Printed circuit boards (PCBs) are ubiquitous in modern life, integral to high-tech devices, computing tools, and household appliances. They come in various types and are manufactured using diverse methods. Traditional PCB production involves designing a copper-clad board and selectively etching away excess copper. However, the ability to create custom copper boards opens up creative possibilities. Glass PCBs represent a unique category within this diverse landscape of PCB types.
What is a glass substrate?
Glass substrates form the core of numerous optical devices. They are meticulously ground, coated, and polished to create mirrors and lenses. Glass substrates offer exceptional thermal stability and uniformity, crucial for maintaining optical transmission quality. They are utilized across a spectrum of sizes, from microns thick to meters in diameter.
Glass PCB
Glass PCBs are commonly used in LCD and LED technologies. These boards are crafted from raw glass materials. In manufacturing glass PCBs, the circuit pattern is transferred onto a copper board using a UV curable mask. UV curable masks are preferred for their ability to achieve precise results, especially with narrow track widths, making them suitable even for mass production.
During the process, a thin layer of UV curable etching resist is applied to the copper board. An opaque film, containing the circuit image, is placed over the resist-coated board and exposed to UV light. This exposure cures the resist in the areas where the circuit is designed. Subsequently, the uncured resist is removed using a developer solution, which dissolves the unexposed parts of the resist while leaving the cured areas intact.
UV resist films are categorized into positive and negative types. In this context, a negative resist is utilized, where the exposed portions of the resist remain after development, forming the circuit pattern.
The layout of Circuit
Once the material is ready, begin by creating a schematic diagram of your circuit, followed by designing the PCB layout. To transfer this layout onto a clean sheet, typically OHP (Overhead Projector) paper is used. For routing the circuit paths, utilize the auto-routing tool, which may initially appear complex for beginners but becomes manageable with practice.
In PCB design, only the tracks (routes) and pads (connection points) are essential. Once satisfied with the design, isolate the tracks and pads layer in the layer palette and export the monochromatic image as a PNG file.
For our process, we use a negative type of photoresist. This requires exposing the resist to light in areas where we intend to retain the copper traces. When exporting the image, select the “white on black” option to ensure the tracks and pads appear white against a black background. This format ensures clarity, as white areas on OHP paper remain transparent after printing.
Printing and Layout
The objective of the OHP print is to create a mask that effectively blocks UV light exposure to undesired areas of the resist. To achieve this, ensure that the black areas of the OHP print completely block UV light to prevent exposure. For optimal blocking, three prints are aligned and securely bonded together to maintain stability during the exposure process.
Adhering Glass to Copper Foil
When using copper foil on glass, it is essential to securely bond the copper to the glass surface. Start by thoroughly cleaning both the glass and the copper foil with rubbing alcohol to ensure proper adhesion and prevent copper delamination. Apply a generous layer of adhesive evenly onto the glass surface, then firmly press the copper foil onto the adhesive. Remove any air bubbles by applying uniform pressure across the foil surface. Trim excess foil as necessary and allow sufficient time for the adhesive to cure completely.
Applying Photoresist
Begin by cutting the photoresist to the required size. The photoresist film is sandwiched between two protective covers, which must be carefully removed before application. Securely adhere the exposed side of the photoresist film onto the copper surface. Use strips of scotch tape on both sides of the film to facilitate easy removal of the protective covers. Press the photoresist firmly onto the copper to ensure complete adhesion and eliminate any trapped air bubbles between the film and the copper surface.
Setting Up for Light Exposure
Position the previously prepared OHP print onto the copper board, ensuring correct orientation to prevent mirrored prints. Place a piece of glass over the setup to secure the OHP print firmly onto the copper surface. Use clips to stabilize the arrangement and prevent movement during exposure.
UV Light Exposure
Expose the setup to UV light, either from a dedicated UV source or sunlight. If using sunlight, approximately 5 minutes of exposure is typically sufficient. Maintain stability of the setup throughout the exposure process using clips to prevent any movement.
After exposure, carefully remove the setup from the light source and dismantle the arrangement. The photoresist should show signs of drying, indicating successful exposure.
Developing the Photoresist
After applying the photoresist, there is a protective film covering it which needs to be removed. Use scotch tape to carefully peel off this protective cover. Prepare a developer solution using baking soda or a similar alkaline substance dissolved in water. Immerse the board in the developer solution for about a minute, then gently rinse it under running water. The unexposed areas of the photoresist will wash away, revealing the cured resist tracks. Repeat the rinsing process until all exposed areas are fully developed, leaving behind the desired circuit pattern on the copper board.
Etching
Prepare a solution of ferric chloride by dissolving ferric chloride powder in approximately 150ml of water until the solution turns black. Add more ferric chloride if necessary to ensure effectiveness. Submerge the copper board into the ferric chloride solution and agitate it regularly to facilitate even etching. After about 10-15 minutes, all unwanted copper should be dissolved, leaving only the protected copper tracks intact.
Final result
Remove the remaining photoresist from the copper tracks using acetone or warm water. PCBs fabricated on glass substrates may not offer electrical advantages but are suitable for applications requiring transparency. Applications like LED placements on such circuits can benefit from their unique properties.
Advantages of glass PCB
With the 360-degree light-emitting package and clear glass invisible wire decorating, transparent glass PCB is utilized in LED, 5G, LCD, and other applications.
The glass substrate has distinct advantages in terms of flatness, transparency, deformation, heat resistance, tear resistance, and so on; the deformation rate is very low when working at high temperatures for extended periods; the glass PCB can emit 360-degree luminescence, with an 80 color rendering index of 140lmw or more; it does not require a heat sink, and there is no light attenuation. Currently, glass PCB is extensively utilized in LED displays, solar panels, 3D printers, and other applications.