duration
3. Poor design (e.g., large copper area connected via thin traces)
4. Inadequate brown oxide treatment

Solutions:

• Select high-quality laminate materials
• Optimize soldering parameters (<260°C, <5 seconds)
• Use thermal relief connections in designs
• Verify brown oxide process parameters with the manufacturer
• Conduct thermal stress testing when necessary (288°C, 10 seconds, 3 cycles)

Q2: How to address layer-to-layer misregistration in multilayer PCBs?

Misregistration Sources:

• Material expansion/contraction inconsistencies
• Layer shifting during lamination
• Insufficient exposure registration accuracy
• Drilling position deviations

Improvement Measures:

• Design Stage:

• Add registration targets (minimum 3)
• Maintain even copper distribution
• Account for material properties (special handling for high-frequency materials)

• Manufacturing:

• Use high-precision LDI exposure equipment
• Implement X-ray drilling alignment
• Apply material shrinkage compensation algorithms
• Consider sequential lamination for high aspect ratio boards

• Material Selection:

• Use low CTE materials
• Select dimensionally stable prepreg

Q3: How to resolve rough hole walls in small holes (<0.2mm)?

Technical Solutions:

• Drill Selection:
• Specialty drills (e.g., UC-type)
• Point angle 130-140°
• Helix angle 35-40°
• Parameter Optimization:
• Increase RPM to 120,000-150,000
• Reduce feed rate to 1.0-1.5m/min
• Change drills every 500 hits
• Auxiliary Materials:
• High-density aluminum entry material
• Specialty backup boards (e.g., phenolic)
• Post-Processing:
• Enhanced desmear (plasma treatment optional)
• Optimize etchback before electroless copper

Q4: How should solder mask openings be designed for BGA areas?

Design Specifications:

• Standard BGA:

• Solder mask openings 0.05mm larger than pads per side
• Minimum solder mask bridge 0.1mm
• NSMD (Non-Solder Mask Defined) design

• Fine-Pitch BGA (≤0.5mm pitch):

• Solder mask openings equal to or slightly smaller (0.02-0.03mm) than pads
• SMD (Solder Mask Defined) design
• Consider the LDI (Laser Direct Imaging) process

• Special Treatments:

• Prevent solder mask from climbing BGA spheres
• Control solder mask thickness to 10-15μm
• Implement solder mask dams when necessary

Common Issue Resolution:

• Thick solder mask causing soldering issues: Use thin solder mask inks
• Broken solder mask bridges: Optimize exposure energy and development
• Misaligned openings: Verify phototool or LDI data

Q5: Why does ENIG plating sometimes result in “Black Pad”? How to prevent it?

Black Pad Causes:
Black Pad refers to brittle interfaces between nickel and solder in ENIG finishes, primarily caused by:

• Over-etching of nickel during gold deposition
• Abnormal nickel phosphorus content (should be 7-9%)
• Excessive gold thickness (>0.15μm) causing nickel passivation
• Improper post-treatment (inadequate cleaning)

Prevention Methods:

• Process Control:

• Maintain bath pH 4.5-5.5
• Control gold thickness 0.05-0.10μm
• Add post-dip treatment (e.g., mild acid wash)

• Quality Monitoring:

• Regular nickel phosphorus content testing
• Cross-section analysis of nickel-gold interface
• Solder ball shear testing (>5kg/mm²)

• Alternative Solutions:

• Consider ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)
• Use electrolytic nickel/gold for high-reliability applications

Q6: How to address signal integrity issues in high-speed PCBs?

Design-Manufacturing Co-optimization:

• Material Selection:

• Low Dk (dielectric constant), low Df (dissipation factor) materials
• Smooth copper foils (e.g., HVLP)

• Design Optimization:

• Tight impedance control (±10%)
• Minimize via stubs (back drilling)
• Use microstrip or stripline structures

• Manufacturing Controls:

• Etch precision (±15μm trace width)
• Dielectric thickness control (±10%)
• Surface finish selection (prefer Imm Ag or OSP)

• Testing Verification:

• TDR (Time Domain Reflectometry) testing
• Insertion/return loss measurements
• Eye diagram testing (for high-speed signals)

Typical Parameters:

• 10 Gbps signals: Materials with Df<0.010
• 28Gbps+: Consider Megtron6 or Rogers materials
• Impedance: 50Ω single-ended, 100Ω differential (adjust per protocol)

Conclusion

PCB manufacturing represents a multidisciplinary technology combining materials science, chemical processes, and precision mechanical engineering. As electronics evolve toward higher frequencies, speeds, and densities, PCB fabrication processes continue advancing accordingly. Understanding these manufacturing workflows not only facilitates designing more manufacturable PCBs but also enables rapid troubleshooting and effective communication with manufacturers.

Whether working with conventional FR-4 materials for consumer electronics, specialized high-frequency materials for 5G equipment, or high-reliability automotive electronics, selecting appropriate PCB manufacturers and thoroughly understanding their capabilities proves critical. We hope this guide provides valuable insights to support your informed decision-making in PCB manufacturing.