Causes and elimination of spray spots and moist spots (mottling) during the coating process

of the film and reduce the pass rate of the product. This article will discuss the causes of spray spots and moist spots (variegations) and how to eliminate them.

Reasons for the formation of spray spots

There are several reasons for spray spots during the coating process:

1. The coating material is not high in purity and contains many impurities. These impurities cannot be removed during the pre-melting process. The impurities splash on the surface of the workpiece during the evaporation process to form spray spots.

2. The material is relatively moist. The electron gun spot cannot melt all the materials on the surface during pre-melting, and it is easy to produce spray spots during the evaporation process (this situation is more likely to occur when the domestic electron gun is used to plate MgF2 and some directly sublimated materials).

3. The pre-melting of the material before coating is not sufficient, and the fine particles in the material splash on the surface of the workpiece during the evaporation process to form spray spots.

4. During the coating process, the spray spots caused by the material splash caused by the electron gun beam flow.

Reasons for the formation of wet spots:

1. The cleaning solution is not well-proportioned and contains a lot of water. There are residues on the surface of the workpiece after cleaning, and mottling is formed after coating.

2. Standardize the use and storage of materials, (drying tank, desiccant) to ensure that the materials are not damp, and the properties of the materials do not change.

3. The water contained in the workpiece itself (detailed explanation of the white sheet production process of the workpiece) escapes during the baking process before vapor deposition, and moisture spots are formed after vapor deposition.

4. The coating fixture will emit water vapor and other exhaust gases during the baking process, and moisture spots will be formed on the surface of the workpiece after evaporation and will affect the optical properties of the coating.

 In view of the causes of the above spray spots and moisture spots (variegation), the following measures have been taken in production to eliminate spray spots and moisture spots (variation):

1. Choose a reliable material supplier and do a test for each batch of purchased materials before putting them into use to determine the reliability of the materials.

2. Standardize the use and storage of materials, (drying tank, desiccant) to ensure that the materials are not damp, and the properties of the materials do not change.

3. Standardize the coating operation and strictly implement the operating procedures of pre-melting and evaporation. The pre-melting of the material before evaporation must be thorough, and the method of multiple pre-melting can be used to eliminate spray spots when plating some thicker layers.

4. Standardize the cleaning operation, strengthen the inspection after cleaning, improve the cleaning quality, and eliminate the moist spots (variegation) caused by cleaning.

5. Adjust the baking temperature to eliminate moisture spots caused by baking.

6. Standardize the management of coating fixtures. Before each use, the fixture needs to be cleaned and baked (300 ℃, more than 3 hours) before it can be put into production.

7. When designing the coating fixture, try to use materials with a small amount of outgassing in the vacuum chamber, and at the same time, use Ni plating on the surface of the fixture to reduce the amount of outgassing of the fixture and eliminate the resulting moisture spots (variegation).

Through the above measures, the spray spots and moist spots (variegations) produced during the coating process can be very effectively controlled.