Transparent glass technology is bringing revolutionary changes in the printed circuit board or PCB industry lately. Various LED and LCD devices are using transparent PCBs for their optical characteristics. However, the latest use of transparent PCBs is in the 5G network, and this is likely to change the communication scenario forever.
Transparent PCB Substrates
At present, the industry uses two main substrates for transparent PCBs — glass and ceramic. Transparent substrates based on ceramic materials offers higher thermal conductivity compared to those made from glass. However, the glass substrate offers higher optical transmissivity. The transmissivity of a ceramic transparent substrate is about 75 – 80%, while that for a glass transparent substrate is about 99.5 – 99.99%.
Glass is typically derived from a mix of sand, ash, limestone, and soda. These four materials are abundantly available in nature, and therefore, they are eco-friendly and recyclable. The most common make of glass is the tempered glass. This is reprocessed and pre-stressed glass with high durability. In addition, tempered glass has high capacity for absorbing shock and impact.
Sapphire glass is the strongest and hardest of all glass variants. High-end electronic equipment use sapphire glass for its high dielectrics and thermal properties. This type of glass is useful even in devices meant for use in satellites and space.
Printed circuit boards manufactured from transparent glass can transmit light in almost all directions. Therefore, it is possible to see light through the transparent PCB from almost all sides.
Advantages of Glass PCB Substrate
A glass PCB substrate offer many advantages:
- Highly resistant to wear and tear
- Highly resistant to heat
- Highly resistant to deformation
- Highly transparent
- Extremely flat surface
Although the glass PCB substrate is transparent, copper traces on the PCB are not. Therefore, copper traces on the PCB are visible through the glass. However, some manufacturers are using new technology that makes copper traces also transparent, like the glass substrates are.
Because of the characteristics of the glass substrate, glass PCBs subjected to high temperatures for long duration show very low rate of deformation. This is one of the most desirable characteristics of glass substrates for PCBs. Glass can withstand temperatures of about 1000 °C, and since the raw materials are abundantly available, transparent PCBs are easy to manufacture. This also makes glass PCBs cheaper than ceramic ones.
Transparent Flexible PCBs
Using PET thin films, PCB manufacturers are able to make transparent flexible PCBs. They make them in several colors like transparent, light blue, white, light green, and more. The transparent PET materials allow making transparent flexible PCBs that are different from regular flexible PCBs.
Fully Transparent PCBs
Special innovative techniques allow making the conductive traces also completely transparent. The fabricator clamps the circuit with conductive traces between two PET films to protect it and making it totally invisible from the outside.
Advantages of Transparent Flexible PCBs
- Offers many optical features
- Possible to fold, bend, twist, and move arbitrarily in 3-dimensional space
- Dimensions are very stable
- Reduces weight of PCB by about 90% compared to regular rigid PCBs of same dimensions.
- Useful for reducing form factor, weight, and for miniaturization of products.
Applications of Transparent Flexible PCBs
Flexible transparent PCBs have many uses. Applications include medical devices, electronic calculators, mobile phones, cameras, ABS systems, instrument panels, automatic control systems, and for PCB requirements in restricted spaces. Applications of transparent flexible PCBs also include smart contact lenses, electronic skins, and flexible transparent electronic devices.
Transparent Ceramic Circuit Boards
Manufacturers make transparent ceramic circuit boards from various materials like Al-Mg spinel, Aron, yttrium oxide, magnesium oxide, and alumina or aluminum oxide. For instance, they use fine alumina powder at nano levels, adding it to resin water liquids at 5 – 10% levels. This helps to increase the resin hardness from 6H to 8H. Some manufacturers use oil-based alumina liquid, while others use water-based alumina liquid to manufacture transparent ceramic circuit boards.
Most transparent ceramic PCBs use transparent substrates with a solder mask based on glass. However, the conductors are not transparent. The transparent substrate have about 75 – 85% transparency. The glass solder mask covering the traces helps in maintaining the transparency of the substrate.
Advantages of Transparent Ceramic Circuit Boards
- Offers many optical features
- High temperature resistance. For instance, 2000+ °C is normal. Glass transition temperature of thorium oxide and yttrium oxide circuit boards can reach 3100 °C.
- High thermal conductivity of over 24 – 31.4 W/mK.
- Very low and very stable coefficient of thermal expansion
- High electrical insulation
- Very low water absorption
Applications of Transparent Ceramic Circuit Boards
Applications of transparent ceramic circuit boards includes optical sensors, high power LEDs, high pressure sodium vapor lamps, infrared windows in missiles, aircraft etc.
Conclusion
several revolutionary changes are occurring within the PCB industry. Manufacturers are using many innovative techniques to meet the needs and requirements of different electronic devices. One of these innovative techniques is the development of transparent PCBs. With the advent of 5G, LCDs, and LEDs, we will be seeing more uses of transparent PCBs.