Copper weight is a dimension, not a dropdown
On a quote form, copper thickness is a dropdown that runs from 0.5oz to over 12oz and takes a second to fill in. In the factory it is a dimension, and it shows up in three places at once: the cross-sectional area of the copper (current carrying and heat spreading), the amount of copper etching has to remove (how fine a trace you can hold), and the thickness budget of the stackup (board thickness, and therefore hole aspect ratio). Changing that dropdown changes all three.
First, a terminology check. Base foil and finished copper are not the same number. The cover cross-section labels the outer layer "Cu 18µm + plating" and dimensions it at 35µm. Outer-layer copper equals the foil laminated before pressing plus the copper deposited during through-hole plating. Inner layers get no such plating step, so base foil is finished thickness. When you say "1oz outer," the figure to design trace width against is the plated result, not the foil spec. That is also why inner and outer rules are listed separately.
The overall window published on Sunking's /capabilities/ page is 0.5oz to over 12oz, tiered as: Standard 3oz, Advanced 12oz, R&D above 12oz, for both inner and outer layers. The heavy copper product page gives its working range as 3oz to over 12oz, 1 to 16 layers, 0.8 to 6.0 mm board thickness, 6 mil minimum trace width. Read those together: that 6 mil floor is what "copper goes up, trace width backs off" looks like once it reaches a product page.
Where the Standard/Advanced boundary sits
The detailed specification table on /capabilities/ lists windows at Standard, Advanced and R&D levels. The copper-relevant rows:
| Item | Standard | Advanced | R&D |
|---|---|---|---|
| Max copper (outer) | 3oz | 12oz | >12oz |
| Max copper (inner) | 3oz | 12oz | >12oz |
| Min line/space (inner) | 4/4 mil | 3/3 mil | 2/2 mil |
| Min line/space (outer) | 4/4 mil | 3/3 mil | 2/2 mil |
| Mechanical hole min | 0.2 mm | 0.15 mm | 0.15 mm |
| Laser hole min | 0.1 mm | 0.075 mm | 0.075 mm |
| Max aspect ratio (PTH) | 10:1 | 10:1 | 10:1 |
| Layer count | 40 | 64 | 64 |
| Max board thickness | 4 mm | 6 mm | 6 mm |
| Backdrill diameter | 0.50 mm | 0.35 mm | 0.25 mm |
The common misreading: this is a list of per-item ceilings, not a menu you can order by column. "Advanced does 12oz" and "Advanced does 3/3 mil" sit in the same column, but that does not mean 3/3 mil traces on 12oz copper. The 3/3 mil figure holds on thin copper; the 12oz figure holds with relaxed geometry. Both are extremes of the same column and cannot be taken together.
The rule of thumb is simple: where two ceilings in a column conflict, you get one of them, and the other has to back off to a level that cooperates. How far it backs off is something engineering confirms against your material, layer count and pattern density; it is not a formula you can apply at design time. Any conditional capability figure should be treated as "Advanced level, subject to engineering review."
Why thicker copper forces wider traces
Etching removes unwanted copper from the top down, and the sidewalls are being attacked the whole time. Thicker copper means more vertical etch time, which means more lateral loss on the sidewall, so the finished conductor is a trapezoid rather than a rectangle. Holding the width at the top of the trace means drawing it wider in the artwork. That is etch compensation.
Two consequences land on the designer:
Tolerance is proportional, not absolute. The /tolerance/ page gives trace width at ±20% and copper-to-copper spacing at ±20%. Proportional tolerance means the finer the trace, the smaller the absolute band you have to work in. A 4 mil trace at ±20% is ±0.8 mil. Put that trace on a heavy copper layer and normal heavy-copper etch variation can consume the whole 0.8 mil. That is not a yield problem, it is a geometry problem: the variation does not fit inside the band. Traces on heavy copper layers have to be wider, not out of conservatism but so the tolerance band can hold the process.
Spacing backs off with width. As the sidewall is eaten back, the gap between adjacent conductors is squeezed from both sides. Relaxing width without relaxing spacing just moves the risk from open circuit to short. On heavy copper layers, treat line and space as a pair.
We are not publishing compensation coefficients or a copper-weight-to-trace-width conversion table. Those move with material, pattern density and etch line condition, and engineering confirms them against the actual stackup. No general table circulating online is a valid basis for an order.
How copper reaches the holes: thickness budget and 10:1 aspect ratio
Copper weight reaches your holes through board thickness. Every copper layer counts toward the total, and heavy copper layers additionally need high-resin-content prepreg to fill the gaps between thick copper features, so that dielectric is thicker too. Keep the layer count and just thicken a few layers, and board thickness rises.
When thickness rises, aspect ratio follows. The maximum aspect ratio on /capabilities/ is 10:1, identical at Standard, Advanced and R&D. It is the physical limit on plating copper evenly down a barrel, and it does not relax with tier. Aspect ratio is board thickness divided by smallest drilled diameter, so:
- Thicker board, larger minimum hole.
- To keep small holes, hold the thickness down: thicken fewer layers, or confine heavy copper to the layers that need it.
- Mechanical holes floor at 0.2 mm (Standard) / 0.15 mm (Advanced), laser holes at 0.1 mm (Standard) / 0.075 mm (Advanced) — but those floors are only available inside the thickness range 10:1 permits.
The classic failure here: drawing holes at the Advanced 0.15 mm mechanical floor while pushing board thickness past 4 mm for copper reasons. At 4 mm, 0.15 mm is far beyond 10:1. That combination never existed. The heavy copper page runs to 6.0 mm and /capabilities/ lists max thickness at 4 mm Standard / 6 mm Advanced — the thickness window is open, but spending all of it means giving up small holes.
Backdrilling sits on the same chain: 0.50 mm (Standard) / 0.35 mm (Advanced) / 0.25 mm (R&D). Greater thickness makes depth control harder, so state heavy copper and backdrill requirements together at RFQ.
Solder mask and surface finish move too
Heavier copper means taller copper features standing off the laminate, and the mask has to climb those steps. /tolerance/ gives solder mask registration at ±0.003" (≈0.076 mm), depth ±0.005", requires mask openings 0.003" larger than the pad (mask swell), and flags that pads within 0.010" of each other may need special processing.
On heavy copper these read as one chain:
- The 0.003" swell is headroom for registration error, not decoration. Taller copper steps mean mask coverage on the sidewall is naturally thinner than on flat areas, so drawing an opening exactly equal to the pad usually ends with mask creeping onto the pad or copper exposed at the edge.
- The 0.010" pad-pitch flag triggers earlier in dense heavy copper areas. When fine pitch and heavy copper appear in the same region, mark that region at RFQ rather than waiting for DFM feedback.
- Finish options narrow. The comparison tables on
/surface-finish/and/capabilities/mark HASL and lead-free HASL as unsuitable for fine-pitch SMT and BGA/μBGA because their pads are domed; ENIG, ENEPIG, hard gold and immersion silver are flat. If a heavy copper board also carries fine-pitch parts, a domed finish worsens coplanarity that is already tight, so favour the flat finishes.
Impedance meets the same junction. /capabilities/ lists impedance control at ±10% (standard) / ±5% (advanced). Impedance is set jointly by trace width, dielectric thickness and copper thickness — and on a heavy copper layer width must be relaxed while the dielectric shifts to meet fill requirements. Making a heavy copper layer an impedance-controlled layer is usually the most expensive choice on the board. The workable arrangement is controlled-impedance signals on thin inner layers, with heavy copper carrying current and heat only.
Combinations to give up
These are not "difficult." Against the published windows they are mutually exclusive. Ruling them out at design time saves a DFM round trip.
- 12oz copper with 3/3 mil line/space. Two ceilings from one column that cannot coexist; heavy copper must relax both width and spacing.
- Board thickness above 4 mm with 0.15 mm mechanical holes. Aspect ratio exceeds 10:1 regardless of tier.
- Full 6.0 mm thickness with minimum hole size. The thickness window and the hole window cannot both be spent.
- Heavy copper layers with ±5% controlled impedance. Width relaxation plus fill-driven dielectric variation make the advanced impedance tier unrealistic on that layer; move controlled impedance to thin inner layers.
- Dense heavy copper pads with HASL. Domed pads on top of tall copper steps; use ENIG or another flat finish for fine pitch and BGA.
- Heavy copper with sub-0.010" pad spacing, undeclared. Not forbidden, but it must be stated at RFQ or it will come back through DFM.
- Etch compensation copied from a generic conversion table. Compensation moves with material, density and etch line condition and must be confirmed by engineering against the real stackup.
What to state before you ask for a quote
Put these in the RFQ and engineering can answer in one pass instead of three.
- Copper weight per layer, inner and outer separately. Say whether the figure is base foil or finished copper; if outer is specified as finished, call that out.
- Which nets need heavy copper, and why — current or heat, and over which areas. This determines whether heavy copper can be confined to some layers.
- Minimum line/space by region. Separate heavy copper areas from signal areas instead of applying one global minimum.
- Minimum hole size and target board thickness together, so engineering can check aspect ratio instead of guessing which is the hard requirement.
- Which layers carry impedance requirements, with target value and tolerance tier (±10% or ±5%).
- Surface finish and a list of fine-pitch parts.
- Acceptance class: IPC-A-600 / IPC-6012 Class 2 or Class 3. The
/quality/page states acceptance to IPC-A-600 Class 2/3; annular ring, plating and visual criteria all differ by class.
Direct entry points: process windows on Capabilities and Heavy Copper PCB, tolerance figures on Manufacturing Tolerance, layer structures on Multi-Layer PCB, finish comparison on Surface Finish, base materials in the Materials Library, and when the parameters are settled, Get Quote.
FAQ
How thick is 1oz copper?
1oz is one ounce of copper per square foot by area density, roughly 35µm. Note that finished outer copper is base foil plus plating — the cover cross-section labels "Cu 18µm + plating" and dimensions it at 35µm for exactly this reason. Inner layers have no plating step, so base foil is the finished thickness.
What is Sunking's copper thickness range?
/capabilities/ publishes 0.5oz to over 12oz overall, tiered as Standard 3oz, Advanced 12oz and R&D above 12oz for inner and outer layers. The heavy copper line works in 3oz to over 12oz, 1 to 16 layers, 0.8 to 6.0 mm thickness.
Why can't I have 12oz and 3/3 mil if they are in the same column?
Because the table gives per-item ceilings, not orderable column packages. Where ceilings conflict you take one: 12oz means relaxed line and space; 3/3 mil means lighter copper. Engineering confirms the exact trade against your stackup.
Does heavier copper limit my minimum hole size?
Yes, through board thickness. Copper weight and the fill dielectric heavy copper requires both raise thickness, while maximum aspect ratio is fixed at 10:1 across all three tiers. Thicker board, larger minimum hole. To keep small holes, thicken fewer layers or hold total thickness down.
Can heavy copper boards be impedance controlled?
You can have impedance requirements, but do not put controlled-impedance traces on heavy copper layers. Width must be relaxed and dielectric thickness shifts with fill needs, so the ±5% advanced tier on /capabilities/ is hard to hold there. Route controlled impedance on thin inner layers and leave heavy copper for current and heat.
What should I watch on solder mask for heavy copper?
Taller copper steps mean naturally thinner mask coverage on sidewalls. Per /tolerance/, registration is ±0.003", openings need to be 0.003" larger than the pad, and pads within 0.010" may need special processing — a flag that triggers earlier in dense heavy copper areas, so mark those regions at RFQ.
How do I know if my combination needs engineering review?
Any time you approach two window edges at once — near 12oz while asking for fine lines, or near 6 mm thickness while asking for small holes. Conditional capability figures should be treated as "Advanced level, subject to engineering review," not locked in at the ceiling during design.