Introduction to PCB Imaging
Printed Circuit Board (PCB) imaging is a critical step in the PCB manufacturing process. It involves transferring the circuit pattern onto the Copper-Clad Laminate board. The accuracy and precision of the imaging process directly impact the functionality and reliability of the final PCB product.
There are several methods for PCB imaging, including screen printing, inkjet printing, and photolithography. Among these, dry film photolithography has emerged as the most widely used technique due to its high resolution, excellent repeatability, and compatibility with large-scale production.
What is Dry Film Photolithography?
Dry film photolithography is a process that uses light-sensitive polymeric films to selectively expose and develop the desired circuit pattern on the copper surface of the PCB. The process involves the following key steps:
- Lamination of the dry film resist onto the copper surface
- Exposure of the resist through a photomask
- Development of the exposed resist
- Etching of the unwanted copper
- Stripping of the remaining resist
The dry film resist is a three-layer structure consisting of a polyester cover sheet, a photosensitive resist layer, and a polyethylene separator sheet. The photosensitive layer contains diazo compounds or photopolymers that undergo chemical changes upon exposure to ultraviolet (UV) light.
Advantages of Dry Film Imaging
Compared to other PCB imaging methods, dry film photolithography offers several advantages:
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High Resolution: Dry film resists can achieve fine feature sizes down to 25 microns, enabling the fabrication of high-density interconnects (HDI) and miniaturized PCBs.
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Excellent Uniformity: The dry film lamination process ensures a uniform and consistent resist thickness across the entire PCB surface, minimizing variations in the etched pattern.
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High Throughput: Dry film imaging is a high-speed process that can handle large panel sizes and multiple PCBs simultaneously, making it suitable for mass production.
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Cost-Effective: The materials and equipment used in dry film imaging are relatively inexpensive compared to other high-resolution imaging techniques like direct laser imaging (DLI).
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Environmental Friendliness: Dry film resists generate less chemical waste compared to liquid photoresists, and the process can be optimized to minimize the use of developers and strippers.
Dry Film Lamination
The first step in dry film imaging is the lamination of the resist onto the copper surface of the PCB. This is typically done using a hot roll laminator, which applies heat and pressure to bond the resist to the copper.
Lamination Parameters
The key parameters that affect the quality of dry film lamination are:
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Temperature: The lamination temperature should be high enough to soften the resist and ensure good adhesion to the copper, but not so high as to cause resist flow or deformation. Typical lamination temperatures range from 100°C to 130°C.
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Pressure: The lamination pressure should be sufficient to eliminate any air gaps or voids between the resist and the copper, but not so high as to cause resist thinning or damage to the PCB substrate. Typical lamination pressures range from 1 to 3 kg/cm².
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Speed: The lamination speed should be slow enough to allow sufficient heat transfer and bonding, but fast enough to maintain productivity. Typical lamination speeds range from 1 to 3 m/min.
| Lamination Parameter | Typical Range |
|---|---|
| Temperature | 100°C – 130°C |
| Pressure | 1 – 3 kg/cm² |
| Speed | 1 – 3 m/min |
Lamination Defects
Improper lamination can lead to various defects that affect the quality and reliability of the imaged circuit pattern, such as:
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Air Bubbles: Entrapped air between the resist and the copper can cause localized exposure and development issues, leading to circuit opens or shorts.
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Uneven Thickness: Non-uniform lamination pressure or temperature can result in variations in the resist thickness, affecting the exposure and development times and causing dimensional inaccuracies.
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Edge Lifting: Poor adhesion at the edges of the PCB can cause the resist to lift or delaminate during subsequent processing steps, exposing the copper to unintended etching.
To minimize these defects, it is important to optimize the lamination parameters for each specific resist and PCB type, and to maintain proper equipment maintenance and calibration.
Exposure and Development
After lamination, the next steps in dry film imaging are exposure and development. These steps are critical for accurately transferring the circuit pattern from the photomask to the resist.
Exposure
Exposure is typically done using a UV light source, such as a mercury vapor lamp or a metal halide lamp. The photomask, which contains the circuit pattern, is placed in contact with the laminated PCB, and the assembly is exposed to UV light for a specific duration.
The exposure time depends on several factors, including the resist thickness, the light intensity, and the desired feature size. Overexposure can cause resist hardening and difficulty in development, while underexposure can result in incomplete pattern transfer and poor resist adhesion.
| Resist Thickness | Exposure Energy (mJ/cm²) |
|---|---|
| 15 μm | 30 – 50 |
| 25 μm | 50 – 80 |
| 38 μm | 80 – 120 |
| 50 μm | 120 – 160 |
Development
After exposure, the PCB is immersed in a developer solution, which selectively dissolves the unexposed areas of the resist, revealing the underlying copper. The developer solution is typically an aqueous alkaline solution, such as sodium carbonate or potassium carbonate.
The development time and temperature are critical parameters that affect the quality of the developed image. Overdevelopment can cause resist erosion and loss of fine features, while underdevelopment can lead to residual resist in the unexposed areas.
| Developer Concentration | Development Time (seconds) | Temperature (°C) |
|---|---|---|
| 0.5 – 1.0% | 30 – 60 | 25 – 35 |
| 1.0 – 1.5% | 20 – 40 | 25 – 35 |
| 1.5 – 2.0% | 10 – 30 | 25 – 35 |
After development, the PCB is rinsed with water to remove any remaining developer and resist residues, and then dried using compressed air or a hot air dryer.
Post-Development Inspection
After development, it is important to inspect the PCB for any defects or irregularities in the resist pattern. Common post-development defects include:
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Incomplete Development: Residual resist in the unexposed areas can cause shorts or poor copper etching.
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Overetching: Excessive development can cause resist erosion and loss of fine features, resulting in open circuits or reduced conductor width.
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Underetching: Insufficient development can lead to residual resist in the exposed areas, causing incomplete copper removal during etching.
Post-development inspection can be done visually using a microscope or by using automated optical inspection (AOI) systems that can detect and classify defects based on predefined criteria.
Etching and Stripping
After the resist pattern is developed, the next steps are etching and stripping. These steps remove the unwanted copper and the remaining resist, respectively, leaving behind the final circuit pattern.
Etching
Etching is typically done using an acidic copper etchant, such as ferric chloride or acidified cupric chloride. The PCB is immersed in the etchant solution, which selectively dissolves the exposed copper while leaving the resist-protected areas intact.
The etching time depends on the etchant concentration, temperature, and the copper thickness. Overetching can cause undercutting of the resist and loss of fine features, while underetching can result in residual copper between the conductors.
| Copper Thickness | Etchant Concentration | Etching Time (minutes) | Temperature (°C) |
|---|---|---|---|
| 18 μm | 30 – 40 Baume | 1 – 2 | 45 – 55 |
| 35 μm | 30 – 40 Baume | 2 – 3 | 45 – 55 |
| 70 μm | 30 – 40 Baume | 4 – 6 | 45 – 55 |
After etching, the PCB is rinsed with water to remove any remaining etchant and copper residues.
Stripping
Stripping is the final step in the dry film imaging process, where the remaining resist is removed from the PCB surface, exposing the final circuit pattern. Stripping is typically done using an alkaline stripping solution, such as sodium hydroxide or potassium hydroxide.
The stripping time depends on the resist type, thickness, and the stripper concentration and temperature. Overstripping can cause damage to the copper conductors or the PCB substrate, while understripping can leave residual resist on the surface.
| Resist Thickness | Stripper Concentration | Stripping Time (minutes) | Temperature (°C) |
|---|---|---|---|
| 15 μm | 2 – 5% | 1 – 2 | 40 – 60 |
| 25 μm | 2 – 5% | 2 – 3 | 40 – 60 |
| 38 μm | 2 – 5% | 3 – 4 | 40 – 60 |
| 50 μm | 2 – 5% | 4 – 5 | 40 – 60 |
After stripping, the PCB is rinsed with water and dried using compressed air or a hot air dryer. The final PCB is then inspected for any defects or irregularities before proceeding to the next steps of the manufacturing process, such as soldermask application and surface finishing.
Frequently Asked Questions (FAQ)
- What is the difference between dry film and liquid photoresist?
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Dry film photoresist is a solid, polymeric film that is laminated onto the PCB surface, while liquid photoresist is a liquid that is coated onto the PCB surface by spinning or spraying. Dry film offers better thickness uniformity and ease of handling, while liquid photoresist offers higher resolution and conformity to surface irregularities.
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Can dry film imaging be used for both inner and outer layers of a multilayer PCB?
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Yes, dry film imaging can be used for both inner and outer layers of a multilayer PCB. However, the process parameters and resist types may differ depending on the layer position and the desired feature sizes.
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What is the typical resolution of dry film imaging?
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Dry film imaging can achieve feature sizes down to 25 microns, depending on the resist type, thickness, and exposure and development conditions. Higher resolutions can be achieved using thinner resists and optimized processing parameters.
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How does dry film imaging compare to other PCB imaging methods in terms of cost?
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Dry film imaging is generally more cost-effective than other high-resolution imaging methods, such as direct laser imaging (DLI), due to the lower equipment and material costs. However, for very high-volume production or very fine feature sizes, other methods may be more economical.
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What are the environmental considerations for dry film imaging?
- Dry film imaging generates less chemical waste compared to liquid photoresist processes, as the resist is applied as a solid film rather than a liquid. However, the process still involves the use of developers and strippers that must be properly handled and disposed of in accordance with local regulations. Many dry film resists and processing chemicals are now available in more environmentally friendly formulations that minimize the use of hazardous substances.
Conclusion
Dry film imaging is a critical process in the manufacturing of high-quality and reliable PCBs. By selectively exposing and developing a photosensitive resist, the desired circuit pattern can be accurately transferred onto the copper surface of the PCB.
The key steps in dry film imaging include lamination, exposure, development, etching, and stripping, each of which requires careful control of the process parameters to ensure optimal results.
Compared to other PCB imaging methods, dry film photolithography offers several advantages, including high resolution, excellent uniformity, high throughput, cost-effectiveness, and environmental friendliness.
As PCB designs continue to evolve towards higher densities and smaller features, dry film imaging will remain a vital tool for the PCB manufacturing industry, enabling the production of advanced electronic devices and systems.
