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Copper Weight PCB Specification: Foil Ounces and Finished Copper
Copper weight in a PCB specification usually expresses copper mass per unit area, commonly ounces per square foot. It is a convenient way to describe nominal foil, but a complete fabrication requirement must also identify the layer and whether the value refers to starting or finished copper.
This guide helps designers and buyers write an unambiguous copper schedule. The converter shows the theoretical thickness associated with a mass per area and an entered density. It does not replace supplier tolerances, finished-thickness requirements or measurement agreements.
Copper mass per area to nominal thickness
Copper mass per area to nominal thickness
Use this worksheet within its stated scope. Keep its assumptions with the result; downloads remain local and must be attached manually to an enquiry.
| Input | What to establish before using it |
|---|---|
| Review reference | Use a nonconfidential scope or revision. |
| Copper mass 路 oz/ft虏 | Mass per area, not board weight. Standard avoirdupois ounce. |
| Copper density 路 g/cm鲁 | Uniform dense foil; use the specified material density. |
Say which copper you mean
Separate inner-layer foil, outer starting foil, finished outer copper and copper in plated holes. They have different processing histories. A note that says 2 oz copper without a layer schedule may be interpreted differently by different quoting teams.
Provide a stack table with the intended copper requirement on each layer. If a minimum finished thickness is essential for a particular current path, state it explicitly and agree how it will be verified. Do not assume that a nominal foil callout guarantees the same minimum at all trace and hole locations.
Convert mass per area carefully
An avoirdupois ounce is 28.349523125 grams and a square foot is 929.0304 square centimeters. At an entered density of 8.96 g/cm鲁, 1 oz/ft虏 corresponds to a theoretical uniform thickness of about 34.06 碌m. Industry descriptions often round a nominal ounce to about 35 碌m.
The difference between a theoretical conversion and a rounded commercial label is not a reason to reject a board by itself. Acceptance depends on the agreed material and finished construction specification. Keep the conversion tool's result labeled theoretical rather than presenting extra decimals as manufacturing accuracy.
| Review item | What to establish |
|---|---|
| Definition | Mass per area versus finished planar and hole copper. |
| Units | Theoretical density conversion distinguished from rounded nominal foil. |
| Acceptance | Measurement location, minimum and method agreed. |
| Process | Artwork, lamination and soldering reviewed with the copper schedule. |
Distinguish nominal and minimum
Nominal thickness supports design communication, while minimum requirements define a boundary that may affect process selection and verification. A nominal 70 碌m description and a requirement for at least 70 碌m everywhere are not automatically equivalent purchasing instructions.
Discuss the locations and method of measurement. Cross sections, foil certificates and process records answer different questions. If a requirement applies to a plated hole wall, it must be defined for that structure instead of inferred from the copper weight on a planar layer.
Account for etching and feature density
Thicker copper changes the relationship between conductor height, finished width and spacing. Fine patterns that are comfortable in a thinner copper construction may require a different process window or geometry in heavier copper. Local density and isolated features also deserve review.
Use standard versus heavy copper thickness for the electrical comparison and heavy-copper tradeoffs for process decisions. A copper schedule should be checked against the actual artwork rather than selected independently from routing density.
Review stack balance and pressing
Copper distribution affects how the construction is laminated and supported. A heavy copper layer with large open areas can present resin-filling and thickness-control questions different from a nearly solid plane. Symmetry should consider the actual construction, not just equal ounce labels on opposite sides.
Provide copper-density information and local thickness constraints where they matter. Keep the dielectric target and any impedance requirements together with copper changes. Increasing copper can alter more than resistance, particularly when the original design relies on a controlled cross section.
Consider assembly consequences
More copper can conduct heat away from a solder joint and change the assembly process required to achieve a suitable joint. Review thermal reliefs, pads, large terminals and rework access. The electrical reason for a solid connection and the process need for controlled heating should be resolved together.
The board finish and component terminal geometry also matter. A fabrication-only copper choice may need confirmation with the assembly provider. Do not use a generic instruction to remove every thermal relief without reviewing the actual current and soldering requirements.
Compare quotations with one schedule
Send the same layer-by-layer copper requirements, board size, quantity, finish, hole and inspection scope to each quoting route. Ask whether proposed values are starting foil, finished nominal or minimum finished requirements. Identify any alternatives as alternatives rather than silently mixing them into a price comparison.
For current-path studies, use the trace resistance calculator with the geometry you intend to specify. That arithmetic can help explain why a requirement exists, while the manufacturer determines whether the complete pattern and stack can meet it consistently.
Put the final requirement on the drawing
Release a copper schedule that can be understood without email history. Include layer identifiers, units, relevant minima, tolerances and the agreed evidence. Preserve the revision when a substitution or copper change is approved.
Continue to heavy copper PCB manufacturing for a construction review where thick conductors are necessary. The nominal conversion is only the first step; a useful order describes the actual board and how its critical copper requirements will be accepted.
Turn a conductor calculation into a fabrication note
Sketch the complete electrical path and divide it into sections where geometry or material changes. Record the finished dimensions used in each calculation. Keep planar copper, plated barrels and contact interfaces distinct so that a convenient rectangular model does not hide a dominant connection.
State current waveform and temperature assumptions. If a calculation uses a fixed conductor temperature, do not present the result as a solution of self-heating. If a current guideline is used, retain its construction and thermal boundary. The arithmetic and the current-carrying assessment should be traceable as separate steps.
Review critical minima and measurement locations with the manufacturer. A nominal ounce label, a drawing minimum and an actual section measurement are different records. Agree how the ordered construction will be accepted, including any features affected by etching, plating, lamination or assembly.
Build a copper schedule around the actual current path
A copper schedule should explain what the electrical calculation depends on. Separate the inner-layer foil requirement, outer-layer finished conductor requirement and plated-hole requirement into different rows. Add the measurement basis for any critical minimum. This avoids the common procurement error of requesting a heavier nominal ounce value while leaving the narrowest finished conductor undefined. More copper can be useful, but the design decision needs a geometry and an acceptance basis.
Consider a hypothetical 40 mm copper path, 2 mm wide and 35 碌m thick at 20掳C. Using a resistivity of 1.724 脳 10鈭�8 惟路m, its rectangular-path resistance is about 9.85 m惟. At 5 A, the calculated drop is 49.3 mV and the dissipation is 0.246 W. Doubling the thickness to 70 碌m halves those two values if width, length, temperature and material stay unchanged. This is a DC comparison, not a prediction of the board temperature or a current rating.
Now add a 5 mm long, 0.5 mm wide neck with the original 35 碌m thickness. That short section contributes another 4.93 m惟. It is only one eighth as long as the wide section but adds half as much resistance. The combined path is about 14.78 m惟 before vias, terminals and the return path. This example shows why increasing the copper across an entire board may be a less targeted change than reviewing a local neck. It does not prove the neck can be widened: pad geometry, routing space and manufacturing constraints still need checking.
| Region | Quantity to specify | Evidence to agree |
|---|---|---|
| Inner plane | Specified foil construction and relevant thickness basis | Material record and agreed finished construction |
| Outer high-current route | Finished width and thickness requirements at critical locations | Measurement method and acceptance interval |
| Plated transition | Hole geometry and barrel copper requirement | Applicable cross-section or process evidence |
| Fine-pitch region | Finished spacing and conductor geometry | Artwork/process review for the selected copper |
The copper decision also changes the fabrication question. Thick copper next to fine-pitch routing may require additional geometry allowance, and a thicker outer conductor changes the physical transmission-line cross-section. Keep impedance, etching and insulation spacing in the same review. Do not accept a proposal to change copper merely because it improves one current calculation while assuming every other constraint stays satisfied.
Record temperature separately from the copper conversion. The ounce-to-thickness calculation describes an areal mass and density. The resistance calculation describes a finished conductor at an assumed temperature. Neither calculation establishes the minimum delivered metal. A measured cross-section is also local evidence, so agree how measurements represent the ordered construction rather than treating one favorable location as a universal value.
Download the copper-schedule worksheet and identify each critical segment before requesting a heavy-copper construction review. If ordinary copper is sufficient, retain it instead of selecting a thicker construction for its label. The useful enquiry states the load envelope, allowable drop, relevant thermal conditions and fixed routing constraints. This lets the fabrication discussion distinguish a required finished feature from a preliminary design preference.
Compare two copper proposals without changing the question
Suppose proposal A identifies 2 oz starting foil while proposal B specifies a finished copper requirement. Do not rank them as equivalent from the number two or a rounded thickness conversion. Ask each proposal to identify the same finished layer geometry and acceptance basis. If the intended load calculation depends on a minimum cross-section at a neck, record that location and the width definition. A material certificate for the starting foil does not describe every finished outer trace after processing.
Also separate a necessary electrical requirement from a convenient purchasing shorthand. If the complete path already meets the product's verified drop and thermal requirements, increasing all layers may add process consequences without addressing a real constraint. If a local transition dominates, define that transition first. The final schedule should explain why the selected copper is needed and which evidence preserves that reason on a repeat order.
Ask how the critical cross-section will be verified on the delivered construction. A coupon can help only if its geometry and processing represent the feature being accepted. Record the measurement location and acceptance method before comparing inspection results between suppliers or between prototype and production lots.
Standards, source context and limits
For fabrication scope and the agreed construction requirements, see PCB manufacturing.
Download the editable engineering review worksheet (CSV). This blank template stays on your device; attach it manually if useful.