Solder resist is applied to a PCB (Printed Circuit Board) in a multi-step process involving coating, exposure, development, and curing. Here's a breakdown:
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Cleaning and Preparation: The PCB surface is thoroughly cleaned to remove any contaminants like dust, oil, or fingerprints. This ensures proper adhesion of the solder resist.
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Applying the Solder Resist: A layer of liquid epoxy solder resist is applied to the PCB. This can be done using various methods, including:
- Screen Printing: A stencil with the desired pattern is used to selectively apply the solder resist. This is a common and cost-effective method.
- Spray Coating: The solder resist is sprayed onto the PCB, covering the entire surface. Excess resist is then removed during the development process.
- Curtain Coating: The PCB is passed through a "curtain" of solder resist, creating a uniform layer.
- Electrostatic Deposition: A charged solder resist is sprayed onto the PCB, attracted to the grounded board, providing a very even coating.
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Drying (Pre-bake): After applying the solder resist, the PCB is dried (or "pre-baked") to remove solvents and make the resist tacky enough to adhere properly during exposure.
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Exposure to UV Light: A photomask (a transparent film with opaque areas defining the solder resist pattern) is placed over the PCB. The board is then exposed to ultraviolet (UV) light. The UV light cures (hardens) the solder resist in the areas not blocked by the photomask. The areas blocked by the photomask remain uncured.
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Development: The PCB is then run through a developer solution (typically an alkaline solution). This removes the uncured solder resist, leaving the desired solder resist pattern on the board.
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Curing (Post-bake): Finally, the PCB is heat-cured (or "post-baked") to fully harden the solder resist and ensure a durable, long-lasting finish. This process improves the resist's chemical resistance, mechanical strength, and adhesion to the PCB.
In summary, solder resist application involves coating the PCB with a photosensitive epoxy resin, exposing it to UV light through a photomask to define the desired pattern, developing the unexposed areas, and curing the resist for a durable finish.
