Why and How to Use an Aluminum PCB Substrate for Your Next Stackup Design
Aluminum can be used for more than just soda cans
I don’t drink soda so much anymore now that I’m in my thirties, but I know aluminum has plenty of uses outside of making coke cans. One use is in the core of your PCB as a material for thermal management. Aluminum has high thermal conductivity and can be used to carry heat away from active components on a PCB when other passive or active cooling measures cannot bring component temperatures to a sufficiently low level.
Using an Aluminum PCB for Thermal Management
Active components dissipate a significant amount of power, thus the use of a cooling fan on a CPU or other components with a large number of switching transistors. If the ambient temperature is excessively high, active cooling measures will only be useful for bringing the temperature of the board back down near the ambient level. In addition, you can only dissipate so much heat with active cooling. This is where some additional strategies need to be used to dissipate heat away from your active components.
Aluminum is one alternative material that can be used in the core of a PCB, commonly referred to using the misnomer “aluminum pcb”. Using aluminum as the metal core in a PCB allows it to easily dissipate heat away from active components thanks to its high thermal conductivity. The high thermal conductivity of aluminum or another metal in the core of a PCB allow heat to be distributed more uniformly throughout a board.
Contrast this with FR4, which is a relatively poor thermal conductor compared to a number of other alternative materials for PCB substrates. Hot spots on a PCB can form near active components, thus the use of active and passive cooling measures to dissipate heat and bring the temperature to a safe level. Heat generated from active components can also be transported away from the component layer and into an interior ground or power plane using thermal vias and lands.
In boards with an FR4 core, the ground/power planes are limited in the amount of heat they can transport around the board, as the cores have low thermal conductivity. Thermal vias and lands can help with heat dissipation, but other strategies often need to be used to bring the operating temperature of components down to a safe level.
An Aluminum PCB Stackup
Although an aluminum PCB might seem like an odd choice from a manufacturing standpoint, the stackup that can be used with an aluminum PCB actually resembles the stackups that can be used with FR4 substrates. An example stackup is shown in the image below:
Example layer stack with an aluminum PCB
An aluminum PCB stackup should be designed with the following considerations:
Surface layer: This is a standard copper foil layer. Some manufacturers will recommend you use heavier copper (up to 10 oz) than would be used on FR4.
Dielectric layer: The interior dielectric layer can be any thermally conductive layer that functions as a prepreg. This can be a polymer or ceramic layer. Opting for a material with higher thermal conductivity, particularly a ceramic with high ratio of thermal to electrical conductivity, will aid thermal management while also providing sufficient insulation. The typical thickness of the dielectric layer is 0.05 to 0.2 mm.
Aluminum membrane layer: The aluminum membrane layer plays a protective role in that it protects the aluminum core from unwanted etching. This is a very thin insulating layer that plays an important role for any vias that are drilled through the core (see below).
Aluminum core: The interior layer is the aluminum core with high thermal conductivity. Most aluminum boards are 0.5 mm thick, although thicker boards can be used to provide greater structural stability.
Note that vias can be drilled through the aluminum core, although the aluminum membrane layer will need to cover the interior of the via hole in order to form an insulating layer between the copper via wall and the core. If you opt to use a thicker aluminum core, your material costs and your fabrication costs will increase.
Some Applications of Aluminum PCBs
Because the laminates used in an aluminum PCB dissipate heat faster than FR4, they can be used in a variety of systems that generate significant heat. One excellent example is in LED lighting arrays. SMD LEDs that operate at high power produce a significant amount of heat. The heat dissipated by the aluminum core quickly moves heat away from the LEDs, which extends their lifetime.
Lighting array with SMD LEDs
Thin aluminum core PCBs can even be fabricated as flexible PCBs. Both static and dynamic flex PCBs can be fabricated from aluminum PCBs. As ceramics are much less ductile than aluminum, they should not be used in flexible aluminum PCBs. Therefore, polymer laminates should be used as the dielectric layer in these boards.
Aluminum PCBs provide other benefits beyond thermal management. Rigidity and higher strength against bending and shock were mentioned above. In addition, the metal core provides better EMI shielding, thus an aluminum PCB is also useful in an electrically noisy environment. Metal cores are also more environmentally friendly than FR4 or other materials, as aluminum is recyclable.
If you opt to use an aluminum PCB for your next device, you’ll need design tools that include an extensive library of layer stack materials and an intuitive stackup manager. Altium Designer contains all these features and many more, and provides the industry’s leading design and analysis tools in a single program. These important design features also interface with the native 3D design tools in Altium Designer.