The minimum size for a solder mask, specifically the width of the solder dam between mask openings, is 100 microns (4 mils).
A solder mask is a protective layer applied to a printed circuit board (PCB) to prevent solder bridges during the soldering process. The "solder dam" refers to the narrow strip of solder mask material between two pads or components. If this dam is too small, it can flake off, leading to manufacturing defects.
Here's a breakdown of why this minimum size is important:
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Preventing Solder Mask Slivers: Narrow solder mask segments, especially between closely spaced surface mount device (SMD) pads, are prone to becoming "solder mask slivers." These are small pieces of solder mask that detach from the board.
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Manufacturing Reliability: A minimum width of 100 microns ensures the solder mask adheres properly and can withstand the heat and chemicals involved in the soldering process. This reduces the risk of defects and improves the overall reliability of the PCB.
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Design Considerations: PCB designers must adhere to this minimum width specification to ensure manufacturability. Violating this rule can lead to increased manufacturing costs due to rework or scrapped boards.
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Material Properties: The type of solder mask material used can influence the minimum size requirements. Some materials may be more prone to flaking than others. Always consult with your PCB manufacturer for their specific recommendations.
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Manufacturing Tolerances: PCB manufacturing processes have inherent tolerances. Specifying a larger solder dam width than the absolute minimum provides a safety margin to account for these variations.
In summary, adhering to a minimum solder dam width of 100 microns (4 mils) is crucial for preventing solder mask slivers, ensuring manufacturing reliability, and minimizing defects in PCB assembly.
