Dry film solder resist is not liquid or paste, it is in the form of a photosensitive polymer film sandwiched between two protective layers that protect the emulsion film in the middle from damage during operation. The steps of applying dry film anti-welding film are described as follows:

1. Surface preparation

In this stage, the tin/tin-lead metal solder resist is stripped from the surface of the circuit board and cleaned with water, thoroughly dried, followed by a series of cleaning procedures for the copper surface, such as:

1) Remove grease with hot strong alkali lotion;

2) Rinse with water;

3) The surface copper layer was microetched 1-2.5 μm in order to enhance the adhesion of the dry photosensitive polymer film to the surface;

4) Conduct tests to ensure that tin/tin-lead is completely removed;

5) Rinse with water;

6) Dip in 10% sulfuric acid for 1-2 min.

7) Rinse with water;

8) Grinding cleaning (320 grinding brush and pumice stone scrub)

9) Rinse and dry under high pressure water.

Controlled oxidation of the copper layer with brown oxide and black oxide is used to increase the adhesion of the surface to the anti-welding film. The oxidation process must be strictly controlled to keep the thickness of the oxide layer at 0.5-1.0 μm. If the oxidized circuit board is stored for too long, the surface must be completely degreased before pressing.

2. Pre-press and dry

Absorbing moisture and moisture residues are the most common causes of bubbling and delamination of PCB boards, so it is especially important to remove all standing water from PCB surfaces and holes. A high pressure hair dryer can be used to physically remove water from the surface and holes of the circuit board. A high pressure true air knife can also be used in this step.

The circuit board can be baked at (1 10±10) "C for 15-20 minutes to remove moisture absorbed by the circuit board, or an infrared oven equipped with a transport belt can be baked at 80-120 "C for 30 seconds. Boards with different substrates and thicknesses require different drying times. Excessive baking times and temperatures should be avoided as this can increase copper oxide formation and lead to poor bonding.

3. Pressing

Vacuum pressing ensures that all wires are completely encapsulated by photosensitive polymer film and the circuit board is free of residual bubbles.

4. Selection of mask width

The choice of dry film width should be based on the width of the process board, the dry film width should not exceed the width of the process board more than 10 mm to avoid edge dressing loss. Pressing can be pressed on both sides at the same time can also be pressed on one side and then pressed on the other side, which is mainly determined according to the type of laminating machine used and the type of dry film.

5. Maintenance time after pressing

The holding time between pressing and exposure should be carefully controlled for good results. The exposure energy required at the beginning of the pressing process is low, but it increases rapidly as the holding time increases and gradually stabilizes to a constant value. Therefore, the holding time after pressing should be constant to avoid drastic changes in exposure time.

6. Exposure (negative)

Generally, the photosensitive polymer anti-welding film works negatively, so it requires a positive negative film (the pad area is opaque) for exposure. Because of the high energy required, the film must have a very high density

The minimum Dmax value should be greater than 4 to avoid the production of anti-welding film in the welding pad area, and the minimum Dmin of density in the blank area must be less than 0.15. Because the density of the mask changes with use, the film must be checked periodically to ensure that the Dmax is greater than 4.

Usually, the single-sided azo brown sheet with 7mIL drug film can achieve good results. For the counterpoint system that does not need to be aligned through welding pads, silver halide mask can be used to tightly stick its drug film side to the circuit board to obtain good exposure effect and edge verisimilar alignment. The minimum opaque pad area of the film should be 0.1-0.15mm, which should be slightly larger than the pad diameter on the printed circuit board.

The exposure time and subsequent degree of aggregation can be determined and monitored by using the Stouffer step 21 exposure rule θ. In order to determine the correct exposure value, the duration and developing conditions must be compatible. The Stouffer 21 exposure ruler used defines the latter stage as covering more than 50% of the barrier. A certain amount of compatibility maintenance time is required for the polymerization process to complete and enter a stable state. Printing ink dry film manufacturer

7.)

The development process is to wash the unexposed (unpolymerized) solder resist off the PCB surface. The development time of a particular type of photosensitive polymer depends on the chemical used, and the correct development time

Overcontrol unexposed inhibitors can be identified from the point in time when the PCB surface is completely washed off. Welding inhibitors need to be completely removed from the pads and holes of the PCB to achieve good weldability. Common chemicals are: aqueous solution and developer.

In aqueous solution development, dilute uranium carbonate solution is usually added to 99% pure water at 45 'C. The scour distance experienced by the printed circuit board for full development with this solution is between 40% and 60% of the length of the developing chamber. The developing point shall be set at 25% of the full length of the developing room.

8. Drying

In order to obtain a satisfactory curing effect, the board must be thoroughly dried after the development, by using a hot air knife to heat the board at 90't for 15 minutes, you can completely remove the moisture absorbed by the solder resistance and substrate in the development and cleaning stage of the circuit board.

9 curing in the wave soldering and gas phase welding process in order to achieve good welding performance and prevent the mask warping, photosensitive polymer inhibitor must be properly cured.

Ultraviolet (UV) curing of the photosensitive polymer anti-welding film can be accomplished by conventional curing equipment using soft lamp. The required energy level is obtained by controlling the density of the soft lamp and the speed of the conveyor belt, and sufficient energy is absorbed in one pass to make one side of the circuit board completely lumped. Uv lamps should use 200W/in high pressure mercury vapor lamps. Since the heat emitted by uv lamps forms an essential part of the overall curing process, it is not recommended to use "cool" UV units to reduce the heat reaching the board. In the process of uv curing temperature on the surface of the circuit board is less than 105 ℃ often leads to poor curing effect, at the same time also note in the uv curing process cannot excessive heating circuit board, the typical temperature should be between 110-140 ℃, excessive heat can cause photopolymer brittle, and foaming phenomenon occurring on the surface.

During the thermal curing process, the temperature of the oven used to bake the circuit board should be completely uniform throughout the oven, and these ovens should not be used for other purposes to prevent pollution and anti-welding film. An oven without a fresh air inlet should not be used because hot air may condense to the surface of the circuit board, reducing weldability, and if the air in the oven is not diluted, this hot air can reach a dangerous temperature where a crash may occur. For the oven to work safely, there should be a thermostatically controlled timer and temperature recorder, exhaust fans to allow heat to flow throughout the oven, and exhaust pipes with a hair dryer.

The thickness of dry film anti-welding film manufactured under certain controlled conditions is uniform. When applying this mask, high-pressure laminating machine is used to ensure uniform and consistent coating of the entire process board area. This method can also make the process board coating of different batches and furnace remain uniform and consistent.

Screen printing and curtain coating processes are single-sided processing systems, in other words, the use of anti-welding film can only be operated on one side at a time, before printing the other side of the circuit board must be dried to remove ink adhesive, which makes the treatment of the second side will also take a long time. Using a vacuum laminator, dry film flux can be applied on both sides of the board at the same time.