Etching defines what your PCB layout can and cannot achieve. Understanding the relationship between copper thickness, trace width, and spacing is not optional — it directly affects what your board costs and who can make it.
How Etching Works — Downward and Sideways#
The etchant does not only remove copper downward. It simultaneously attacks laterally — inward beneath the etch resist at the edges of every copper structure. This lateral attack is called undercut. The etch medium itself can be alkaline or acidic depending on the process — both attack copper selectively through the use of photo and metal resists as protective layers.
The result is not a simple trapezoid. The actual copper profile after etching has an hourglass or bow-tie shape — the narrowest point is in the middle of the copper height, not at the top or bottom.
AFTER ETCHING — actual cross section (hourglass/bow-tie)
|<—— wider ————>| <- TOP — protected by resist longest
╔══════════════════╗ <- Sn etch resist (stripped after etch)
\ /
\ /
) narrowest ( <- MIDDLE — maximum combined lateral attack
/ \
/ \
/ \
════════════════════ <- BOTTOM — wider than middle
|<——— widest ————>|
LaminateWhy this shape forms:
- The top is wider — protected by the etch resist for the entire etch duration, the resist limits lateral attack here
- The middle is narrowest — exposed to lateral attack from both the downward-working etchant AND the gap etchant simultaneously — maximum combined attack at this level
- The bottom is the widerst — the etchant reached this level last, minimal lateral exposure time to clear out the gap
Two consequences follow from this:
- Tracks lose width — because the etchant undercuts both edges
- Gaps widen — because the etchant undercuts both copper sides, increasing the gap
Both effects increase with copper thickness. Thicker copper means longer etching time, more lateral attack, and larger minimum structure sizes.
Etch Allowance — The CAM Correction#
To compensate for the lateral undercut, the CAM data preparation adds an etch allowance to every copper structure before exposure. The structure is drawn wider in the layout so that after the undercut the lower portion of the structure reaches approximately the required final width.
The etch allowance equals the base copper thickness — rounded up slightly for process tolerance. It is a practical engineering approximation — the real hourglass profile means the actual cross-sectional area is slightly less than the theoretical rectangle, but the lower portion width is what matters for electrical conductivity and the formula gives the correct practical value.
What happens to the gap:
When the etch allowance widens the track in the layout data, the gap between adjacent structures shrinks by exactly the same amount.
Example — 18 µm base copper, 20 µm etch allowance:
Layout data after compensation:
Track: 100 µm + 20 µm = 120 µm
Gap: 100 µm - 20 µm = 80 µm
After etching:
Track lower portion reaches: ~100 µm (target achieved)
Track middle (narrowest): slightly less than 100 µm
Gap cleared: 80 µm (etchant must clear this fully)During CAM processing, copper features are enlarged to compensate for lateral etching. This compensation reduces the temporary gap between adjacent structures. The amount of compensation depends on copper thickness and process capability. Therefore the minimum manufacturable line/space is ultimately determined by the fabricator’s ability to reliably clear the remaining copper channel during etching.
Minimum Structure Width — The Formula#
The minimum structure width applies equally to track width and spacing. Both must meet the same minimum value in the original layout.
Outer Layers#
Outer layers of a plated through board carry electroplated copper on top of the base foil, plus a chemical tin (Sn) etch resist:
Minimum structure width (outer layer) =
Base copper
+ Plated copper
+ Sn etch resist (~8 µm)
+ Etch allowance (= base copper, rounded up)Example — 0.5 oz base copper (18 µm):
Base copper: 18 µm
+ Plated copper: 20 µm
+ Sn etch resist: 8 µm
+ Etch allowance: 20 µm (18 µm rounded up)
————————————————————————
= 66 µm = 0.066 mmBoth track width and spacing in the layout must be at least 0.066 mm.
Inner Layers#
Inner layers are etched on base copper foil only — no electroplated copper. The etch resist is a dry film photoresist (~40 µm) rather than chemical tin.
Minimum structure width (inner layer) =
Base copper
+ Photoresist (~40 µm)
+ Etch allowance (= base copper, rounded up)Example — 0.5 oz base copper (18 µm):
Base copper: 18 µm
+ Photoresist: 40 µm
+ Etch allowance: 20 µm
————————————————————————
= 78 µm = 0.078 mmReference Table — Moderate and Safe Values#
The calculated value represents a theoretical minimum. In practice, CAM engineering uses a rounded manufacturing limit. The rounded value provides process margin and establishes a clear decision boundary during data review. This avoids unnecessary data modifications, reduces customer queries, and minimizes the risk of introducing new errors while still maintaining manufacturability.
The table below shows recommended design values — conservative enough to be producible by a wide range of fabricators without process risk.
Outer Layers#
| Base Copper | Plating Copper | Sn Etch Resist | Etch Allowance | Calculated Minimum | Rounded CAM Limit |
|---|---|---|---|---|---|
| 9 µm | 20 µm | 8 µm | 10 µm | 47 µm | 50 µm |
| 12 µm | 20 µm | 8 µm | 15 µm | 55 µm | 60 µm |
| 18 µm | 20 µm | 8 µm | 20 µm | 66 µm | 70 µm |
| 35 µm | 20 µm | 8 µm | 40 µm | 103 µm | 110 µm |
| 70 µm | 20 µm | 8 µm | 75 µm | 173 µm | 180 µm |
| 105 µm | 20 µm | 8 µm | 115 µm | 248 µm | 250 µm |
| 140 µm | 20 µm 0 | 8 µm | 145 µm | 313 µm | 320 µm |
Inner Layers#
Inner Layer#
| Base Copper | Dry Film Resist | Etch Allowance | Calculated Minimum | Rounded CAM Limit |
|---|---|---|---|---|
| 9 µm | 40 µm | 10 µm | 59 µm | 60 µm |
| 12 µm | 40 µm | 15 µm | 67 µm | 70 µm |
| 18 µm | 40 µm | 20 µm | 78 µm | 80 µm |
| 35 µm | 40 µm | 40 µm | 115 µm | 120 µm |
| 70 µm | 40 µm | 75 µm | 185 µm | 190 µm |
| 105 µm | 40 µm | 110 µm | 255 µm | 260 µm |
| 140 µm | 40 µm | 140 µm | 320 µm | 320 µm |
Designing within these values keeps your sourcing options open — more fabricators can produce your board reliably, lead times are shorter, and pricing is more competitive.
Why Pushing Limits Costs More#
Every process running at its absolute minimum compresses manufacturing tolerance to nearly zero. Natural variation in copper thickness, plating uniformity, etch chemistry, and resist adhesion — all real, process-wide variables — pushes individual features toward failure.
The result: higher scrap rates, more rework, tighter process monitoring, and a cost premium that stays with the design for its entire production life.
Fabricators know which designs push limits. It is reflected in the quote.
DFM — Catching Problems Before Production#
Checking the relationship between copper weight and structure dimensions is part of CAM engineering and DFM — Design for Manufacturability. When your files arrive at the fabricator, the CAM engineer checks every track and gap against the minimum structure width for the specified copper weight.
If your design passes this check, it proceeds to production. If not, you receive a DFM query — and your lead time restarts from day one.
Most CAD tools do not enforce copper-weight-aware design rules by default. A global minimum clearance of 0.10 mm passes DRC on a board with 2 oz copper. It fails CAM review.
A DRC pass is not a DFM pass.
Submitting a PCB for quotation? rfq.com ensures your copper specification is clearly defined — so fabricators can assess manufacturability accurately and quote on equal terms.