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4 oz copper thickness: what the number does and does not specify

A 4 oz copper PCB identifies nominal copper weight of 4 oz/ft? on selected conductive layers. The familiar rounded equivalent is 140 ?m, 0.140 mm or 5.51 mil. It is a conductor reference, not the total board thickness, a minimum finished-copper guarantee or a current rating. State the copper requirement for every layer and identify whether the drawing describes starting foil or finished copper.

At 4 oz, the geometry that remains after manufacture deserves particular attention. A conductor's actual cross-section can differ from a simple artwork-width multiplied by nominal-thickness calculation. That matters near narrow tracks, pad exits and thermal-relief spokes. Design the electrical route and the manufacturing dimensions together before committing to a finished board.

Use the heavy copper PCB guide for family-level choices. This page focuses on finished conductor shape, assembly connections and the information needed to quote a 4 oz design. For a simpler upgrade decision from lighter copper, the 3 oz comparison page contrasts increasing thickness with widening the route.

Artwork width, compensated width and finished width

The designer's artwork represents electrical and mechanical intent. The fabricator may prepare compensated features for its process. Inspection then evaluates the finished product against the agreed requirement. These are related dimensions, but they are not automatically the same number. A useful review identifies the minimum finished width, where it is measured and which tolerance or exception applies.

Etched conductors can have different widths at their two faces. The direction and size of a taper depend on the process; it is unsafe to assume one universal undercut per ounce. An ideal trapezoid is useful for estimating area when both finished face widths are known, but it does not reproduce every real conductor profile. Do not use this model to generate artwork compensation.

If compensation is needed, agree who applies it. Adding a guessed allowance in the design and allowing a second factory adjustment can move pads, gaps and features away from the intended geometry. The same issue affects conductor clearances: a process change that recovers width may also affect the space between adjacent features.

Estimate loss from an ideal finished cross-section

Enter the two finished face widths, thickness, length, assumed copper temperature and steady DC current. The tool treats the conductor as a uniform trapezoid along the entire path. Either face can be wider. It reports the equivalent rectangular width and compares the result with a rectangle using the wider face, making the simplification visible.

For a 100 mm path with 4.6 mm and 5.0 mm face widths and 140 ?m thickness, area is (4.6 + 5.0)/2 × 0.14 = 0.672 mm?. At 20°C, resistance is approximately 2.5656 mΩ. At 25 A DC the calculated loss is about 1.6035 W. Using a full 5.0 mm rectangle instead would underestimate this example's resistance by a factor corresponding to about 4.17% relative to that rectangular result.

This example is not a factory tolerance proposal. The 4.6 mm value is chosen to demonstrate the calculation, not to predict the result of etching a 5.0 mm artwork feature. For a trace that changes width along its length, divide it into appropriate series sections. Exclude none of the pad exits or transitions simply because the main pour is large.

Minimum spacing must answer two different questions

The first question is whether the selected manufacturing process can reliably reproduce the conductor and gap. The second is whether the finished assembly has adequate electrical clearance and creepage for its application. A gap that can be etched is not automatically suitable for a working voltage, environment or safety requirement. Keep those decisions separate in the drawing and review.

The older 吃瓜tv page listed 14 mil, approximately 0.356 mm, as a 4 oz spacing guideline. Treat that historical figure as a discussion reference, not a universal current capability or a guarantee for every layer and construction. Confirm the latest project-specific finished width and gap requirements through engineering; do not substitute a different manufacturer's capability table or combine independent limit values.

This page intentionally does not issue a universal trace/space rule from copper weight alone. Layer location, starting and finished copper, pattern density and the actual process matter. Mark critical fine features on the fabrication drawing and use PCB DFM review to resolve them while changes are still practical.

Pad necks and solderability can become the real constraint

A wide power route often narrows as it approaches a terminal or component. That neck adds resistance in series and may be more sensitive to dimensional loss than the main conductor. A thermal-relief pattern adds another electrical and assembly decision. Its spokes may improve soldering access to heat while constraining the conductive route; a direct plane connection may need a different assembly process.

There is no automatic rule that every heavy-copper pad must have a relief or that every power pad must be solid. Review the load, footprint, joint formation and assembly equipment together. Where relevant, ask the assembler to validate a representative thermal profile. Component temperature limits and the approved soldering process remain applicable even when the copper is thick.

The original product photograph below shows a separate 5 oz board edge. It is useful for seeing construction features, but it is not a 4 oz cross-section measurement. Physical appearance alone cannot establish finished copper, hole-wall thickness or dielectric spacing. Those dimensions require the corresponding specification and evidence.

Put the 4 oz layer in a complete stackup

Specify the laminate grade, finished thickness, layer map and dielectric requirements. Copper weight does not determine glass transition temperature or continuous service temperature. A material choice should address the product environment, assembly exposure, dimensional stability and insulation requirements. The current illustrates why grade-specific data and processing information matter.

For mixed power and control designs, consider whether all layers need 4 oz. Fine signal routing and high-current distribution can have different requirements. A mixed-copper construction still needs balance, resin and interconnect review. The 6 oz stackup page explains how copper thickness consumes the overall stackup budget, using an explicit multilayer example.

Power-distribution products may benefit from reduced conductor resistance, but the assembled temperature depends on the whole heat path. Nearby components, planes, interfaces and the enclosure alter heat flow. provides the current, conductor-size and temperature-rise context; the cross-section calculator on this page does not implement a thermal approval method.

Follow the requirement from design to inspection

The engineering sequence below connects copper specification, finished geometry, construction and evidence. It is an explanation, not a recording of a fixed 吃瓜tv production recipe. The actual sequence and controls depend on the approved board construction. You can select each stage or play the explanation; it starts paused and supports reduced-motion settings.

Agree the applicable rigid-board performance and acceptability requirements, including revision, class and any customer-specific notes. Record critical widths, gaps, copper and holes in the same revision as the manufacturing data. Connectivity testing and dimensional inspection serve different purposes; neither proves the assembled current path will meet its temperature requirement.

What should a 4 oz quotation include?

Ask for a quotation against the finished geometry, not a copper label alone. Fine features, mixed copper, mask openings, material, finish, holes and required evidence can affect processing and yield. Dimensions and panel use remain relevant, but multiplying a standard-board price by a generic “4 oz factor” hides these differences.

The budget brief records your geometry notes and adds only the allowances you enter for this construction. It can produce a technical enquiry with costs left blank. Explicit examples are not market prices. Separate setup, special tooling and validation only where they are not already included in the quoted unit amount. Changing the construction clears costs so you can obtain an updated basis.

For procurement, request comparable quantity breaks and acceptance scope. An offer that omits a required cross-section report is not the same deliverable as one that includes it. Use the PCB quote comparison worksheet to collect these differences before choosing on unit price alone.

4 oz copper PCB questions