How to Design a Heavy Copper PCB?

Step 1: Determine where heavy copper is needed

The first step in designing a heavy copper PCB is to analyze the circuit and thermal requirements to determine the optimal placement of heavy copper. You want to allocate the thicker copper only in areas that will benefit, as overuse will unnecessarily drive up costs. Some key factors to consider:

• High current traces – Any traces that will carry substantial continuous current loads are candidates for heavy copper. This includes power rails, motor leads, and other high-power links. Using thick copper will minimize losses and heating on these traces.
• Heat dissipation – Components like voltage regulators, drivers, and processors that generate significant heat are good candidates to have heavy copper plane layers nearby or under them. The thick copper spreads heat efficiently, preventing hot spots.
• RF transmission lines – Thick copper builds robust RF transmission lines with low loss, as the skin effect is reduced. So RF and microwave circuits often utilize heavy copper.
• Mechanical durability – Areas subject to vibration, repeated insertions, or other physical stresses can benefit from reinforced heavy copper traces and pads.
• High voltage – Since the voltage drop across a trace increases proportionally with current, high voltage traces such as those feeding power tubes can utilize heavy copper to minimize drop.

Take time to analyze current paths and layout early in the design process. Consulting with a PCB manufacturer can help identify areas likely to benefit from heavy copper. Careful planning ensures you apply thick copper strategically where it provides performance gains without sending costs too high.

Step 2: Select the appropriate copper thickness

Once you have determined the areas that need heavy copper, the next key design decision is selecting the appropriate copper thickness. Common heavy copper weights are 1oz, 2oz, 3oz, and 6oz, but other custom thicknesses are possible. Consider the following factors when choosing:

• Current capacity – Doubling the copper thickness doubles the current capacity. Calculate required currents and aim for <500 circular mils per amp to avoid overheating.
• Thermal performance – Thicker copper spreads heat better. Use 2oz or thicker under high-power components. Model the thermal behavior to guide thickness.
• Manufacturability – Thicker copper has tighter tolerances and is more prone to issues like fold-over. Consult with your PCB manufacturer on limitations.
• Cost – Heavy copper significantly increases material costs. Only use it where needed, as 6oz could cost 5-10X more than 1oz.
• Weight – Extra thick copper adds substantial weight, which may matter in products like handheld devices.
• Layer count – With more layers, less copper may be needed to meet targets. A 4-layer board needs less heavy copper than a 2-layer.
• Voltage drop – If voltage drops must be minimized, the thickest copper may be required.

Seek to balance performance and cost by using just enough copper thickness in each area to meet requirements. Combining 2oz and 6oz on different areas of the same board can optimize the design. Work closely with your PCB partner to dial in the ideal copper weights.

Step 3: Adjust trace widths

Once the copper weights are established, the trace widths must be adjusted accordingly. The larger cross-sectional area of heavy copper allows narrower traces to handle the same current load. However, higher fabrication tolerances necessitate wider minimum widths. Follow these guidelines:

• Adjust width based on current, using 500 circular mils per amp rule. A 5A trace only needs 20 mils width in 2oz copper versus 50 mils with 1oz.
• Observe minimum trace/space rules, which increase with thicker copper. A PCB manufacturer can provide their design rules.
• Account for greater copper fill which can cause overheating in inner board layers. Expand spacing between traces and planes.
• Add solder mask web width on traces carrying high current to avoid solder leaching. Typically 0.25mm per side.
• Increase pad sizes for parts carrying high current, equal to at least the trace width, for reliable solder joints.
• Ensure wider traces still fit within the routing channels on the board layout. Avoid necking down traces.
• Review impedance requirements as thicker traces increase capacitance to ground. This impacts high-speed signals.

Testing the design rules checklist from your PCB partner is highly recommended when adjusting trace widths and spacing. Don’t assume standard rules apply. The IPC-2221 standard provides general heavy copper guidance.

Step 4: Manage thermal expansion

One critical design consideration with heavy copper PCBs is managing thermal expansion. As copper gets thicker, the difference in expansion and contraction with temperature increases. This can lead to various failure modes if not addressed properly:

• Pad lifting – The thick copper trace can lift pads completely off the board when contracting at extremely cold temperatures.
• Delamination – The shear forces generated can delaminate the board layers during thermal cycling.
• Cracks – Rapid, localized expansion and contraction can crack plated through holes and traces.
• Warping – Uneven thermal expansion forces across a board can lead to warping.

Several techniques can mitigate these failure risks:

• Use intermittent traces – Breaking up long heavy copper traces into segments with gaps allows expansion/contraction in each segment and avoids shear.
• Incorporate relief gaps and voids – Carefully placed voids and gaps alongside traces/planes provide space for expansion while maintaining electrical connection.
• Optimize padshapes – Rounded pad shapes are more fracture resistant than rectangular. Avoid sharp corners.
• Match coefficients of thermal expansion – Pairing heavy copper layers with materials like Invar that expand minimally reduces shear.
• Add reinforcement – Copper-filled vias help tie heavy copper layers together while allowing some movement.
• Simulation – Use thermal modeling software like Ansys Q3D to identify high-risk areas and guide mitigation steps.

With careful design grounded in physics simulation and modeling, heavy copper PCBs can reliably withstand many thermal cycles without failure. Contact JHYPCB’s engineering team for further guidance in this critical area.

Step 5: Include adequate clearances

When working with heavy copper PCBs, sufficient clearances must be included in the design to account for the higher current densities and voltage drops. Follow these guidelines:

• Increase clearance between traces and planes carrying high current to prevent arcing or insulation breakdown. A minimum 2mm is recommended for traces over 5A.
• Adjust clearances for voltage drops. A 1V drop across a heavy copper trace may need 5mm vs 2mm for a thin trace. Verify isolation margins.
• Check any reduced creepage requirements for thick PCBs. Clearance rules may differ from standard 1.6mm PCBs.
• Maintain adequate distance between high current traces and sensitive components like oscillators. The magnetic fields generated can cause interference.
• Ensure clearance from chassis walls, fasteners, and other mounted hardware. Thicker copper flexes less, so less margin is needed.
• In mixed-thickness boards, ensure thinner copper areas meet their normal clearance requirements. Don’t assume heavy copper rules apply globally.
• Account for reduced arc-chute venting on inner layers since heavy copper fills more space. May require additional vent cut-outs.

Be sure to consult IPC-2221 and the PCB manufacturer’s specific design rules. Also consider environmental factors like humidity and pollution levels which could necessitate larger spacings. Planning adequate clearances upfront prevents field failures.

Step 6: Account for manufacturability

Heavy copper PCBs present unique challenges for manufacturing that must be addressed in the design process. Work closely with your PCB supplier to understand their capabilities and limitations. Key factors to discuss include:

• Minimum trace/space – The minimum trace width and spacing increases for thicker copper. Ensure your design rules adhere to these specs. For example, 6 oz copper may require 8 mil trace/space versus 5 mils for 1 oz.
• Hole wall tolerance – Plated through holes have tighter diameter tolerances with thick copper due to etching variability. Account for this in pad designs.
• Fold over – Thick copper is prone to fold over into traces or holes during lamination. Tight from-to spacing should be avoided.
• PTH barrel fill – Filling a 12 mil hole with 6 oz copper requires special plating cycles. Verify your vendor can meet the plating needs.
• Line width variation – Thicker copper traces may see greater line width variation. Budget adequate margins in impedance-controlled lines.
• Panel utilization – Heavy boards occupy more panel space, reducing panel utilization. Ensure minimum slotting requirements are met.
• Via temps – High temp cycles may be required to fully heat thick copper vias for effective soldering. Confirm your PCB assembler can meet the thermal needs.

Early engagement with your fabrication partner is key to avoiding manufacturability issues. Be ready to make design adjustments if needed to enable reliable, high-yield production. Don’t let the PCB process become the weak link.

Step 7: Perform thermal analysis

Robust thermal analysis is critical for ensuring heavy copper PCBs perform reliably under temperature extremes. Simulation and modeling during design provides invaluable feedback for optimization before reaching manufacturing. Follow this best-practice thermal analysis workflow:

• Create preliminary layout with identified heavy copper areas.
• Build a 3D model incorporating copper weights, die sizes, board shape, and component placement.
• Set up the proper material properties for each layer and component.
• Apply the required power profiles and load conditions. Consider peak and continuous loads.
• Run simulations across expected ambient temperature ranges. Consider convection and radiation.
• Identify hot spots that exceed target temperatures and heat density limits.
• Adjust copper thicknesses, thermal vias, coolant channels, or other aspects to improve the temperature distribution.
• Iterate with additional simulations until thermal targets are reliably met with adequate guardband.
• Perform sensitivity analysis to quantify robustness to variances in manufacturing or operating conditions.

Tools like Ansys Icepak, Solidworks Flow Simulation, and SimScale provide advanced thermal modeling capabilities. Leverage these platforms early in the design process to optimize use of heavy copper and achieve a robust thermal design. Contact the engineering team at JHYPCB for further thermal analysis assistance.