Innerlayer imaging for multilayer PCB

What is Innerlayer Imaging in PCB Manufacturing?

Innerlayer imaging, also known as primary imaging, is a critical step in the manufacturing process for multilayer printed circuit boards (PCBs). It involves transferring the circuit patterns onto the inner copper layers of the board before laminating them together into the final multilayer stack.

Accurate and high-quality innerlayer imaging is essential for ensuring the electrical performance, reliability, and functionality of the finished PCB. Any defects or inaccuracies in the imaged circuit patterns can lead to short circuits, open circuits, or other issues that compromise the board’s integrity.

Key Aspects of Innerlayer Imaging

1. Photoresist coating: The innerlayer copper surfaces are coated with a light-sensitive photoresist material, which hardens when exposed to UV light.

2. Exposure: The coated panels are aligned with photographic films or digital artwork containing the circuit patterns, then exposed to UV light. The light passes through the clear areas of the artwork, hardening the corresponding areas of the photoresist.

3. Developing: The unexposed portions of the photoresist are dissolved away using a chemical developer solution, leaving behind the hardened resist in the shape of the circuit pattern.

4. Etching: The exposed copper areas not protected by the hardened photoresist are etched away using an acidic solution, typically cupric chloride or ammoniacal. This leaves behind the copper traces that form the actual circuitry.

5. Resist stripping: After etching, the remaining hardened photoresist is stripped away using a chemical solution, revealing the final copper pattern on the innerlayer.

Photoresist Types for Innerlayer Imaging

There are two main types of photoresists used in innerlayer imaging:

1. Dry film photoresist (DFR): This is a solid, film-based resist that comes in rolls. It is laminated onto the copper surface using heat and pressure. DFR offers good conformance to surface topography and is suitable for fine-pitch designs.

2. Liquid photoresist (LPI): This is a liquid resist applied to the copper surface by dip coating, roller coating, or spraying. After application, the panel is baked to remove the solvent and harden the resist. LPI provides excellent resolution and is often used for ultra-fine pitch designs.

The choice between DFR and LPI depends on factors such as the desired feature size, Aspect Ratio, surface topography, and manufacturing capabilities.

Comparison of Dry Film and Liquid Photoresists

Exposure Systems for Innerlayer Imaging

The exposure step in innerlayer imaging can be performed using either conventional UV exposure units with photographic films or direct imaging (DI) systems with digital artwork.

Conventional UV Exposure

In conventional exposure, the photoresist-coated panel is aligned with a photographic film containing the circuit pattern, then exposed to UV light. The light passes through the clear areas of the film, hardening the corresponding areas of the photoresist.

Advantages of conventional exposure:
– Lower equipment cost compared to DI systems
– Well-established and widely used technology

Disadvantages of conventional exposure:
– Requires the creation and handling of physical photographic films
– Lower resolution and accuracy compared to DI systems
– Potential for misalignment and registration errors

Direct Imaging (DI)

Direct imaging systems use digital artwork instead of physical films. The circuit pattern is directly projected onto the photoresist-coated panel using a high-resolution UV laser or LED light source.

Advantages of direct imaging:
– Eliminates the need for photographic films, reducing material costs and handling
– Higher resolution and accuracy compared to conventional exposure
– Improved registration and alignment, especially for multilayer designs
– Faster setup and changeover times
– Enables quick design changes and prototyping

Disadvantages of direct imaging:
– Higher equipment cost compared to conventional exposure units
– Requires specialized software and data preparation

Developing and Etching

After exposure, the next steps in innerlayer imaging are developing and etching.

Developing

The developing process removes the unexposed portions of the photoresist, leaving behind the hardened resist in the shape of the circuit pattern. This is typically done using a chemical developer solution, such as sodium carbonate or potassium carbonate.

Key factors in the developing process:
– Developer concentration and temperature
– Developing time
– Agitation or spraying of the developer solution
– Rinsing and drying of the developed panel

Proper control of these factors is essential for achieving a clean and well-defined resist pattern.

Etching

After developing, the exposed copper areas not protected by the hardened photoresist are etched away using an acidic solution. The most common etchants used in PCB manufacturing are:

• Cupric chloride (acidic)
• Ammoniacal (alkaline)

The etching process is carefully controlled to ensure complete removal of the unwanted copper while minimizing undercut and maintaining the desired trace widths and spacings.

Key factors in the etching process:
– Etchant concentration and temperature
– Etching time
– Agitation or spraying of the etchant solution
– Rinsing and drying of the etched panel

After etching, the remaining hardened photoresist is stripped away using a chemical solution, revealing the final copper pattern on the innerlayer.

Quality Control and Inspection

Ensuring the quality and accuracy of the imaged innerlayers is crucial for the overall performance and reliability of the Multilayer PCB. Several inspection and quality control methods are used throughout the innerlayer imaging process:

1. Visual inspection: Operators visually check the innerlayers for defects such as incomplete etching, shorts, opens, or resist residues.

2. Automated optical inspection (AOI): AOI systems use high-resolution cameras and image processing algorithms to detect and classify defects on the innerlayers.

3. Electrical testing: Continuity and isolation testing can be performed on the innerlayers to verify the electrical integrity of the circuits.

4. Microsectioning: Cross-sections of the innerlayers can be examined under a microscope to assess the quality of the resist profile, etch uniformity, and trace geometries.

Implementing a robust quality control system helps identify and address any issues early in the manufacturing process, reducing the risk of defects in the final multilayer PCB.

Frequently Asked Questions (FAQ)

1. What is the difference between innerlayer imaging and outerlayer imaging?
   Innerlayer imaging refers to the process of transferring circuit patterns onto the inner copper layers of a multilayer PCB, while outerlayer imaging is done on the outer layers of the board. Innerlayer imaging is performed before laminating the layers together, while outerlayer imaging is done after lamination and drilling.

2. Can both dry film and liquid photoresists be used for innerlayer imaging?
   Yes, both dry film (DFR) and liquid (LPI) photoresists can be used for innerlayer imaging. The choice depends on factors such as the desired feature size, aspect ratio, surface topography, and manufacturing capabilities.

3. What are the advantages of using direct imaging (DI) over conventional exposure for innerlayer imaging?
   Direct imaging offers several advantages over conventional exposure, including higher resolution and accuracy, improved registration and alignment, faster setup and changeover times, and the elimination of physical photographic films. However, DI systems have a higher equipment cost compared to conventional exposure units.

4. How does the etching process work in innerlayer imaging?
   During the etching process, the exposed copper areas not protected by the hardened photoresist are chemically removed using an acidic solution, typically cupric chloride or ammoniacal. This leaves behind the copper traces that form the actual circuitry on the innerlayer.

5. What quality control methods are used to ensure the accuracy of imaged innerlayers?
   Several quality control methods are used, including visual inspection, automated optical inspection (AOI), electrical testing, and microsectioning. These methods help identify and address any defects or issues early in the manufacturing process, ensuring the quality and reliability of the final multilayer PCB.

Conclusion

Innerlayer imaging is a critical step in the manufacturing of Multilayer PCBs, directly impacting the quality, reliability, and performance of the final product. By understanding the key aspects of photoresist selection, exposure systems, developing and etching processes, and quality control methods, PCB Manufacturers can optimize their innerlayer imaging capabilities to meet the ever-increasing demands of modern electronic devices.

As technology advances and circuit designs become more complex, the importance of accurate and high-resolution innerlayer imaging will only continue to grow. Investing in state-of-the-art equipment, processes, and quality control systems will be essential for PCB manufacturers to stay competitive in this rapidly evolving industry.