Guide

Heavy Copper PCB: Construction, Properties, and Applications

What is Heavy Copper PCB?

Heavy copper PCB is a printed circuit board used for applications that carry high currents. Copper is generally a good conductor of electricity as well as heat. Heavy copper PCBs are printed circuit boards with thicker copper thickness in the inner or outer layers, usually ranging from 2 oz (70um) to 40 oz (1400 um) copper thickness. 

The manufacturing process varies from one manufacturer to another. The layers can be from 2 to 20 layers in general stacked over each other with thermal and electrically stable lamination sheets. Heavy copper PCB has lower resistance than multilayer FR4 copper PCB. 

The cross-section of the heavy copper PCB is shown below. A clear-cut view of isolation layers and thickness of copper is shown. In this type of heavy copper PCB, the isolation layer is kept thick to provide electrical isolation and mechanical reliability.

The Cross-Section of Heavy Copper PCB

Construction of Heavy Copper PCB

There are two ways to manufacture heavy copper PCBs. The most commonly used method is the Embedded heavy copper method where thick copper is added to the prepreg layer to match the thickness of the copper. The second method is the blue bar method where a thick rod of copper is added to the circuit boards, the resin flows into the empty traces of copper to achieve a smooth surface and fill between inner layers. In any manufacturing process, the following construction materials are considered based on design applications.

  • Base Material: Copper layers in general FR4 PCB can be up to 1 Oz. When it comes to heavy copper PCB then the construction is purely dependent on base materials like FR$ or Halogen free or Rogers or Aluminium or in some cases, hybrid base materials are used. 
  • Copper Thickness:  The thickness of the copper varies from 3 Oz to 40 Oz in some special cases. The maximum inner layer copper thickness is 10 Oz whereas the outer layer thickness can be up to 20 Oz.
  • Number of Layers: The number of heavy copper PCBs varies from 2 to 20 layers depending on the manufacturing capability. 
  • Board Thickness:  The thickness of the board is more as the copper ounce increases; it can vary from 2 mm to 50 mm in special cases.
  • Hole Size: Higher ounce copper is used in multi-layer PCB so the drill hole size is also 0.1 mm and in higher current carrying Copper Printed circuit Board the minimum thickness is 0.5mm
  • Line Width & Spacing: Line width and spacing depend on the copper layer thickness. The minimum trace width is 30 mils for 4 Oz copper and spacing is 50 mils, it also depends on the manufacturer’s capability. 
  • Surface Finish: Surface finish can be OSP, HASL, HASL Lead-Free (HASL LF/ ROHS), Tin, Immersion gold (Au), Immersion Silver (Ag), ENIG, ENPIG as per standards and few hybrid finishes are also available like Golden finger + HASL, ENIG + OSP, OSP + Golden finger for better conductivity on the surface as huge current has to make contact with the external component’s terminal.
  • Solder Mask: Solder mask finish in Heavy copper Printed circuit board can be classified into standard mask, Wet film, and peelable solder mask. 
  • Testing: PCB manufacturing testing is standard as X-ray or Automated Optical Inspection (AOI), while the component continuity or In-Circuit Test (ICT) or Functional circuit test (FCT) is done using a flying probe or bed of nails fixture. 

 

Properties of Heavy Copper PCB

The property of the heavy copper PCB determines the major advantage of various applications. The copper PCB contains 3 areas of copper that are in the outer layer, the inner layer, and the weight of the copper that is used in the copper plating. 

Heavy Copper PCB Designs

  • Mechanical Stability: Heavy copper PCBs are more capable of carrying currents than any standard FR4 PCB with a smaller size as the copper sheets are stacked up over each other with a more mechanically stable structure similar to a copper busbar.
  • Corrosion Resistance: Copper PCBs are more reliable than other materials over some time as the corrosion resistance is higher in extreme environments. 
  • Anti-Interference: Metals conducting huge amounts of current tend to cause interference and become unstable during heat transfer but copper is efficient in many ways. Isolation of layers is also maintained with no or minimal signal interference issues. 
  • Thermal conductivity:  Copper has better current carrying capacity and better thermal conductivity than any normal metal, the heat transfer is also more efficient as the surface of the copper tends to radiate more to the thermal material or air or other medium.
  • Lifecycle: The lifecycle of copper is more; it does not cause much harm to the environment. The copper in Heavy copper PCB weighs more and it can be recycled easily. Heavy copper printed circuit PCB is more. The leverage of using heavy copper PCB also increases the lifespan of the components as the thermal stability is managed better. 

 

Design Considerations of Heavy Copper PCB

Heavy copper PCB design needs predetermined calculations to determine the stack up and achieve the desired design requirements which are mentioned below.

  • Current Calculation:  To determine the layer stack up of the Heavy copper PCB the understanding and specifications of the entire circuit must be calculated. This results in the numerical value of the current that is required to pass through the circuit. With these data, the width of the PCB can be determined with the ounce of copper that is needed.  
  • Resistance:  Resistance is an important parameter for any metal. As the trace length increases the resistance of the copper also increases which is also determined by I(square)R. 
  • Etching Type:  The etching of heavy copper PCB is more difficult; it could lead to over-etching or under-etching as the thickness of the copper is high. Based on this the manufacturers use chemical etching or laser etching or plasma etching for minimal tolerance and good precision.
  • Placement of the Components:  The length of the circuits determines the power loss as the trace length increases. The resistance is going to increase, as the position of sensitive components determines them. The PCB can also be designed for single-layer or double-sided PCB to replace the busbar with lower component density.
  • Base and Substrate Materials: The main parameter of the substrate material should withstand the heat generated by the heavy copper as per design. The degradation factor should also be considered. Some additional base materials are also used to transfer the heat more effectively. 

 

Heavy Copper PCB Board

Disadvantages of Heavy Copper PCB

Heavy copper PCB also has its disadvantages, it entirely depends on the cost, manufacturing process, and density of design.

  • Manufacturing Difficulty: The manufacturing process when compared to FR4 PCB or other metal PCB as the etching process and plating of heavy copper needs more precise design considerations. High-speed signal circuits are difficult to manufacture, and impedance matching and signal integrity are difficult to achieve.  
  • High Cost: The cost of manufacturing is also high as the copper metal cost is high. This is one major disadvantage of heavy copper PCB.
  • Complex Designs:  Complex high-density boards cannot be designed, and high-frequency switching signals cannot be etched. The Heavy copper PCB designs are difficult to achieve as the nature of the thick copper layer does not allow freedom.

 

Applications of Heavy Copper PCB

  • Military & Aerospace: Optimal performance, thermal conductivity, and reliability make them apt for military and aerospace designs where high current and high power are needed. They are also implemented in wireless, satellite communication devices, and radar appliances.
  • Industrial Equipment: Industrial equipment uses heavy copper PCB which can be used in harsh environments as it is corrosion resistant to many chemicals. The current carrying capability is more so that electrical substations use heavy copper PCB. 
  • Inverter/ Traction Control: Inverter applications that use more than 100kW designs need more current carrying capability. High power PMSM or BLDC motors that operate with higher currents not just require current carrying capability but also thermal reliability to extend the life span of the components.
  • Automotive: Battery management systems, Motor controllers, onboard chargers (OBC), or PDU work in harsh environments. The automotive environment needs long life and corrosion resistant High-power PCB which is achieved by heavy copper PCB. 
  • Medical Electronics: Special medical equipment like laser operation or robotic machines, imaging devices like scan machines, X-ray, etc need high currents with unique applications, in these cases copper heavy copper printed circuit boards are used. 

Renewable Energy:  Copper PCB has more heat transfer and lower resistance to current. Power transfer is efficiently achieved from a huge Photovoltaic plant that runs over a huge area.

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