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What are the conditions for making PCB solder mask? What are the main issues?

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The production of PCB solder mask is an important part of the PCB production process. Its main function is to cover a protective film on the copper surface where soldering is not required to prevent solder paste from being applied to areas where it should not be applied, ensuring that soldering only occurs in specific areas, thereby avoiding short circuits and other welding defects. The following are some conditions and main issues for making solder mask:

Production Conditions:

Pretreatment: 

Ensure that the copper surface is clean and free of oil and dirt. Roughening treatment may be required to enhance ink adhesion.

Choose the Right Ink: 

Choose the right solder mask ink according to the specific needs and end use of the PCB. The color can be green, blue, black, white or red, etc.

Screen Printing or Spraying: 

Use the correct method to evenly cover the ink on the PCB surface.

Pre-drying: 

Remove the solvent in the silk screen ink so that the ink reaches a certain degree of dryness.

Exposure: 

Use a UV light source to cure the ink covered on the PCB. The exposure time and intensity need to be precisely controlled.

Development: 

Remove the uncured ink part to expose the copper surface.

Post-drying: 

Completely cure the solder mask to ensure its hardness and adhesion.

Main Matters:

Precise Alignment: 

Ensure that the opening of the solder mask is precisely aligned with the pad to avoid covering the solder joint or exposing the copper surface that should not be exposed.

Ink Thickness: 

Control the thickness of the solder mask. Too thick may affect the solder paste printing, and too thin may not provide adequate protection.

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Environmental Control: 

Dust and pollutants should be avoided during the solder mask production process to ensure the cleanliness of the working environment.

Avoid Bubbles and Pinholes:

Avoid bubbles and pinholes during screen printing or spraying. These defects will affect the quality and consistency of the solder mask.

Complete Curing: 

Ensure that the solder mask is fully cured to provide good physical and chemical protection.

Inspection and Testing: 

After the production is completed, carry out necessary inspections and tests, such as thickness testing, hardness testing, and adhesion testing.

Solder Mask Mmaterial: 

Select a suitable solder mask material, which can be a liquid photosensitive solder mask material or a dry film solder mask material. Select the appropriate type according to the application requirements.

Solder Mask Opening: 

For the position where welding is required, the size of the solder mask opening should be reasonably designed to ensure the feasibility of welding and the quality of the solder joints.

The production of solder mask is a delicate process that requires precise control of each step to ensure that the quality of the final product meets the design and functional requirements.

Selection of Solder Mask Material: 

The solder mask material can be liquid photosensitive solder mask material or dry film solder mask material. Liquid photosensitive solder mask material is suitable for surface mount technology, especially for fine pitch components, while dry film material may be more suitable for some applications that do not require very fine window openings.

Solder Mask Thickness: 

The thickness of the solder mask is usually between 0.07-0.1mm (0.003-0.004 inches), but surface mount PCBs may require a thinner solder mask to avoid affecting the application of solder paste.

Opening Design of Solder Mask: 

The opening design of the solder mask should be 0.15mm (0.006 inches) larger than the pad to allow a 0.07mm (0.003 inch) gap on all sides of the pad.

Environmental Control: 

The production of the solder mask needs to be carried out in a dust-free or low-dust environment to prevent dust and other contaminants from affecting the quality of the solder mask.

Ink Viscosity Adjustment: 

In the use of liquid photosensitive solder mask materials, viscosity adjustment is crucial to the quality of screen printing.

Exposure Time and Intensity: 

The exposure process needs to be precisely controlled, and overexposure or underexposure will affect the quality of the solder mask.

Development Process: 

Use appropriate chemical solutions for development to ensure that uncured ink is completely removed.

Post-processing: 

After the solder mask is completed, post-baking treatment may be required to improve the hardness and wear resistance of the solder mask.

Quality Inspection: 

Including tests of solder mask thickness, hardness, adhesion and surface tension to ensure that the solder mask meets product quality requirements.

Type of Solder Mask: 

In addition to standard solder masks, special types of solder masks such as white solder masks may be used to meet specific visual or process requirements.

Coordination of Solder Mask and Solder Paste Layer: 

The coordination of solder mask and solder paste layer (Paste Mask) needs to be considered during design to ensure that the opening of the solder paste layer corresponds correctly to the opening of the solder mask.

Maintenance of Solder Mask: 

After the solder mask is completed, it needs to be properly maintained to avoid damage during subsequent PCB processing.

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