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I. Key Considerations for Thick Copper Board Design
1. Circuit Design
Minimum Line Width/Spacing:
Designing at the absolute limit of standard board process capabilities is strictly prohibited. Thick copper exhibits severe side etching effects during etching, causing “waist-thinning” in traces while simultaneously reducing spacing.
Rule of Thumb
Final design line widths/spacings must be at least 1.5 to 2 times larger than standard process limits, adhering to the principle of maximizing dimensions whenever possible.
Consequences: Insufficient spacing readily causes incomplete etching and bridging/short circuits.
Copper Surface Uniformity:
Avoid isolated large copper pads or excessively thin traces surrounded by large pads. This causes uneven current density during plating, resulting in overplating at thin areas (exceeding copper thickness limits) and underplating at thick areas (risking via breakage).
Optimization Method:
Connect large copper pads using “grid patterns” or “cross-shaped heat dissipation pads” to balance current distribution.
2. Via and Ring Width Design
Minimum Ring Width:
Due to lamination misalignment and pattern transfer deviations, ring widths for thick copper boards must be increased.
Recommendations:
Outer layer ring width ≥ 8 mil, inner layer ring width ≥ 10 mil. For high-current through-holes, ring widths should be further increased to provide adequate pull-ring strength.
Consequences: Insufficient ring width risks copper foil peeling from the hole wall under thermal or mechanical stress.
3. Solder Mask and Surface Treatment
Solder Mask Bridges:
Fine solder mask bridges are nearly impossible to preserve unless pad spacing is sufficiently large. On thick copper boards, solder mask ink may flow down the sides of traces. To prevent ink from sinking between copper traces and causing extremely thin or broken bridges, solder mask ink typically requires 2-3 applications.
Design Recommendations:
Ensure sufficient trace spacing (e.g., >12 mil) and verify the manufacturer's solder mask process capability.
Consequences: Solder mask bridge failure may cause solder bridging during assembly.
Solder Mask Thickness and Coverage:
Thick copper trace sidewalls and corners are vulnerable to inadequate solder mask coverage, potentially leading to insufficient protection, chemical residue retention, or high-voltage breakdown.
Recommendation: Implement a “multiple printing” process at the board manufacturer to ensure complete coverage of trace sidewalls.
Surface Finishing:
HASL: The most common and reliable option, though pad flatness is suboptimal. Not recommended for thick copper boards. If used, select high-quality substrate with Tg ≥ 170°C.
Gold Plating/Electroless Gold: Offers good flatness, but requires sufficient nickel layer thickness (typically ≥150μm) to prevent failure due to “nickel migration” when high currents pass through thin nickel layers after gold layer damage. However, avoid designing electrolytic gold for thick copper boards, as severe etching can cause neck breaks.
Electroless Tin/OSP: Generally not recommended for high-current thick copper boards due to relatively weak thermal resistance and current-carrying capacity.
4. Laminating and Materials
Substrate Thickness:
Using multiple layers of PP is one of the most critical principles. Thin substrates cannot adequately fill the gaps between thick copper traces, leading to insufficient resin filling after lamination, vacuum bubbles causing board failure, and board warpage.
Rule of thumb:
Post-resin-filling dielectric thickness should exceed the adjacent inner layer copper thickness. For example, with 3oz inner layers, use at least 3 PP sheets—adjust based on copper area ratio and manufacturer capability.
Consequences:
Insufficient dielectric thickness → Resin underfilling → Reduced insulation, insufficient voltage withstand, delamination/board failure.
• Board Selection:
Prioritize high-Tg materials (e.g., Tg ≥ 170°C) to enhance thermal resistance and stability.
For scenarios with extreme heat dissipation requirements, consider metal substrates.
5. Thermal Management
Heat Dissipation Holes:
Densely arrange heat dissipation holes beneath heat-generating components to conduct heat to inner layer copper or rear heat dissipation layers. For example, design heat dissipation vias on the IC center ground plane.
Thermal vias can undergo “via filling plating” to maximize thermal conductivity, though current copper paste filling suppliers are limited.
Copper Area:
Fully utilize thick copper as a heat dissipation medium by designing large copper planes connected to ground or power networks.
II. Design Optimization Recommendations
1. Early Communication is Critical
Before finalizing the layout, communicate design intent and key parameters (e.g., target copper thickness, current density, voltage rating) to the PCB manufacturer. Their engineers can propose laminate solutions and DFM rules best suited to their production capabilities.
2. Chamfering Treatment
At corners of thick copper traces, employ 45° chamfers or rounded corners instead of 90° right angles. Right angles act as stress concentration points prone to cracking under thermal shock.
All corners of pads and copper planes should be chamfered.
3. Stepwise Copper Thickness Design
If only localized areas require high current, consider buried copper blocks or spot thickening techniques.
Spot thickening involves selectively augmenting copper thickness (e.g., from 1 oz) only on critical traces/zones. This reduces manufacturing complexity and cost but adds process steps and expenses. Note that some manufacturers may not offer this service—weigh the trade-offs carefully.
4. Design for Manufacturing
Panelization
Thick copper boards are inherently heavy, and V-CUTS are challenging to execute and prone to damaging traces. We recommend panelization using “stamp holes + bridging” instead of V-CUTS.
Warpage Control
During design, strive for symmetrical copper distribution across layers to prevent warping after lamination due to uneven copper loading. Large blank areas can be balanced by adding copper fill in the empty regions.