As cutting-edge technologies continue to emerge, there is an escalating demand for superior signal transmission performance in circuit boards, especially in high-frequency environments where susceptibility to electromagnetic interference is a critical concern. To address this challenge effectively, the focus shifts to the substrate material, and Ceramic PCB have emerged as a highly regarded solution. Ceramic PCB, with their exceptional thermal conductivity, high-frequency characteristics, dimensional stability, and chemical robustness, are gaining significant attention. Among the various ceramic substrate materials, beryllium oxide is a relatively rare material. In this article, TechSparks provides a detailed exploration of this relatively uncommon material and its unique properties.
What Is Beryllium Oxide Ceramic PCB
Beryllium oxide (BeO), also known as Beryllia, is an advanced material made of Beryllium and oxygen. Beryllium (Be) is a rare element with the atomic number 4 and belongs to the alkaline earth metals group on the periodic table of elements.
Beryllium oxide is not normally found in significant quantities as a naturally occurring material and is mostly produced artificially. The primary source of Beryllium is bertrandite, a mineral that contains beryllium in the form of beryllium silicate, from which Beryllium is extracted.
Although beryllium oxide has some advantages, it may produce gases harmful to the human body at high temperatures of 300 degrees. Currently, most countries no longer use beryllium oxide to build PCB but rather for semiconductor components.
Beryllium Oxide Substrate Material Characteristics
Advantages
Mechanical
- Density: BeO has a density of around 2.85 g/cm3.
- Melting Point: The melting point is high at around 2530℃ (4586℉)
- Hardness: BeO is very hard, ranking between 8.0 and 9.0 on the Moh’s scale of hardness.
- Thermal Conductivity: tHe thermal conductivity of BeO is high ranging from 220 to 350 Watts per meter-kelvin (W/mK). This thermal conductivity rivals that of metals and it is one of Beryllium oxide’s major selling points
Chemical
- Stability: BeO is a stable material at room temperature and is resistant to corrosion by chemicals like acids and bases.
- Toxicity: BeO powder is highly toxic and should be handled with care
Electrical
- Dielectric Constant: BeO has a low dielectric constant ranging from 6 to 7. This is essential when dealing with PCBs that propagate high-frequency signals to minimize losses and improve signal integrity
- Volume Resistivity: BeO has a relatively high-volume resistivity of between 1014 to 1016 Ohm-cm. This makes BeO an excellent electric insulator
Disadvantages
Cost
BeO is more expensive than most other substrate materials, leading many users to avoid its use and resulting in a shortage of manufacturing expertise among manufacturers.
Toxicity
In our earlier discussion, we mentioned that BeO can release toxic gases at 300°C, as high temperatures cause it to decompose into boron dioxide. Inhalation of this substance can lead to chronic beryllium disease and lung cancer. While 300°C is not easily reached in everyday life, during PCB assembly, high-temperature soldering is necessary, potentially reaching this temperature, posing a severe risk to workers.
Limited Availability
BeO is a relatively rare material that is not found in large quantities this limits the amount of Beryllium oxide ceramic PCB that can be manufactured.
Do You Need Beryllium Oxide Ceramic PCB
As mentioned earlier, beryllium oxide possesses numerous advantages such as thermal conductivity, reliability, and high-frequency applicability. However, its most critical issue lies in its toxicity. In order to assist you in making a better choice for ceramic substrates, we will now compare beryllium oxide with several other commonly used materials.
| Property | BeO | Al2O3 | AlN | Si3N4 |
|---|---|---|---|---|
| Electrical Conductivity | 10^3−10^6 S/m | 10^-14−10^2 S/m | 10^-15−10^3 S/m | 10^-12−10^2 S/m |
| Dielectric Constant | 6—7 | 9—11 | 8—9 | 7—9 |
| Thermal Conductivity | 220 – 350 W/m·K | 20 – 30 W/m·K | 90 – 200 W/m·K | 20 – 50 W/m·K |
| Coefficient of Thermal Expansion | 6.5 – 7.5 ppm/°C | 7 – 9 ppm/°C | 4.5 – 5.5 ppm/°C | 2.5 – 3.5 ppm/°C |
| Chemical Stability | High | High | High | High |
| Environmental Impact | Toxic | Non-toxic | Non-toxic | Non-toxic |
Beryllium Oxide Ceramic PCB Manufacturing Process
- Preparation of Materials
The main ingredient of the Process is the Beryllium oxide powder, which is measured, weighed, and passed through quality assurance to assure adherence to specifications. When ready, the powder is mixed with a binder to form a slurry.
- Shaping
The Beryllium oxide slurry needs to be molded into a sheet to make the PCB substrate, several methods are used to shape the slurry into its final shape, they include.
- Dry pressing
- Injection molding
- Extrusion
- Drying
The resulting product after shaping, also known as the green body, still contains the liquid binder fluid that needs to be gotten rid of. The shaped slurry needs to be dried to form the desired Beryllium oxide ceramic substrate. The green body is dried in a furnace, the resulting Beryllium oxide structure is a porous white material that is fragile and needs careful handling.
- Sintering
Sintering involves heating the fragile Beryllium oxide to a temperature near its melting point which is usually above 1200℃. This causes the ceramic particles to fuse and create the material with the desired properties to be used as a substrate.
- Machining
Machining involves the use of various mechanical machines to bring the material to the final dimensions that are described by the designer of the Gerber files and other production files. These machining processes include: drilling, grinding, cutting, lapping or precision machining using CNC machines.
- Metallization
Once the beryllium oxide ceramic substrate has been shaped, it is ready to be made into a PCB by the addition of conductive material on the surface. The thin layer of a conductive material like metals such as copper, gold, and silver is used. This conductive layer will form the conductive traces that connect the various components of the PCB.
- Plating and Etching
Plating is the process of adding a conductive layer on top of an existing conductive layer to enhance the solderability and conductivity of the material. The plating material is normally thinner and has superior conductivity than the underlying material.
Etching on the other hand is the process of precisely creating the individual conductive traces. This step enhances the performance of the PCB and ensures that meets the precision that the designer expects.
- Surface Finish
The PCB is basically ready once all the conductive traces and plating layers have been added. PCB surface finish is the treatment applied to the exposed conductive material.
The three primary functions of the surface finish are; protection from corrosion due to oxidation, to improve solderability, and to provide an even surface for easy attachment of surface mount components.
Some of the most common surface finish techniques on a PCB include:
- ENIG: This surface finish consists of a layer of electroless nickel followed by a thin layer of immersion gold on top. This method provides excellent solderability and a very flat surface.
- HASL: This surface finish is achieved by applying molten solder to the PCB surface and hot air is used to level and remove excess. This is an old technology and very cost-effective.
- Immersion Tin: This process involves the immersion of the PCB in molten Tin. This creates a thin layer of thin on the exposed pads. Tin provides excellent solderability and a flat surface.
- Inspection and Testing
The BeO ceramic PCB is ready and needs to be tested for defects to ensure that it meets customer requirements, the testing process is mainly automated and uses automatic visual inspection devices and an automated testing rig that tests for continuity on all the tracks.
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