Purpose and Importance
PCB fabrication drawings, also known as PCB mechanical drawings or PCB assembly drawings, are essential technical documents that provide detailed specifications for manufacturing printed circuit boards. These drawings serve as a communication bridge between PCB designers and manufacturers, ensuring accurate production of the intended design.
Basic Components of a Fabrication Drawing
| Component | Description | Purpose |
| Title Block | Project information and revisions | Identification and version control |
| Board Outline | Physical dimensions and shape | Define board boundaries |
| Layer Stack-up | Material and copper layer details | Specify board construction |
| Drill Table | Hole sizes and quantities | Define drilling requirements |
| Notes | Special requirements and specifications | Clarify manufacturing details |
| Tolerances | Acceptable variation ranges | Quality control parameters |
Title Block Information
Essential Title Block Elements
| Element | Description | Example |
| Drawing Number | Unique identifier | DWG-2024-001 |
| Revision Level | Version control | Rev. A |
| Date | Creation/revision date | 25-Nov-24 |
| Engineer | Designer name | John Smith |
| Scale | Drawing scale ratio | 1:01 |
| Sheet Number | Page information | 1 of 3 |
Board Outline Specifications
Dimensional Information
| Aspect | Common Units | Typical Tolerance |
| Length | mm/inches | ±0.1mm/±0.004″ |
| Width | mm/inches | ±0.1mm/±0.004″ |
| Thickness | mm/inches | ±10% |
| Corner Radius | mm/inches | ±0.1mm/±0.004″ |
Board Profile Features
| Feature | Description | Specification Method |
| Cutouts | Internal board openings | Dimensions from datum |
| Notches | Edge indentations | Size and position |
| Chamfers | Edge angles | Angle and length |
| Slots | Linear openings | Width and length |
Layer Stack-up Information
Standard Layer Stack-up Table
| Layer | Material | Thickness | Description |
| Top Solder Mask | LPI | 0.01mm | Green |
| Top Copper | Copper | 1oz (35µm) | Signal |
| Prepreg | FR4 | 0.2mm | Dielectric |
| Inner Layer 1 | Copper | 0.5oz (17.5µm) | Ground |
| Core | FR4 | 0.4mm | Substrate |
| Inner Layer 2 | Copper | 0.5oz (17.5µm) | Power |
| Prepreg | FR4 | 0.2mm | Dielectric |
| Bottom Copper | Copper | 1oz (35µm) | Signal |
| Bottom Solder Mask | LPI | 0.01mm | Green |
Material Specifications
| Property | Unit | Common Values |
| Dielectric Constant | Er | 3.5-4.5 |
| Glass Transition Temp | Tg | 130-170°C |
| Thermal Conductivity | W/m·K | 0.25-0.5 |
| Copper Weight | oz/ft² | 0.5-2.0 |
Drill Information
Drill Table Components
| Column | Description | Example |
| Symbol | Drill identifier | ⌀1, ⌀2 |
| Size | Hole diameter | 0.3mm |
| Quantity | Number of holes | 150 |
| Plating | Plating requirement | PTH/NPTH |
| Tolerance | Size variation | ±0.05mm |
Common Drill Types
| Type | Description | Typical Use |
| PTH | Plated Through Hole | Component leads |
| NPTH | Non-Plated Through Hole | Mounting holes |
| Blind Via | Partial depth connection | HDI designs |
| Buried Via | Internal layer connection | Complex routing |
Special Requirements
Surface Finish Specifications
| Finish Type | Thickness | Application |
| HASL | 0.5-1.0µm | General purpose |
| ENIG | 3-6µm Ni, 0.05-0.1µm Au | Fine pitch |
| OSP | 0.2-0.5µm | Lead-free |
| Immersion Tin | 0.8-1.2µm | Good solderability |
Solder Mask Specifications
| Parameter | Specification | Notes |
| Color | Green, Red, Blue, etc. | UV blocking |
| Type | LPI, dry film | Process dependent |
| Thickness | 0.02-0.04mm | Coverage |
| Registration | ±0.1mm | Alignment |
Manufacturing Notes
Standard Note Categories
| Category | Purpose | Example Notes |
| Material | Specify board material | “Use TG170 FR4” |
| Finish | Surface treatment | “ENIG per IPC-4552” |
| Testing | Test requirements | “100% E-test required” |
| Marking | Board identification | “UL mark required” |
Critical Parameters
| Parameter | Typical Range | Description |
| Impedance Control | ±10% | Signal integrity |
| Copper Thickness | ±10% | Current capacity |
| Registration | ±0.075mm | Layer alignment |
| Hole Quality | Class 2/3 | IPC standards |
Quality Specifications
IPC Standards Reference
| Standard | Topic | Application |
| IPC-6012 | Rigid PCB qualification | General requirements |
| IPC-A-600 | Acceptability criteria | Visual inspection |
| IPC-TM-650 | Test methods | Quality verification |
| IPC-4101 | Base materials | Material specs |
Inspection Requirements
| Aspect | Method | Criteria |
| Registration | Optical | ±0.075mm max |
| Hole Size | Gauge pins | +0.075/-0.0mm |
| Surface Finish | XRF measurement | Per specification |
| Cleanliness | Ion chromatography | <1.0 µg/in² NaCl |
Frequently Asked Questions
Q1: What are the most important elements to check in a PCB fabrication drawing?
A: The critical elements to verify include:
- Board dimensions and tolerances
- Layer stack-up details
- Drill specifications
- Material requirements
- Surface finish specifications
- Special notes and requirements Always cross-reference these with your manufacturer’s capabilities.
Q2: How do I interpret drill table symbols and specifications?
A: Drill tables typically include:
- Drill size (diameter)
- Quantity of each size
- Plating requirements (PTH/NPTH)
- Special requirements (depth for blind vias)
- Tolerance specifications Look for the corresponding symbols on the drawing to locate each hole type.
Q3: What is the significance of the layer stack-up table?
A: The layer stack-up table defines:
- Number of copper layers
- Layer thicknesses
- Material types
- Dielectric specifications
- Overall board thickness This information is crucial for impedance control and manufacturing process planning.
Q4: How do I verify dimensional tolerances?
A: To verify tolerances:
- Check the general tolerance note
- Look for specific critical tolerances
- Compare with manufacturer capabilities
- Consider cumulative tolerances
- Verify against assembly requirements
Q5: What common mistakes should I watch for when reading fabrication drawings?
A: Common oversights include:
- Missing or incorrect revision numbers
- Conflicting specifications
- Unclear special requirements
- Incomplete drill information
- Missing critical dimensions Always cross-reference with other documentation and verify completeness.
Conclusion
Reading PCB fabrication drawings requires attention to detail and understanding of various technical specifications. The ability to interpret these drawings accurately is crucial for successful PCB manufacturing. Always verify critical parameters, cross-reference specifications with manufacturer capabilities, and ensure all special requirements are clearly communicated. When in doubt, consult with your PCB manufacturer for clarification on any aspects of the fabrication drawing.