Metal Core PCBs

There are many important design guidelines in a metal core PCB that should be considered when creating a stackup with a metal layer. Metal-core boards are normally built with the metal layer used as a core layer in a low-layer count stackup. Aluminum is normally used as the metal layer in the stackup, although copper or stainless steel could also be used.

After determining the metal material and selecting standard dielectric laminates, the layer arrangement needs to be determined. The layer stack will then determine some important aspects of routing and component selection:

Metal-backed PCB
The metal layer is placed on the back side of the board, and components are present on the opposite layer. Traces cannot be routed on the back layer, although through-holes can be used to route between dielectric layers and bring ground to the metal layer.

Metal core PCB
This version can be a double-sided board, and routing can pass between each side of the metal core using plated through-holes. Multilayer dielectric stacks can be used on each side of the metal core, which will support routing on multiple layers.

The metal layer should be grounded so that it can provide shielding and so that it will not act like a big monopole antenna. Specific applications may require mourning the metal core to an enclosure, an important point to consider when designing for aerospace and defense PCBs.

Conductivity
The thermal conductivity of the standard PCB is low, about 0.3w. The mcPCB has a higher thermal conductivity, in the range of 1-2w.

Electroplating through hole
In standard PCBS, we often have to use electroplating through holes. However, we don't necessarily need them in MCPCB.

Heat dissipation
We must use VIAS in the standard PCB for heat dissipation. This means longer drilling cycles and additional processes. However, MCPCB does not require drilling, electroplating or deposition processes. This is because the metal core allows effective heat dissipation.

Solder mask
The solder resist layer in a standard PCB is usually dark. Therefore, the colors are usually black, green, blue, and red. Therefore, we apply solder resist layers on the top and bottom. On the other hand, in MCPCB, we only apply a solder resist layer on the top, which is usually white.

Thickness
Due to the use of layer installation and various material combinations, standard PCBS have a wide thickness range. However, due to the available dielectric sheet thickness and backplane thickness, the thickness variation of MCPCB is usually limited.

Processing process
Standard PCB uses traditional processing techniques, such as wiring, countersunk holes, drilling, and v-marking. However, we need a diamond-coated saw blade to v-mark MCPCB. This is because cutting metal requires sharp tools.

Copper layer
This is the first layer, which is common in all PCBs. The copper layer is a conductive layer that transmits the signals. It is also known as the circuit layer as the whole circuit or conductive paths are made through it. Therefore, it is the same layer you will find in other PCBs.

The thickness of this layer depends on your requirements. You can select the thickness of this layer according to your project.

Dielectric layer
This is an extremely important layer in a metal core PCB. You will not see this layer in FR-4. We know that the top layer is the copper layer, and the bottom layer is the base layer, which is made from metal. These two layers are conductive, and the insulation between them is the dielectric layer. Thus, this layer plays a very important role in separating the copper layer and the metal layer. It prevents short circuits.

The most vital function of the dielectric layer is the dissipation of heat. The layer has the ability to take heat from the components on the top layer and pass that heat through itself to the metallic base layer. Then, the metallic base layer dissipates the heat.
Due to this layer, heat dissipation becomes more efficient as it can improve the heat dissipation up to 9 times. Thus, a metal core PCB is 6-9 times better than the traditional FR-4 PCB. There can be different components on the board, and their heat will be easily dissipated through this layer. Thus, it keeps the component cool and increases their life.
Keep in mind that the thickness of dielectric in the metal core PCBs should be as minimum as possible. Usually, it is no more than 0.003-0.006 inches. Thus, it won't affect the overall thickness of the board. The reason behind the thin layer of dielectric is to transfer the heat in the shortest possible time. The transfer of heat depends on the thickness of the dielectric and the material. There are different dielectric materials available, and every material has a different heat transferring capacity, which is mentioned in Watts per Meter per Kelvin (W/mK).

Base layer
The base layer of metal core PCBs is made of metal, which is why the PCB is named as metal core PCB. As the base layer is metal, it can withstand more pressure stress and vibrations compared to FR-4 PCBs. It is strong, durable, and long-lasting.

The metal layer of the PCB works like heat sinks. It takes the heat from the conductive layers and disperses it to keep the PCB cool. Such cooling cannot be attained using cooling fans, so metal core PCBs enjoy more importance.
The base layer can be of different metals. However, the most commonly used metals are copper and aluminum. Iron is also used in a few cases. Some buyers also request brass and other alloys. In the LCD industry, the preferred metal is aluminum as it offers a reasonable cost. On the other hand, copper is superior in performance, but it is costly.
The thickness of the base depends on the customer's requirements. Typically, the thickness is in the range of 1-4mm, or it can be as you like.

Choose the right metal core material
The metal core material is the foundation of the mcpcb. It provides thermal conductivity, allowing the pcb to dissipate heat effectively. The most commonly used metal core materials are aluminum, copper, and brass. Aluminum is the most popular choice because of its high thermal conductivity, low cost, and light weight. Copper has better thermal conductivity than aluminum, but is more expensive and heavier. Brass is a good compromise material between aluminum and copper, but is less common.

Determine thermal requirements
The thermal requirements of your application will determine the thickness of the metal core and the thermal vias required. Thermal vias are small holes drilled through the metal core that are used to transfer heat from the pcb to the metal core. The number and size of thermal vias depends on the thermal conductivity of the metal core material and the power dissipation of the component.

Optimize electrical performance
The electrical performance of an mcpcb is just as important as its thermal performance.
The metal core can act as a ground plane or a power plane, depending on the circuit design.
The metal core also affects signal integrity and electromagnetic interference (emi) performance.

Ensure mechanical stability
The metal core provides mechanical stability to the pcb, but can also cause mechanical stress if not designed properly. The metal core can expand and contract due to temperature changes, causing the pcb to bend or crack. The metal core can also affect solder joint reliability and component placement.