Ceramic printed circuit boards for aerospace applications

Ceramic materials have been especially useful for highly reliable electronic applications. In the 19th century, ceramic material applications were the standards for isolators and light bulb sockets as well as the development of the use of high-technology application ceramics in radio tubes, early pacemakers and military electronics extensively used in the 1930s.

Since then, increased manufacturing technologies have enhanced the material class incredibly from plain materials through to new mixtures and nanotechnology, to the level of today’s technical ceramics.

Properties and materials

Compared to the earlier standard ceramic materials, new technical ceramics have improved on their durability, inertness and chemical characteristics. Even the physical properties have undergone various changes. For example, they do not shatter as easily as they would have previously — a common issue with previous ceramic applications. In most application cases, specifically for applications in aerospace, there are a large variety of purposes for ceramics as the appropriate material system for printed circuit boards. However, it must be noted that ceramic materials are only a category of material and are not associated with the technology or a specific chemistry utilised in their application and function.

Ceramics are a large group of technical materials providing good opportunities for enabling advanced requirements. The greatest advantage of ceramic materials is their thermal mechanical behaviour. Among its thermal characteristics is the coefficient of expansion, thermal conductivity, thermal capacity, ageing under the influence of thermal cycling and the ability to withstand higher temperatures. The above characteristics are advantageous to electronic applications, especially for aerospace. For instance, unlike polymers and epoxies, ceramic materials do not show decomposition, and their chemical bonding does not break down from heat and UV radiation compared to other substances such as organics. Moreover, ceramics do not soak or absorb humidity in a significant scale and do not outgas in the extreme vacuum of deep space.

Function

In comparison with FR type PCBs, ceramic materials need structuring for electronic functionalities. This requires different technologies and the use of other materials. For instance, PCBs made of ceramic and copper may use alumina or aluminium nitride covered by copper foils using epoxy adhesives. While this is beneficial to most applications, this would not be beneficial in various thermal applications. This and other restrictions have led to product solutions such as direct bonded copper (DBC), including comparable covering techniques for aluminium nitrate, which is widely used for power chips such as an insulated-gate bipolar transmitter.

Materials and layer stack up

Alumina (Al203) is the most cost-effective ceramic material and also the most commonly used as it has great thermal conductivity at 24 to -28 W/mK, compared to metal core PCBs which have 1 to 4 W/mK.

Copper — 70 µm

Al203 — Alumina 0.5 mm

Copper — 70 µm

                            0.65 mm ±0.1 mm

Aluminium nitrate (AlN) provides superior thermal performance at 140 to 170 W/mK also a much higher raw material cost and is generally only designed in the highest of technology products.

Copper — 70 µm

AlN — Aluminium nitrate 0.5 mm

Copper — 70 µm

                            0.65 mm ±0.1 mm

Aerospace application

Aerospace applications usually do not have reduction, as their main target as the use of ceramic PCBs is mainly as a base for power dominated technology. To directly benefit from this group of materials, engineers and designers must have full knowledge and understanding of the limits and restrictions these materials possess and how they interact. The evaluation of necessary process conditions in combination with calculations and balancing of the advantages and disadvantages is required when deciding if the use of ceramics is beneficial to the required application.

Some advantageous characteristics of ceramic materials for electronics in aerospace include:

• Coefficient of thermal expansion — very close to silicon and far below that of most usual metals.
• Excellent electrical isolation — even in elevated temperatures and over lifetime.
• Good thermal conductivity as an isolator — useful for heat spreading and distribution.
• Stable dielectric properties and low losses at high frequencies.
• Chemical stability against many chemicals, moisture, solvents and consumables.
• Very slow ageing due to consistency of substance.
• Compatibility to noble metal paste sintering technology — resulting in highly reliable conductors.
• High processing temperatures — far removed from normal operating range.
• Thermal resistance — showing no classic melting, decomposition or softening.
• Mechanical stiffness — allowing rigid carriers, hardness and wear resistance for sensors working in vacuum, fluids and in industrial pollution.
• Resistance to EUV, plasma and ion bombardment as well as practically no outgassing in high vacuum, ideal for sensors for EUV semiconductor equipment.

Conclusion

At PCB Global, we have the technology and the capabilities not only to fabricate ceramic PCBs, but to also assist you with any design specifications you may have regarding the application, use and outcome of the purpose of your ceramic PCB. For any enquiries or if you would like to arrange a quote for your ceramic PCB, please don’t hesitate to contact us as sales@pcbglobal.com or through our online contact portal: www.pcbglobal.com/contact-us.html.

Top image credit: ©stock.adobe.com/au/jules