A stack-up drawing reads Core 300µm, Cu 18µm + plating. Those are nominal values, not acceptance values. The column between them is tolerance.
The three blanks that show up most often in engineering review sit in exactly that column: what applies when the drawing does not state a tolerance, which fields the fab will never fill in for you, and what evidence backs a claim that an item was met. This article answers only those three. It does not walk through the process control behind each individual tolerance — board thickness control, outline and V-cut tolerance and impedance tolerance classes each take one of those.
1. Nominal and accepted are two different numbers
Stack-up drawings, spec tables and capability pages all publish nominal values: what the board is meant to be. Acceptance values state whether what came out is conforming. On the same board those two almost never match — nominal thickness 1.6mm is accepted at ±10% or ±0.005", whichever is greater; a nominal 8mil trace is accepted at ±20%.
Getting this backwards costs money in both directions. Checking incoming material or finished boards against nominal values rejects conforming lots. Supplying nominal values with no tolerance hands the acceptance boundary to whatever default the other side applies.
2. Tolerance items the drawing omits default to the tolerance table
This is the first rule worth writing down: any tolerance item not stated on the drawing is accepted against the published values in our manufacturing tolerance table. Silence is not the absence of a limit, it is acceptance of the default.
The items with the most direct design impact:
| Item | Tolerance |
|---|---|
| Thickness | ±10% or ±0.005", whichever is greater |
| Trace width | ±20% |
| Copper-to-copper spacing | ±20% |
| Board outline | ±0.005" (≈0.13mm) |
| Hole location (annular ring / pad size) | ±0.003" (≈0.076mm) |
| Hole size | Plated <0.247" ±0.003"; non-plated <0.250" ±0.002"; larger holes ±0.005" |
| Slots | ±0.005" (≈0.13mm) |
| Internal cutouts | Non-plated ±0.005", plated ±0.010" |
| Scoring | Location ±0.003", depth ±0.005" |
| Soldermask registration | ±0.003", depth ±0.005" |
| Inner layer hole to copper | Hole size + 0.020" (≈0.51mm) |
Three reading notes that get missed: imperial values are authoritative, metric is a convenience conversion; the values are given per IPC-6012 acceptability standards; and the table carries design tips alongside each item — pads for plated holes must be at least 0.010" larger than the hole to meet the IPC-6012 annular ring, soldermask clearance should be 0.003" larger than pads to allow for mask swell, and pads within 0.010" of each other may need special processing.
Tighter tolerances can be requested — but request them per item, see section 5.
3. Three fields are never inherited and must be declared
Some fields have no sensible default to inherit. If the drawing is silent, there is nothing to judge against:
1. Target impedance. The tolerance class on the capabilities page is ±5% (±10% standard), but a class states the allowed deviation, not the target in ohms. The tolerance table says it plainly: for impedance-controlled boards, specify target impedance so trace width can be adjusted. "Impedance controlled" without a target value is a tolerance with no datum.
2. IPC-A-600 class. The quality assurance page states inspection to IPC-A-600 Class 2; the capabilities page lists both Class 2 and Class 3. A drawing with no class is accepted at Class 2. Asking for a Class 3 re-judgement afterwards swaps the whole acceptance boundary. How that judgement maps to rework is covered in defect classification and no-rework points.
3. The specific items you want tightened. "High precision required" is not an acceptance criterion; "hole location tightened to ±0.002"" is. Tightening has to land on an item, and it should carry a reason, because tightening one item usually changes the process route and the inspection method.
These fields get confirmed together with stack-up and material at release review — see the stack-up drawing release checklist and the Gerber file preparation checklist.
4. Which process step produces the evidence
Writing a tolerance into the drawing is the first half. The second half is who measures it, at which step, and on what. Of the ten in-process checkpoints published on the quality page, four map directly onto tolerance items:
- After lamination (05): every production panel is measured for finished thickness and logged to the job file. That record is the evidence for board thickness tolerance.
- Drilling (06): drill diameters are verified automatically by the machine; multilayer boards carry a dedicated test coupon to check hole position relative to the inner layers, confirming registration margin.
- Outer layer etching (08): a test coupon on every production panel verifies trace width against tolerance. The coupon runs the same process in the same batch, so measuring it measures the board.
- After routing (09): outline tolerance and hole-position accuracy are confirmed on a vision system; the same step confirms soldermask registration and thickness plus surface-finish thickness, gold and nickel on ENIG for instance.
One more relates to copper: plating (07) uses non-destructive sampling at five or more points per production panel, and heavy copper builds and high-aspect-ratio holes get an additional microsection to confirm hole-wall copper uniformity.
Those measured values consolidate into a per-job quality record (laminate grade and batch, measured values at each checkpoint, tests passed), and the full production record can be supplied on request. So a tolerance here means a measured number traceable to a specific production panel, not a promise.
5. Why tightening everything is the wrong move
The capabilities page splits specifications into Standard, Advanced and R&D, and the same item is not the same number across the three — min line/space runs 4/4mil, 3/3mil, 2/2mil; mechanical hole minimum runs 0.2mm, 0.15mm, 0.15mm; backdrill diameter runs 0.50mm, 0.35mm, 0.25mm.
That split says one thing clearly: tightening is not free, it pushes the product into a narrower process window. Tightening every item raises the process class on all of them, and most of those items have nothing to do with your function. The workable approach is to tighten only what the function depends on — trace width and dielectric thickness for impedance, hole location and outline for assembly, copper weight for thermal — and accept defaults elsewhere.
On the material side, cross-check Tg / Td / Dk / Df item by item in the materials library, because impedance is set by dielectric thickness together with Dk, not by trace width alone.
6. Seven tolerance fields a drawing can sign off on
- Thickness: nominal value plus whether the default ±10% / ±0.005" (greater of) is accepted.
- Impedance: target value in ohms, tolerance class (±5% or ±10%), controlled layers.
- IPC-A-600 class: Class 2 or Class 3, stated, not left blank.
- Items to tighten: item name, target tolerance, reason.
- Hole chart: plated or non-plated marked per hole, since the tolerances differ.
- Outline and scoring: outline tolerance plus whether V-cut location and depth follow defaults; note edge clearance (outer copper ≥0.005", inner copper ≥0.010", scored arrays excepted).
- Electrical test method and record requirements: whether a coupon microsection report or the full job quality record is needed.
7. Self-check before submitting
- For every tolerance item, can you say whether you declared it or inherited it?
- Does an impedance-controlled board carry a target value in ohms?
- Is the class stated?
- Do the tightened items carry reasons?
- Are the measurement records you need written into the documents?
Answer those five and tolerance questions rarely come back during review. To start a project, submit your data for a quote, or review the board type overview first.
FAQ
Q: If the drawing states no tolerance, will it be rejected or built to defaults?
A: Built and accepted against the tolerance table defaults. Nothing is rejected for a missing tolerance, but if a default turns out not to suit your assembly, the responsibility boundary is hard to draw afterwards.
Q: Are the table values guaranteed?
A: They are standard tolerances. Tighter values can be requested per item and need to be confirmed in the order documents.
Q: Imperial or metric when they disagree?
A: Imperial is authoritative; metric is a convenience conversion.
Q: Does a trace width measured on a coupon represent my board?
A: The coupon runs the same process in the same batch, so measuring it measures the board. That is also why destructive conclusions are taken on coupons only.
Q: Can I just specify "high precision"?
A: Not as an acceptance basis. Put it on an item — hole location, trace width, thickness — with a target value.
Sources
- SKPCB manufacturing tolerance table: all tolerance items, design tips, imperial authority and the IPC-6012 reference.
- SKPCB capabilities: basic parameters, impedance tolerance class, IPC-A-600 Class 2/3, Standard/Advanced/R&D specification table.
- SKPCB quality assurance: ten in-process checkpoints, coupon logic, post-lamination thickness record, five-point plating sampling, post-routing vision measurement, per-job quality record.
Image
- Cover
/images/products/2a2.jpg - alt: Multilayer PCB stack-up cross-section diagram, left column labelling the solder mask, copper, prepreg and core layer order, right column labelling the nominal thickness of each layer.
- Self-check: the alt describes only what is drawn (layer order and nominal thickness labels); it does not present the drawn values as an SKPCB tolerance commitment, does not claim a layer count or copper weight as a capability limit, and does not claim the image shows any measurement result.
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