Ceramic vs. FR4 Multilayer PCBs: When to Use Either and How
For most folks, hearing the word “ceramics” conjures images of a middle school or high school art class, where you proudly created a deformed coffee cup for your parents. Fast forward to your days as an engineer, and ceramics material play a critical role in electronic components. A ceramic material PCB design with capacitors can be a valuable circuit to know.
Industries that require high speed/high frequency boards that can withstand harsh environments can benefit from ceramic PCBs. Aerospace and heavy industrial equipment can see huge reliability benefits by switching to ceramic PCBs over FR4. The primary drawback is cost, which can become significant in high volume manufacturing runs.
Ceramic boards offer some particular advantages over FR4 boards and these advantages can be exploited in certain applications. There is no single “ceramic” material; this term refers to a class of materials with similar chemical structure and physical properties. The ceramic substrate used for circuit boards are aluminum oxide, aluminum nitride, and beryllium oxide. Substrate such as silicon carbide and boron nitride are two other ceramics with similar performance.
One massive difference between FR4 and ceramic materials is their thermal conductivity. FR4 has appallingly low thermal conductivity compared to the ceramic materials used for a circuit. Aluminum oxide is about 20 times as thermally conductive as FR4. Aluminum nitride and silicon carbide are about 100 times as thermally conductive, and beryllium oxide has even higher thermal conductivity. Boron nitride has the highest thermal conductivity by far.
FR4 PCBs that have high thermal demands often compensate for the low thermal conductivity using metal structures to transport heat. Thermal vias, metal planes on the inner layers, active cooling elements like fans, and thermal landings are all used to direct heat away from surface layers. Ceramic boards do not require these elements except in extreme cases, and heat can be easily transported to a thermal landing, active cooling element, or device packaging.
If you are familiar with chemistry and physics, you may know that thermally conductive materials tend to also be good electrical conductors. Ceramics buck this trend somewhat, meaning their electrical conductivity is still low enough that these boards can be used for PCB substrates. The electrical conductivity of ceramic boards can also be adjusted through doping; this is the same process used to set the resistance of ceramic resistors.
You’ll need more than ice cubes to manage thermal demands in your PCBs
Ceramic boards have other benefits that are particularly useful in multilayer boards. The high thermal conductivity helps prevent the formation of hot spots on the surface and inner circuit layers as heat transport is more uniform throughout the board. In contrast, FR4 relies on metal structures or active cooling to transport heat away from certain locations of the board or between layers, and hot spots are more likely to form on an FR4 PCB.
Multilayer boards use vias to access the inner layers of a circuit, and vias in FR4 boards are susceptible to fracture during thermal cycling. The risk of fracture arises due to mismatches in the thermal expansion coefficient between copper and FR4. Thermal cycling of these boards creates stress along the via barrel and butt joints on in-pad vias. These points are prone to fracture, and designers need to take special design considerations to prevent via failure.
Ceramic circuit boards have thermal expansion coefficients that are closer to the values for their conductor structures, thus reducing the stress that accumulates in these structures during thermal cycling. The higher thermal conductivity across the entire ceramic board ensures that thermal expansion is also more uniform, preventing large stress from being exerted on any vias in a particular part of the board.
Ceramics have desirable mechanical strength and can withstand high mechanical loads, including strong vibrations and shock. They have lower Young’s modulus than FR4, meaning a ceramic board will tend to deform less than an FR4 for the same applied force.
The manufacturing process for ceramic boards allows the use of silver or gold conductive pastes for laying trace connections in each layer. These metal elements or substrate are typically placed in each layer using a layer-by-layer screen printing process.Vias can also be mechanically punched in an unfired layer or microvias can be drilled with a laser.
Your ceramic PCB might be fired in an oven like this
Once the ceramic layers have been printed and stacked, the entire stack is fired in an oven. The firing temperature required to bake the ceramic board is typically below 1000 °C, which matches the sintering temperature of the material of gold and silver pastes. This low temperature bake process is what allows the use of gold and silver in ceramic PCBs.
The hot pressing/baking and sintering process for multilayer PCBs makes it easy to integrate passive components directly into the inner layers of a ceramic PCB. This is not possible in a PCB made from FR4 material. This allows designers to increase component and connection density on inner layers.
The extensive design, simulation, and analysis features in Altium Designer® give you the power to build PCBs from any material, including ceramics. Now you can download a free trial and find out if Altium Designer is right for you. Talk to an Altium expert today if you want to learn more.