From the perspectives of manufacturability and reliability, this article outlines the core precautions and optimization suggestions for designing Heavy Copper PCBs (typically defined as having outer layer copper thickness ≥2oz and inner layer copper thickness ≥3oz).
☆☆ The core challenges of heavy copper boards lie in heat concentration/stress imbalance and graphic pattern precision. All design efforts must address these points. ☆☆
I. Core Design Considerations for Heavy Copper PCBs
1. Circuit Line Design
Minimum Trace Width/Spacing: ❗ Never design based on the standard PCB process capabilities. The side-etching effect during the etching of thick copper is severe, leading to an "hourglass" shape in traces and reduced spacing. ? Rule of Thumb: The finished trace width/spacing should be at least 1.5 to 2 times larger than standard process capabilities. Follow the principle of "bigger is better." ? Consequence: Insufficient spacing极易导致 incomplete etching, bridging, and short circuits.
Copper Surface Uniformity: ❗ Avoid isolated large copper pours or very fine traces surrounded by large copper areas. This causes uneven current density during plating, leading to over-plating (excessive copper thickness) in thin areas and under-plating (risk of hole voids) in thick areas. ? Optimization Method: Use a "gridded" or "cross-hatched" pattern or "thermal relief pads" to connect large copper areas, balancing current distribution.
2. Hole and Annular Ring Design
Minimum Annular Ring: Due to layer-to-layer registration and pattern transfer deviations in thick copper boards, the annular ring must be increased. ? Recommendation: Outer layer annular ring ≥ 8 mils, inner layer annular ring ≥ 10 mils. For high-current plated through-holes, the ring should be further increased to provide sufficient pull-off strength. ? Consequence: Insufficient annular ring width risks the copper barrel being pulled off under thermal or mechanical stress.
3. Soldermask and Surface Finish
Soldermask Bridge (Dam): It is nearly impossible to retain fine soldermask bridges unless the spacing between pads is sufficiently large. On thick copper boards, the soldermask ink tends to flow on the trace sidewalls. To prevent the ink from sagging between copper lines—which causes the bridge to be extremely thin or broken—the soldermask typically needs to be printed 2 to 3 times. ? Design Suggestion: Ensure trace spacing is large enough (e.g., > 12 mils) and confirm the manufacturer's soldermask process capabilities. ? Consequence: Broken soldermask bridges can lead to solder bridging during assembly.
Soldermask Thickness and Coverage: The sidewalls and corners of thick copper traces are weak points for soldermask coverage, which can lead to inadequate protection, chemical entrapment, or high-voltage breakdown. ? Recommendation: Require the manufacturer to use a "multiple printing" process to ensure thorough coverage on trace sidewalls.
Surface Finish:
4. Lamination and Materials
Dielectric Layer Thickness: ❗ Using multiple sheets of prepreg (PP) is one of the most important principles. Thin dielectric layers cannot fill the gaps between thick copper traces, leading to insufficient resin fill after lamination, vacuum bubbles/voids, delamination, and board warpage. ? Rule of Thumb: The post-fill dielectric thickness should be greater than the adjacent inner layer copper thickness. For example, for a 3oz inner layer, it is recommended to use at least 3 or more sheets of PP. The exact number depends on the copper area ratio and the manufacturer's capability. ? Consequence: Too thin a dielectric layer -> insufficient resin fill -> reduced insulation, inadequate withstand voltage, delamination, and blistering.
Laminate Selection:
5. Thermal Management
Thermal Vias:
Copper Area: Utilize thick copper as a heat spreader by designing large copper pours connected to ground or power nets.
II. Design Optimization Recommendations
1. Early Communication is Key Before finalizing the layout, communicate design intent and key parameters (e.g., target copper weight, current levels, voltage withstand requirements) to your PCB manufacturer. Their engineers can provide stack-up proposals and DFM rules best suited to their production line capabilities.
2. Corner Treatment (Chamfering)
Use 45° chamfers or rounded corners at bends in thick copper traces, avoiding 90° right angles. Right angles are stress concentrators prone to cracking under thermal shock.
Apply corner treatment to all pad and copper pour corners.
3. Adopt "Step-like" or Selective Copper Thickness Design
If only localized areas require high current, consider designing with embedded copper blocks or selective copper plating.
4. Design for Manufacturing (DFM)
Panelization: Heavy copper boards are inherently heavy, and V-CUT scoring is difficult and can damage traces. It is recommended to use "breakaway tabs (mouse bites) + bridges" for panelization, avoiding V-CUT.
Warpage Control: Strive for symmetrical copper distribution across layers during design to prevent warpage after lamination caused by imbalance. Add balance copper (thieving) in large blank areas.
Summary Checklist
Before releasing the design for fabrication, confirm the following:
Have trace width/spacing been significantly increased based on copper weight and confirmed with the manufacturer?
Is the annular ring width (especially inner layers) sufficient (≥ 8-10 mils)?
Is the dielectric thickness sufficient to fill the gaps between thick copper traces?
Is the soldermask bridge width adequate?
Does the laminate meet the required Tg value (recommended ≥170°C)?
Have all trace corners been chamfered or rounded?
Is the copper distribution even, with no isolated copper?
Does the panelization method avoid V-CUT, using breakaway tabs instead?
Have all special requirements (hole copper thickness, step copper, surface finish, etc.) been clearly stated in the fabrication notes?
Confirming the above points will significantly enhance the design success rate, production yield, and long-term reliability of your heavy copper PCB. Heavy copper board design is a systematic engineering task that must consider the manufacturer's process capabilities as a core design input.