Blog  /  Transparent PCB: See-Through Circuit Board with High Optical Transmissivity

Transparent PCB: See-Through Circuit Board with High Optical Transmissivity

Emerging electrical and electronic devices have different PCB requirements. Therefore, it is not uncommon to spot aluminum PCBs, ceramic, and transparent boards placed inside them. Transparent PCBs are no longer new in the market. However, they are vital in new electrical devices and technologies like 5G.

If you have a project requiring this see-through circuit board, it is vital to understand what it is all about. We have covered this in detail below, so take a look!

 

What is a Transparent PCB?

 

A transparent PCB is a see-through circuit board made using either ceramic or glass, providing superb optical fields. Also, it can have 360° luminescence and a rendering index value of 140lmw or higher. The PCB does not experience light attenuation and requires no heat sink. Also, it contains transparent or translucent components and conductive traces.

 

A transparent PCB

A transparent PCB

 

Transparent PCB Substrate

 

As stated above, transparent PCBs can have a glass or ceramic substrate. Ceramic material transparent substrates have a higher thermal conductivity than their glass counterparts. However, glass PCB substrates offer better optical transmissivity than the ceramic type. The optical transmissivity of glass PCB substrates is about 99.5-99%, while that of ceramic ranges from 75-80%.

 

The Base Materials of Transparent PCBs

 

Transparent PCBs can be rigid or flexible. For instance, the rigid ceramic PCBs contain an aluminum substrate, usually in the form of the following:

  • Aluminum oxide
  • An amalgam of magnesium and aluminum
  • Magnesium oxide
  • and more

These materials assure roughly 80% transparency. Also, an oily, water-based nano-blue phase alumina enhances the hardness of the resin materials from 6H to 8H.

However, the base materials in the flexible type of transparent PCBs are polyimide and polyester. These have almost 95% transparency.

 

A transparent flexible PCB

A transparent flexible PCB

Source: Wikimedia Commons

Flexibility does not mean the boards are weak. On the contrary, the Polyimide-based types are strong enough to handle mechanical and electrical stresses. Additionally, they are resistant to thermal pressure.

 

Clear PCB: Transparent Glass PCB

 

Glass is a material made using a mix of soda, limestone, ash, and sand. Additionally, these materials are abundant in nature and easily recyclable. Therefore, glass is an eco-friendly material.

However, regular glass is not very sturdy, so you will most likely see tempered glass. Tempered glass is a reprocessed and prestressed glass to maximize durability. Besides, it has better shock and impact absorbing capabilities, making it useful for extreme applications.

 

Tempered glass

Tempered glass

Source: Wikimedia Commons

But tempered glass isn't the sturdiest glass variety. Sapphire glass is the strongest and most durable of them all. Therefore, it is the go-to glass PCB material for high-end electronics and space/satellite devices because of its thermal properties and high dielectrics.

 

Sapphire glass ingots

Sapphire glass ingots

Source: Wikimedia Commons

 

In addition, glass transparent PCBs have a 360° luminescence. Therefore, light can flow out in all directions.

 

Benefits of Transparent Glass PCBs

 

  • Extremely flat (ideal for symmetrical functionality)
  • High heat resistance
  • Resistant to deformation
  • Highly resistant to wear and tear
  • Cheap, recyclable raw materials
  • Highly transparent

 

Clear PCB: Ceramic Transparent Circuit Boards

 

PCB manufacturers usually make transparent ceramic circuit boards using materials like aluminum oxide, alumina, magnesium oxide, al-mg spinel, etc. These boards have a transparent substrate and a glass-based solder mask to help maintain the substrate's transparency at 75-85%. However, their conductors and conductive traces are not transparent.

 

Advantages of Ceramic Transparent Circuit Boards

 

  • High thermal conductivity (24-31.4 W/mK)
  • Low and stable thermal expansion coefficient
  • Highly temperature resistant (over 2000°C)
  • No water absorption with superb electrical insulation
  • Multiple optical features

 

Applications of Transparent Ceramic Circuit Boards

 

  • High power LEDs
  • Optical sensors
  • Aircraft and missile infrared windows
  • High-pressure sodium lamps

 

A high-pressure sodium vapor street lamp

A high-pressure sodium vapor street lamp

Source: Wikimedia Commons

 

Clear PCB: Transparent Flexible PCBs

 

Transparent flexible circuit boards contain PET thin films that are either light blue, white, transparent, light green, or other colors. However, the pads and copper traces in these PCBs are not see-through.

 

A transparent flexible PCB

A transparent flexible PCB

But there is a variant that has transparent copper traces. So, making these boards requires a unique technique that involves clamping the circuit between two PET films.

 

Advantages of Transparent Flexible PCBs

 

  • Provides multiple optical features
  • Bends, twists, folds, and moves arbitrarily in 3D space
  • 90% lighter than rigid PCBs
  • Nano/micro level accurateness in PCB manufacturing
  • Ideal for miniaturization, thinning and reducing product weight

 

A clear flexible PCB with copper traces

A clear flexible PCB with copper traces

Transparent Flexible PCB Applications

 

  • ABS systems
  • Automatic control
  • Mobile phones
  • Cameras
  • Instrument panels
  • Medical devices
  • Smart contact lenses
  • Electronic skins

 

An electronic skin with transparent and opaque PCBs

An electronic skin with transparent and opaque PCBs

Source: Network Tigers

How to Make a PCB Transparent?

Since it is unlike the conventional circuit board, you need to use the following steps to make a PCB transparent.

  • First, arrange the required materials (copper foil, ferric chloride, polyester, etc.).
  • Second, prepare the circuit design layout and print it.
  • Third, fabricate the substrate using transparent materials.
  • Fourthly, add copper foil and apply the photoresist.
  • Next, expose the setup to sunlight or an artificial light source.
  • Then, develop the photoresist.

 

Photoresist development after light exposure

Photoresist development after light exposure

Source: Wikimedia Commons

  • After that, etch the board using a ferric chloride solution to eliminate the unwanted copper.
  • Finally, wash and dry the PCB.

 

Design Rules for Transparent Circuit Boards

Transparent PCBs require the following critical design rules to optimize their performance.

  • First, a high volume transparent flexible PCB should have a panel dimension of over 100mm (for PCBs having more than 50 panels)
  • Secondly, the ideal board thickness is 0.38mm
  • Metallization should be a silver-palladium alloy
  • A panelized transparent board should have a five and one-ounce conductor thickness
  • Immersion gold and ENIG are the best surface finishes for these PCBs
  • The standard solder resist minimum opening should be 0.2mm
  • 5-12 mils via diameter
  • 3-12 mils minimum via spacing
  • Short and direct trace length
  • Include a power plane for smooth ground and power plane distribution
  • Also, keep the board components and related test points together
  • Less than 4mm dimensional tolerance
  • 10% and 4 mils thickness and hole tolerances, respectively
  • Less than 0.5 micrometers of surface roughness
  • 0.8 mils minimum line spacing
  • Use decoupling capacitors to minimize the noise in high-speed signal transmission applications
  • Design rules might change, so check regularly

Summary

In conclusion, a transparent PCB can enhance the design quality, durability, and functionality of different electronic products. Consumer electronic devices also benefit immensely from the properties of flexible PCBs, making them thinner and lighter. However, these see-through boards require special attention when designing and manufacturing. If you need one for your project, contact us for more details. We have the expertise and experience to build transparent PCBs in line with industry standards.