PCB Fabrication Drawing Guide: Fab Notes, Drill Chart, Stackup, and Tolerances

FAQ

What information is required on a PCB fab drawing?

A pcb fabrication drawing should specify board dimensions, thickness, material (such as FR4 or polyimide), layer count, copper weight, surface finish, solder mask color, silkscreen, and any special requirements like controlled impedance or blind/buried vias.

What is impedance control in PCB fabrication?

Controlled impedance pcb fabrication requires precise control of trace width, dielectric thickness, and material dielectric constant. Manufacturers use impedance calculators and test coupons to verify target values (typically 50 ohm or 100 ohm differential).

How do I specify PCB tolerances on a fab drawing?

Standard pcb tolerances include +/- 10% on trace width, +/- 0.1mm on drilled hole diameter, and +/- 0.2mm on board outline. Tighter tolerances for high-density designs should be explicitly noted along with acceptable limit ranges.

What are fiducial marks on a PCB?

Fiducial marks are copper features used by automated optical inspection and pick-and-place equipment for panel registration. They should be placed at three corners of the panel or board for reliable optical recognition.

What is the difference between plated and non-plated holes?

Plated through holes (PTH) form electrical connections between layers using copper deposition. Non-plated holes (NPTH) are mechanical only and may be used for mounting standoffs. All holes require specification of final diameter and tolerance.

Contents

• Highlights
• What Is a PCB Fabrication Drawing?
• Why Fab Drawings Cause Engineering Holds
• What Must a PCB Fab Drawing Include?
• How the Fab Drawing Interacts with CAM, Assembly, and QA
• Comparison: Common Options and What You Gain / Lose
• Fab Notes for Impedance, Via Protection, and Flatness
• Where PCB Drawings Are Heading
• Capability & Ordering Guide
• What to Send With Your Fab Drawing for a Quote
• Conclusion

Highlights

• The Legal Contract: Why the fab drawing overrides Gerber files in conflict scenarios.
• Critical Elements: Drill charts, stackup definitions, and IPC class designations.
• Material Control: How to specify laminates without locking yourself into a single vendor.
• Tolerance Management: Balancing mechanical precision with manufacturing cost.
• Future Trends: The shift from PDF drawings to intelligent formats like IPC-2581.

What Is a PCB Fabrication Drawing?

A PCB fabrication drawing (fab drawing, or fab print) is the controlled document that tells the board manufacturer how to build the bare PCB. The Gerber or ODB++ data shows the copper, mask, and legend images. The printed circuit board drawing adds what those images cannot: the board outline with dimensions and tolerances, the drill chart, the stackup and material, the surface finish, the IPC class, and the fab notes that govern inspection. It is separate from the assembly drawing, which shows where components go.

Fab drawing element	What it controls	Typical omission that triggers a hold
Board outline and dimensions	Profile, cutouts, slots, datums	No tolerance on the outline or slot positions
Drill chart	Hole symbols, finished sizes, plated vs non-plated	Drill size given instead of finished size
Stackup and material	Layer order, dielectric, copper weight, laminate spec	Brand named without an IPC-4101 slash sheet or equivalent rule
Fab notes	IPC class, finish, mask and legend, via treatment, impedance	Via plugging or impedance layers not stated
Special features	Gold fingers, bevels, castellations, countersinks	Bevel angle and depth missing

PCB terms used in the notes (annular ring, slash sheet, VIPPO) are defined in the printed circuit board terminology glossary.

Why Fab Drawings Cause Engineering Holds

In the past, a simple "Make to IPC-6012 Class 2" note and a board outline were often sufficient for 2-layer or 4-layer boards. However, the landscape of electronics manufacturing has shifted dramatically. Modern designs feature tighter impedance controls, higher layer counts, and mixed-material stackups that require explicit instruction.

The disconnect often arises because CAD tools (like Altium, Allegro, or KiCad) are excellent at electrical connectivity but require manual effort to generate high-quality mechanical documentation. As density increases, the margin for error decreases. A missing note about via plugging can lead to solder wicking during assembly. A vague material callout can result in a board that functions electrically but delaminates during reflow. For manufacturers like APTPCB (APTPCB PCB Factory), a clear fab drawing is the primary tool for ensuring the final product matches the designer's intent.

What Must a PCB Fab Drawing Include?

A useful fabrication drawing is not just a picture of the board; it is a collection of data tables and technical notes. The following elements are the non-negotiable essentials.

1. The Drill Chart and Hole Schedule

The drill chart is the map for the CNC drilling machines. It must explicitly link a symbol on the drawing to a specific hole size and type.

• Finished vs. Drill Size: The most common error is failing to specify that hole sizes apply to the finished plated hole. Manufacturers must calculate the drill bit size (usually 0.1mm to 0.15mm larger) to account for plating thickness.
• Plated vs. Non-Plated: Clearly distinguishing between Plated Through Holes (PTH) for signals and Non-Plated Through Holes (NPTH) for mounting is vital.
• Tolerance: Standard vias might have a tolerance of ±0.075mm, while press-fit connectors require much tighter control.

2. Stackup and Material Definition

For controlled impedance boards, the stackup is critical. The drawing must specify the layer order, copper weight, and dielectric thickness.

• Generic vs. Specific: Instead of specifying a brand name (which might have long lead times), it is often better to specify the IPC-4101 slash sheet (e.g., /126 for high-Tg FR4). This allows APTPCB to select a compliant material that is in stock, reducing lead time.
• Copper Balance: Specifying the finished copper weight (e.g., 1oz) ensures the etchant process is calibrated correctly.

3. Mechanical Dimensions and Tolerances

The board outline in the Gerber file is precise, but the fab drawing defines the allowable deviation.

• Profile Tolerance: A standard routing tolerance is ±0.15mm. If the board must fit into a precision machined enclosure, this needs to be explicitly stated.
• Chamfers and Bevels: For edge connectors (gold fingers), the angle and depth of the bevel must be dimensioned to ensure proper mating.

4. Fabrication Notes

This is the "catch-all" section that governs quality.

• IPC Class: Defining Class 2 (standard) or Class 3 (high reliability) dictates the inspection criteria for annular rings and plating thickness.
• Surface Finish: Explicitly stating ENIG, HASL, or Hard Gold prevents costly assumptions.
• Solder Mask and Silkscreen: Defining the color and type (e.g., LPI Green) ensures consistency.

For a deeper dive into how these layers come together, review our guide on PCB stack-up design.

How the Fab Drawing Interacts with CAM, Assembly, and QA

The fab drawing does not exist in a vacuum. It triggers a cascade of events across the manufacturing ecosystem.

Interaction with Cam Engineering

When data arrives, Computer-Aided Manufacturing (CAM) engineers compare the fab drawing against the Gerber files. If the Gerbers show a 0.2mm hole but the drawing says 0.3mm, the drawing usually takes precedence (or triggers an engineering query). Clear notes reduce these "EQ" holds, speeding up the PCB fabrication process.

Impact on Assembly

While the fab drawing is for the bare board manufacturer, it impacts assembly. Fiducial markers defined on the drawing are essential for pick-and-place machines. Panelization details (V-score vs. tab-route) defined in the fab drawing determine how the boards are handled during SMT assembly. If the drawing allows for "vendor panelization," the manufacturer can optimize material utilization, lowering costs.

Quality Assurance (Qa)

The final inspection team uses the fab drawing as a checklist. They measure the board thickness, check the hole sizes against the drill chart, and verify the surface finish. If a requirement isn't on the drawing, it likely won't be inspected.

Comparison: Common Options and What You Gain / Lose

Engineers often face trade-offs when creating fab drawings. Should you be prescriptive or descriptive? Should you lock down every variable or allow manufacturer flexibility?

Fab Notes for Impedance, Via Protection, and Flatness

The fab drawing is the primary tool for enforcing reliability standards.

Signal Integrity and Impedance

For high-speed boards, a note stating "Impedance lines to be ±10%" is standard. However, for DDR or PCIe interfaces, you may need ±5%. The drawing must identify which layers and trace widths correspond to which impedance values. Without this, the manufacturer cannot perform the necessary coupon testing. You can estimate trace geometry with the PCB impedance calculator before writing the note.

Thermal Reliability and via Protection

Vias are potential failure points. The drawing must specify via treatment:

• Tented: Covered with soldermask (lowest cost, but risk of chemical entrapment).
• Plugged: Filled with non-conductive epoxy and plated over (VIPPO). This is essential for HDI PCB designs where vias are placed in pads.

Mechanical Flatness (Bow and Twist)

Surface mount components require a flat surface. Standard FR4 allows for 0.75% bow and twist. If you are using large BGA components, your fab drawing should restrict this to 0.5% or less to prevent solder joint fractures.

Where PCB Drawings Are Heading

The traditional PDF fab drawing is slowly being replaced by intelligent data formats. While the PDF is human-readable, it is not machine-readable, leading to manual data entry errors.

Capability & Ordering Guide

When moving from design to production, understanding the manufacturer's limits is as important as the drawing itself. Below is a snapshot of standard versus advanced capabilities to help guide your fab drawing specifications.

Capability Snapshot

Parameter	Standard Capability	Advanced Capability	Notes
Layer Count	2–10 Layers	12–64 Layers	High layer counts require precise stackup design; see high layer count PCB for construction options.
Board Thickness	0.4mm – 2.4mm	0.2mm – 6.0mm	Aspect ratio limits apply to drilling.
Min Trace/Space	3mil / 3mil	2mil / 2mil	Requires thinner copper foil (e.g., 1/3oz or 1/2oz).
Min Mechanical Hole	0.2mm	0.1mm	0.15mm is a common cost-effective breakpoint.
Laser Micro-via	N/A	0.075mm	Essential for HDI designs.
Copper Weight	0.5oz – 2oz	Up to 20oz	Heavy copper requires larger spacing.
Impedance Tolerance	±10%	±5% (Single), ±8% (Diff)	Tighter tolerance requires specific dielectric selection.
Surface Finishes	HASL LF, ENIG	Immersion Ag/Sn, Hard Gold	Hard Gold is best for edge connectors.
Max Panel Size	500mm x 600mm	Custom	Depends on equipment; consult engineering.
Solder Mask	Green, Blue, Black, White	Matte colors, Purple, Red	Matte finishes are better for vision systems.

Lead Time & Moq

Order Type	Typical Lead Time	MOQ	Key Drivers
Prototype	24–72 Hours	1 Piece	Quick-turn assumes standard materials (FR4) and finishes (HASL/ENIG).
Small Batch	5–7 Days	5–50 Pieces	Testing requirements (e.g., flying probe) and complex stackups add time.
Mass Production	10–15 Days	100+ Pieces	Volume pricing kicks in; lead time includes panelization and tooling.

RFQ / DFM Checklist (What to Send)

To get an accurate quote and a clean DFM review, ensure your data package includes:

• Gerber Files: RS-274X format (or ODB++ / IPC-2581).
• Fab Drawing: PDF format with:
  • Board outline and dimensions.
  • Drill chart with symbols and finished sizes.
  • Stackup diagram with material types and copper weights.
  • Notes on IPC Class, color, and finish.
• NC Drill Files: Excellon format (if not embedded in ODB++).
• Panelization: If you need arrays, specify the array drawing or ask for "Vendor Panelization."
• Quantities: Prototype count vs. production forecast.
• Special Requirements: Impedance reports, ionic contamination testing, or specific PCB quality assurance records such as IPC-6012 conformance.

What to Send With Your Fab Drawing for a Quote

When you are ready to validate your design, sending a complete package ensures a faster response. A thorough DFM review at the quote stage can catch issues like impossible drill-to-copper ratios or unavailable materials before they become production delays.

• Attach your Fab Drawing: Ensure all notes are legible.
• Specify Lead Time: Do you need 24-hour turn or standard delivery?
• Material Callouts: Are equivalents allowed?
• Testing: Do you require 100% netlist testing?
• Assembly: If the boards also need assembly, include your BOM, pick-and-place file, and a separate assembly drawing.
• Volume: Quote for 5, 50, and 500 to see price breaks.

Frequently Asked Questions

What is the minimum PCB thickness tolerance APTPCB can hold?

Standard tolerance is ±10% of nominal thickness for boards under 1.6mm. For boards ≥1.6mm, the tolerance tightens to ±0.10mm. HDI and controlled-depth routing jobs use tighter ±0.05mm tolerances.

How does fab drawing affect PCB dimensional requirements?

The fab drawing defines the PCB stack-up, tolerances, surface finish, and testing requirements. APTPCB's engineering review checks for conflicts between stack-up constraints and DFM issues before releasing the job to production.

What drill hit accuracy can be expected for PTH holes?

APTPCB targets ±0.03mm drill hit accuracy on plated through-holes. For controlled-depth drilling and blind/buried via processes, accuracy depends on layer registration which is typically ±0.05mm for 6-layer HDI boards.

How do you specify tolerances for rigid-flex PCBs?

Rigid-flex tolerances are specified separately for the rigid and flexible sections. Minimum bend radius for flex layers is typically 6× to 10× the flex material thickness. APTPCB reviews these constraints in DFM.

What is the standard slot tolerance for PCB milling?

CNC-routed slots hold ±0.10mm tolerance on the slot width. For castellated holes on module boards, IPC standards define Class 2 and Class 3 tolerances. APTPCB reviews slot geometry against panel utilization to avoid stress concentration.

Conclusion

The fab drawing is the bridge between a digital concept and a physical reality. It captures the "unspoken" requirements of a PCB design—reliability, material properties, and mechanical fit—that Gerber files alone cannot convey. A complete PCB fabrication drawing reduces the risk of technical queries, scrap, and field failures.

Whether you are building a simple prototype or a complex aerospace board, clear documentation is the key to success. APTPCB is ready to review your data, offer practical DFM advice, and deliver boards that meet your exact specifications.

A drawing that names its test coupons and inspection notes the way this guide shows lets our PCB prototype service build the verification run without clarification loops.

Related links

• printed circuit board terminology
• PCB stack-up design
• PCB fabrication process
• PCB impedance calculator
• HDI PCB
• high layer count PCB
• PCB quality assurance