Printed circuit boards (PCBs) form the backbone of electronics. They provide the platform to mount and interconnect electronic components using conductive copper traces etched from copper sheets laminated onto insulating substrates.
PCBs with thicker copper layers, known as heavy copper PCBs, offer higher current carrying capacity and better thermal performance compared to standard copper thickness PCBs. This makes them suitable for designing high power circuits and devices.
Heavy copper PCBs find use in various applications including:
- Power electronics
- Industrial automation and control
- Battery management systems
- Automotive electronics
- Aerospace and defense electronics
In this article, we will take a detailed look at heavy copper PCB technology including:
Contents:
- What is Heavy Copper PCB
- Why Use Heavy Copper PCBs
- Heavy Copper PCB Construction
- Applications of Heavy Copper PCBs
- Design Considerations for Heavy Copper PCBs
- Manufacturing Heavy Copper PCBs
- Summary
What is Heavy Copper PCB
Heavy copper PCBs use copper foils thicker than the standard 1 oz (35 μm) foil used on typical PCBs. The typical copper foil thickness used in heavy copper PCBs ranges from 2 oz (70 μm) and can go up to 20 oz (700 μm) or even more for some special applications.
As the name suggests, heavy copper PCBs contain a higher amount of copper, increasing the total copper weight in the finished boards. Standard FR-4 PCBs have a copper thickness of 1/1 oz meaning 1 oz copper on the top and bottom layers. Heavy copper PCBs are denoted for example as 6/6 oz or 12/12 oz indicating symmetrically thick copper on the top and bottom layers.
The thick copper provides benefits like higher current capacity, better thermal performance, and lower power loss. However, it also poses design and manufacturing challenges which need special consideration.
Why Use Heavy Copper PCBs
Heavy copper PCBs provide several benefits making them suitable for high power and high performance electronic systems and devices:
Higher Current Carrying Capacity
- The thicker copper can handle higher electrical currents without overheating.
- Follows relationship:I = k * A/RWhere:I = Current in Ampsk = Constant based on copper propertiesA = Cross sectional area of conductor in mm^2R = Conductor length in mm
- Doubling copper thickness doubles the cross sectional area, allowing approx. double the current handling capability.
Better Thermal Management
- The thick copper acts as a heat spreader helping to dissipate heat from components mounted on the PCB.
- Copper has excellent thermal conductivity helping to transfer heat away from heat sources.
- Larger copper cross sectional area reduces thermal resistance, improving heat spreading.
Lower Copper Loss
- Increased copper thickness reduces resistive losses in the PCB conductors.
- Resistive power loss follows:P = I^2 * R
- Lower resistance R reduces power loss P for a given current flow I.
Heavy Copper PCB Construction
Heavy copper PCB construction involves decisions like copper foil thickness, layer stackup, dielectric materials etc. tailored for the specific application requirements.
Copper Foil Thickness
- Foil thickness from 2 oz (70 μm) up to 20 oz (700 μm) or more.
- Typical thicknesses – 2 oz, 3 oz, 5 oz, 6 oz, 12 oz.
- Symmetric or asymmetric distribution on layers.
- Thicker foils available but can be challenging to handle and fabricate.
Copper Weights
- Total copper weight quantified in ounces per square foot (oz/ft2)
- Increased as more copper layers are added:
- 1 oz – Standard thickness PCB
- 2 oz – First level of heavy copper PCB
- 4 oz – Double layer heavy copper
- 6 oz – Four layer heavy copper
- 8 oz – Six layer heavy copper and so on
Layer Stackup
- Layer configuration optimized for current flow, heat transfer, mechanical stability.
- Important design factors:
- Total number of copper layers
- Foil thickness distribution between layers
- Dielectric materials used
- Layer sequencing
Dielectric Material
- Choices: FR-4, polyimide, ceramic filled, metal core substrates.
- Properties: dielectric constant, loss tangent, thermal conductivity.
- Thicker dielectrics may be required to get desired spacing between thick copper layers.
Copper Fill Areas
- Large continuous copper shapes on multiple layers.
- Help to distribute currents and spread heat.
- Fill areas may be connected with thermal vias.
Applications of Heavy Copper PCBs
Some of the major application areas that benefit from the use of heavy copper PCBs include:
Power Electronics
- Switch mode power supplies, DC-DC converters, inverters.
- High transient currents and thermal cycling stresses.
- Thick copper traces handle high pulsed currents.
- Large copper heat spreading areas and substrates with high thermal conductivity used.
Industrial Automation and Control
- Motor drives, process control systems, power distribution.
- Distributed power and control signals across large PCBs.
- Thick copper traces used for high current motor drives.
Battery Management Systems
- Battery packs for electric vehicles, energy storage systems.
- Interconnect high power battery cells/modules.
- Monitor cell conditions.
- Require robust PCBs to handle high currents and dissipate heat.
Automotive Electronics
- Hybrid and electric vehicle power systems.
- Engine control units, ABS, stability control.
- High ambient temperatures and thermal cycling environment.
- High power LED lighting.
Aerospace and Defense
- Power converters, motor drives, avionics.
- Ruggedness and durability over wide temperature range.
- Withstand high altitude pressure differentials.
- High frequency digital circuits benefit from lower copper loss.
Design Considerations for Heavy Copper PCBs
Heavy copper PCBs require adapting board layout and design practices. Some key considerations include:
Trace Width and Spacing
- Wider traces used with thicker copper to handle high currents.
- Spacing between traces increased due to reduced resistive coupling.
- Typical widths and spacing for common foil thicknesses:
- 1 oz: 8-10 mil width, 8-10 mil spacing
- 2 oz: 15-20 mil width, 15-20 mil spacing
- 5 oz: 30-40 mil width, 30-40 mil spacing
Via Construction
- Plated through hole (PTH) vias need to be drilled larger and plated thicker to match heavy copper layers.
- Typical via diameters range from 12 mil for 2 oz foil up to 30 mil for 20 oz foil.
- Laser drilled microvias may be used for layer-to-layer interconnections.
Plane Layers
- Complete uninterrupted copper planes used on inner layers for current spreading.
- Thermal relief patterns may be incorporated in planes for heat dissipation.
- Multiple plane layers may be stacked depending on total copper thickness.
Component Selection
- Components must be sized appropriately for higher currents.
- Larger packaging, higher power ratings.
- Components should have sufficient clearance from plane layers for heat dissipation.
Thermal Management
- Thermal vias help conduct heat between layers and to the board edges.
- Heatsinks may be mounted to PCB for cooling.
- Metal core PCBs or insulated metal substrates used for high power applications.
Manufacturing Heavy Copper PCBs
Implications on PCB Fabrication
- Panel and equipment capacity to handle thick/heavy panels.
- Drilling capability – spindle speed and torque for larger hole diameters.
- Plating tanks that can plate thick copper in vias and holes.
- Lamination process may need higher temperature and pressure settings.
- Separating panels into individual PCBs gets more difficult with thicker boards.
Testing Considerations
- Need test fixtures that can handle large boards and provide sufficient electrical contacts.
- In-circuit test methods must adapt for increased spacing between test points.
- Additional tests for thermal performance and temperature cycling may be required.
- Electromagnetic interference testing methodology may need modifications.
Summary
Heavy copper PCB technology enables the design and fabrication of high current, high heat boards by utilizing copper foils thicker than the standard 1 oz thickness. The thick copper provides benefits like higher current capacity, improved thermal performance, and lower loss for high power electronic circuits and devices.
However, heavy copper PCBs also pose unique design and manufacturing challenges related to spacing, drilling, plating, lamination, and testing which need careful consideration. The PCB layout practices and assembly processes need to adapt to the specific copper weights and layer stackup configurations employed.
When designed properly, heavy copper PCBs serve as a robust interconnect platform for building efficient power electronic systems applied extensively across automotive, aerospace, industrial, and other sectors constantly pushing the boundaries of electrical power conversion and distribution.
