What materials are used in metal core PCB?

Metal Substrate Materials

The choice of metal substrate is crucial in the performance of a metal core PCB. The most common metals used as substrates include:

Aluminum

Aluminum is the most widely used metal substrate in metal core PCBs due to its excellent thermal conductivity, lightweight nature, and cost-effectiveness. Aluminum substrates are available in various grades and alloys, such as:

  • 5052 aluminum: Known for its high strength and corrosion resistance
  • 6061 aluminum: Offers good machinability and weldability
  • 1100 aluminum: Pure aluminum with excellent thermal conductivity but lower strength

Copper

Copper is another popular choice for metal core PCBs, particularly in applications that demand the highest levels of thermal performance. Copper has a higher thermal conductivity than aluminum, making it ideal for heat-intensive applications. However, copper is heavier and more expensive than aluminum.

Other metals

In some specialized applications, other metals may be used as substrates, such as:

  • Steel: Offers high strength and stiffness but has lower thermal conductivity than aluminum or copper
  • Brass: Provides a balance between thermal conductivity and mechanical properties
  • Invar: An iron-nickel alloy with a low coefficient of thermal expansion (CTE), suitable for applications with strict dimensional stability requirements

Dielectric Layer Materials

To electrically isolate the conductive copper traces from the metal substrate, a dielectric layer is applied to the surface of the metal core. The choice of dielectric material depends on the specific requirements of the application, such as thermal conductivity, electrical insulation, and mechanical properties. Common dielectric materials include:

Polymer-based dielectrics

  • Polyimide (PI): Offers high thermal stability, excellent electrical insulation, and good mechanical properties
  • Epoxy: Provides good adhesion, chemical resistance, and electrical insulation
  • Polyether ether ketone (PEEK): Known for its high temperature resistance, mechanical strength, and chemical stability

Ceramic-based dielectrics

  • Aluminum nitride (AlN): Provides high thermal conductivity, excellent electrical insulation, and a close CTE match to aluminum substrates
  • Alumina (Al2O3): Offers high electrical insulation and thermal stability but has lower thermal conductivity than AlN
  • Boron nitride (BN): Exhibits high thermal conductivity, electrical insulation, and low dielectric constant

Conductive Layer Materials

The conductive layer in metal core PCBs is typically made of copper, which is laminated onto the dielectric layer. The copper layer is then etched to create the desired circuit pattern. The thickness of the copper layer can vary depending on the current-carrying requirements of the application, with common thicknesses ranging from 0.5 oz to 4 oz per square foot.

Solder Mask and Surface Finish

To protect the copper traces and prevent short circuits, a solder mask is applied over the conductive layer. The solder mask is typically a polymer-based material, such as:

  • Liquid photoimageable solder mask (LPISM): Applied as a liquid and then exposed to UV light to cure, forming a protective layer
  • Dry film solder mask: Applied as a dry film and then laminated onto the PCB surface

The exposed copper areas, such as pads and contacts, are then coated with a surface finish to prevent oxidation and improve solderability. Common surface finishes for metal core PCBs include:

  • Hot air solder leveling (HASL): A tin-lead alloy is applied to the exposed copper areas and then leveled using hot air
  • Electroless nickel immersion gold (ENIG): A thin layer of nickel is deposited onto the copper, followed by a layer of gold to protect the nickel from oxidation
  • Immersion silver: A thin layer of silver is deposited onto the copper, providing good solderability and low cost

Thermal Interface Materials

To enhance the thermal transfer between the metal core PCB and the heat sink or other cooling solutions, thermal interface materials (TIMs) are often used. TIMs fill the gaps and irregularities between the surfaces, improving thermal conductivity. Common TIMs include:

  • Thermal greases: Silicone-based pastes filled with thermally conductive particles, such as aluminum oxide or boron nitride
  • Thermal pads: Soft, compressible pads made of silicone or other polymers, filled with thermally conductive particles
  • Phase change materials (PCMs): Materials that change from solid to liquid at a specific temperature, conforming to surface irregularities and improving thermal contact
Material Thermal Conductivity (W/mK) Dielectric Constant Dielectric Strength (kV/mm)
Aluminum (5052) 138
Copper 401
Polyimide 0.2 3.4 120
Epoxy 0.2 – 0.3 3.5 – 4.5 20 – 30
Aluminum Nitride 150 – 220 8.8 20
Alumina 20 – 30 9.8 15

Frequently Asked Questions (FAQ)

  1. Q: What are the advantages of using a metal core PCB over a traditional PCB?
    A: Metal core PCBs offer several advantages, including improved thermal management, better mechanical stability, and reduced electromagnetic interference (EMI). The metal substrate helps dissipate heat more efficiently, preventing components from overheating and extending their lifespan.

  2. Q: Can metal core PCBs be used in high-frequency applications?
    A: Yes, metal core PCBs can be used in high-frequency applications, but proper design considerations must be taken into account. The choice of dielectric material, substrate thickness, and circuit layout can impact the high-frequency performance of the PCB.

  3. Q: How does the choice of metal substrate affect the thermal performance of a metal core PCB?
    A: The thermal conductivity of the metal substrate directly influences the thermal performance of the metal core PCB. Copper has the highest thermal conductivity, followed by aluminum. The choice of metal substrate depends on the specific thermal requirements of the application, as well as cost and weight considerations.

  4. Q: Can metal core PCBs be manufactured with multiple layers?
    A: Yes, metal core PCBs can be manufactured with multiple layers, although the process is more complex than traditional multi-layer PCBs. The additional layers are typically separated by dielectric materials and connected through vias, allowing for more complex circuit designs.

  5. Q: What are the challenges in soldering components onto a metal core PCB?
    A: Soldering components onto a metal core PCB can be challenging due to the high thermal conductivity of the metal substrate. The metal core can act as a heat sink, making it difficult to achieve the required soldering temperatures. To overcome this, specialized soldering techniques, such as localized preheating or the use of thermal barriers, may be necessary.

In conclusion, metal core PCBs are a specialized type of PCB that offer unique advantages in thermal management and mechanical stability. The choice of materials, including the metal substrate, dielectric layer, and conductive layer, plays a crucial role in the performance of the metal core PCB. By understanding the properties and characteristics of these materials, designers can create metal core PCBs that meet the specific requirements of their applications, ensuring optimal performance and reliability.