Why Choose a Thick-Copper PCB?
A thick-copper PCB is a board whose copper foil thickness is ≥ 3 oz (1 oz ≈ 35 μm). It is common in industrial power supplies, new-energy vehicles, medical equipment, and other high-power, high-thermal-load applications. An EV charging pile, for example, must withstand large current surges. A conventional thin-copper board overheats and can burn. Thick copper acts as a wide path for current: it carries current more readily, spreads heat, and also raises the board's mechanical strength against bending and vibration.
Thicker copper is not automatically better. A poor design can produce uneven heating, weak solder joints, and a sharp rise in cost. The real task is to meet electrical and thermal performance while keeping the board manufacturable (DFM).
Core Design Points for Thick-Copper PCBs
1. Copper Foil Thickness: Do Not Pursue Thickness for Its Own Sake
The governing rule is that current determines copper weight. A simplified estimate is:
Allowable current (A) ≈ copper thickness (oz) × trace width (mm) × 0.8 (ambient ≤ 40 °C)
Example: 3 oz copper with a 3 mm-wide trace can carry about 7.2 A, which is enough for many industrial power-supply cases.
A common mistake is to go beyond 10 oz. Ultra-thick copper makes the board warp and makes drilling difficult. Unless there is a special need, such as aerospace hardware, stay with the mainstream 3–6 oz range.
2. Trace Design: Avoid Neck-Down Hot Spots and Keep Current Flowing
Trace width: thick-copper traces cannot be too narrow. For 3 oz copper, the recommended minimum width is ≥ 0.3 mm (0.1 mm is acceptable on ordinary thin copper). Width must increase in proportion to current. For example, 6 oz copper carrying 10 A should use a width of ≥ 5 mm.
Width transitions: do not change width abruptly, such as dropping from 5 mm to 1 mm. Use a tapered transition whose length is ≥ 3 times the width difference. An abrupt step creates a current bottleneck and a local hot spot that can burn.
Thermal relief: under high-power devices such as MOSFETs, use a copper pour plus thermal vias (via diameter 0.8–1.2 mm, pitch 2–3 mm) so heat can conduct quickly into the ground or power plane.
3. Via Design: A Frequent Failure Point on Thick-Copper Boards
Via diameter: barrel copper must be compatible with the foil thickness. A typical 0.4 mm hole cannot meet the plating requirement of 3 oz foil. Use a minimum finished hole of ≥ 0.8 mm, with barrel copper ≥ 20 μm.
Via count: do not put a large current through a single via. For 3 oz copper carrying 5 A, use two or three vias in parallel. Each via can carry about 2–3 A. This reduces the risk of a via heating until it opens.
Solder-mask opening: leave enough solder-mask clearance around the via (0.2–0.3 mm larger than the hole) so solder does not plug the via during assembly and block heat and conduction.
DFM for Thick-Copper PCBs: Design What the Factory Can Build
DFM (Design for Manufacturability) means the design must fit the process. On thick-copper boards, DFM is mainly about the process problems that extra copper thickness creates.
1. Copper Etching: Avoid Uneven Etch
Minimum line width / spacing: for 3 oz copper, minimum line width ≥ 0.3 mm and minimum spacing ≥ 0.3 mm (0.1 mm is acceptable on thin copper). For 6 oz copper, line width / spacing ≥ 0.4 mm is recommended. Tighter geometry is prone to incorrect finished width and shorts during etch.
Pour openings: on large copper areas, use a hatched pour (grid pitch 2–3 mm, line width 0.2–0.3 mm) so foil shrinkage during etch does not bow the board. If a solid pour is required, leave thermal slots (0.5 mm wide, 10–15 mm pitch).
2. Lamination: Prevent Delamination and Blistering
Stack-up order: place thick foil on the outer layers or close to the outer layers. Buried in the middle, the heat cannot escape readily. In a multilayer board, copper weights should be symmetrical (for example, 3 oz on the top layer and 3 oz on the bottom). Asymmetry causes warp after lamination.
Laminate selection: prefer a high-Tg material (Tg ≥ 170 °C), such as FR-4 Tg170 or PI. This reduces softening and delamination during high-temperature soldering. Soldering temperature on thick-copper boards is typically 10–20 °C higher than on thin-copper boards.
3. Soldering: Account for the High Thermal Mass of Thick Copper
Package choice: prefer high-power packages such as TO-220 and D2PAK. A small package soldered onto thick copper may not shed heat, and the solder can remelt.
Pad design: pads on thick copper should be 0.2–0.3 mm larger than a standard pad. For an 0805 resistor, a typical pad is 0.8 × 1.2 mm; on a thick-copper board, 1.0 × 1.5 mm is recommended so the joint is mechanically sound.
Reflow profile: thick copper absorbs more heat. Raise the reflow temperature by 5–10 °C relative to a thin-copper board and extend the soak by 10–15 s to reduce cold joints.
4. Cost Control: The Hidden Value of DFM
Avoid over-design: use 1–2 oz copper in areas that do not carry large current, and reserve thick copper for the critical paths. That lowers material cost.
Standard thickness: stay with common finished board thicknesses such as 1.6 mm and 2.0 mm. Special thicknesses of 3.0 mm and above increase process difficulty and cost.
Confirm capability early: before the layout is frozen, confirm the fabricator's process limits—maximum copper weight, minimum hole size, and etch accuracy—so the design is not finished in a form that cannot be built.
Summary: Three Core Rules for Thick-Copper PCB Design
- Match copper weight to current: do not add thickness blindly. Choose a mainstream 3–6 oz weight from the current requirement.
- Control the details that create risk: use tapered traces, paralleled vias, and line width / spacing that meet the copper weight.
- Put DFM first: design with etching, lamination, and soldering in mind so respins stay down.
Thick-copper PCB design looks complicated, but most failures disappear once current carrying and process fit are treated as the two governing constraints.
