PCB fabrication tolerances are the allowed variation between the board you designed and the board you receive: hole sizes and positions, trace widths, board thickness, outline, impedance and flatness. Every fabricator has standard tolerances it holds at normal cost; tighter ones need more process control and inspection, so they cost more and can lower yield. The IPC standards say where tolerances must be defined (IPC-2221 for design, IPC-6012 for acceptance), but the numbers you get depend on your drawing and the fabricator. This guide explains each tolerance, which ones are worth tightening, and how to write them so the quote matches what you need.
1. Where Do PCB Tolerances Come From?
From three places, in this order:
- Your fabrication drawing. Anything you state there overrides defaults.
- The acceptance standard on your order, usually IPC-6012 Class 2 or 3 for rigid boards. IPC-6012F covers hole size and location (section 3.4.1) and conductor width and thickness (section 3.5.1) among its dimensional requirements (IPC-6012F contents).
- The fabricator’s standard capability for anything you leave open.
IPC-2221C, the generic design standard, has sections on dimensions and tolerances (5.4.1), board thickness tolerance (5.5) and minimum hole location tolerance (Table 9-4) (IPC-2221C contents). The values are in the purchased standards, so if a tolerance matters, write the number on your drawing.
2. Which Tolerances Matter Most?
| Tolerance | What varies | Why it matters |
|---|---|---|
| Finished hole size | Drill size, plating thickness | Press-fit pins and component leads must fit; plated holes finish smaller than the drill |
| Hole position | Drill accuracy, layer registration | Annular ring left around the hole; alignment with mating parts |
| Trace width and spacing | Etching | Current capacity, impedance, minimum spacing |
| Board thickness | Cores, prepreg, copper, plating, finish | Edge connectors, card guides, enclosure fit |
| Outline and cutouts | Routing or scoring | Fit in the enclosure, connector positions |
| Controlled impedance | Trace width, dielectric thickness, Dk | Signal integrity on high-speed nets |
| Bow and twist | Copper balance, lamination | Flat boards for paste printing and placement |
How is board thickness measured?
Specify whether thickness is measured over the bare laminate or over copper, plating and solder mask, and where. A common convention is to measure finished-board thickness on an area of laminate without copper, solder mask or legend unless the drawing says otherwise. Copper weight, plating and finish all add to the measurement at the edge connector.
Why does a plated hole come out smaller?
Because copper is plated onto the hole wall. The fabricator drills oversize to reach your finished size. Always specify finished hole sizes, and mark which holes are plated (PTH) and which are non-plated (NPTH) in the drill table; non-plated holes can often be held tighter because there is no plating variation.
3. What Can Our Process Hold?
Our published fabrication capability, for reference when you set tolerances:
| Capability | QueenEMS |
|---|---|
| Layers | 1–40 |
| Board thickness | 0.2–6.0 mm |
| Maximum board size | 580 × 1050 mm |
| Minimum mechanical drill | 0.15 mm (6 mil) |
| Minimum laser microvia | 0.075 mm (3 mil) |
| Drill position accuracy | ±0.025 mm |
| Aspect ratio | Up to 12:1 |
| Minimum annular ring | 0.075 mm (3 mil) |
| Minimum solder mask bridge | 0.075 mm (3 mil) |
| Impedance tolerance | ±5% for high-speed designs |
| Copper weight | Inner 0.5–6 oz, outer 0.5–10 oz |
Being able to reach a capability is not the same as needing it. Specify the tolerance your design actually requires; the tightest settings belong only on the features that need them.
4. Which Tolerances Are Worth Paying For?
| Situation | Worth tightening | Usually fine at standard |
|---|---|---|
| Press-fit connectors | Finished hole size on those holes | Other holes |
| Edge (gold finger) connectors | Thickness at the fingers, bevel | Overall board thickness elsewhere |
| High-speed nets | Impedance on the specified layers, with a test coupon | Trace width on non-critical nets |
| Tight enclosure fit | Outline and mounting-hole positions | Internal cutouts |
| Fine-pitch BGAs | Registration and solder mask registration | — |
Tighten only the features that need it and say which ones: “holes marked P: finished size ±0.05 mm” costs far less than a blanket tight tolerance on every hole.
5. How Do IPC Class 2 and Class 3 Change Tolerances?
Class 3 tightens acceptance on annular ring, plating thickness and allowed defects, and adds more evidence, such as microsections. It does not by itself tighten your mechanical tolerances (hole size, outline, thickness); those still come from your drawing. See our IPC Class 2 vs Class 3 guide for the differences, and our annular ring guide for pad sizing.
For flatness, IPC-6012 sets bow and twist limits; the commonly cited values are 0.75% for boards with surface-mount parts and 1.5% for others. Balanced copper and a symmetrical stackup are the best ways to stay inside them.
6. What Should Your Fabrication Drawing Say?
| Item | Example note |
|---|---|
| Acceptance standard | “Fabricate and accept per IPC-6012, Class 2” |
| Finished thickness | “1.6 mm ± 10%, measured over laminate” or “over plated copper at fingers” |
| Holes | Finished sizes in the drill table, PTH/NPTH marked, special tolerances by symbol |
| Outline | Tolerance on outline and on critical mounting holes |
| Impedance | Target, tolerance, layer, reference planes, test method |
| Copper | Inner and outer copper weights |
| Everything else | “Unless stated, per fabricator standard tolerances” |
Our trace width calculator, impedance calculator and trace width and spacing guide help set the values; our PCB fabrication cost guide covers how specifications affect price.
How Can We Help?
Send your Gerber files and fabrication drawing; we will check the tolerances against our capability and flag any that drive cost in the quote, which comes back within 2 hours for bare boards. Our free DFM check catches tolerance conflicts before fabrication, and we build multilayer PCBs up to 40 layers.
Conclusion
Fabrication tolerances come from your drawing first, the IPC acceptance standard second and the fabricator’s defaults third. Tighten only what your design needs, such as press-fit holes, edge-connector thickness, impedance on high-speed layers and critical outline dimensions, and say which features the tight tolerance applies to. State finished hole sizes and how thickness is measured. Contact us with your files for a quote.
FAQ
What is the typical tolerance for PCB thickness? It depends on the fabricator and the thickness; state the tolerance and how it is measured on your drawing, for example “1.6 mm ± 10% over laminate.”
What is the standard drill tolerance? It differs for plated and non-plated holes and by fabricator. Specify finished hole sizes and the tolerance you need on the holes that matter.
What impedance tolerance can be held? Ask for the fabricator’s default; we can hold ±5% for high-speed designs. Specify the target, layer and test method.
Does IPC Class 3 mean tighter dimensions? Not by itself. Class 3 tightens acceptance on plating, annular ring and defects; mechanical tolerances still come from your drawing.
What happens if I leave tolerances off the drawing? The fabricator uses its standard tolerances, which may not suit press-fit holes, edge connectors or tight enclosures.
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