PCB liquid photosensitive welding and dry film welding process

1, Curtain Coating method

It is an automatic construction method of photosensitive green paint coating of circuit board, the paint is diluted non-water-soluble green paint ink, the green paint will flow continuously from a long opening, and the automatic transmission of the board vertically intersected, and the board is coated with a uniform film, after the solvent escapes half hardening, and then the board body and the other side of the coating. When both sides are completed, the image transfer of the photosensitive method can be carried out. This "Lian coating method" is not a new innovation in the circuit board industry, and in the early years, it was also used for automatic coating of wooden furniture, but it is now transferred to another use.

2. They would be dirty and infringing

In the PCB industry, when green paint is applied to the board, if traces of green paint appear unexpectedly on the surface of the welding pad that should not be stained with paint (refers to the hole ring wall of the jack or the welding pad of the SMT board), the solderability of the downstream assembly will be seriously affected, which is called "Encroachment".

3. Liquid Photoimagible Solder Mask, LPSM liquid photoimagible solder mask

A kind of anti-welding green paint used for the board surface, due to the thin line of the board day, the early screen printing baked epoxy resin green paint can not adapt, instead of the "empty plate "(or only leave the ink blocking point of the screen) full screen printing on the sensitive green paint construction. After scraping and semi-hardening, you can directly use the film for accurate alignment and exposure, and then after development and hardening, you can get a green paint with accurate position. This active LPSM has been tested by mass production for several years, and its quality has been very good, and it has now become the mainstream in all kinds of anti-welding films.

4, Post Cure follow-up hardening, after baking

In the circuit board industry, the liquid light-sensitive green paint or solderproof dry film needs to be further hardened after the completion of the development to enhance the solderproof properties of its physical properties. This re-work is called "subsequent hardening". In addition, when the polyimide material of the multilayer board after the completion of pressing, in order to make it have a more complete polymerization reaction, it must be put back in the oven to continue to bake for 2 to 4 hours, also known as Postcure.

5, Roller Coating method

Using a roller to paint green paint or "photosensitive line ink" on the surface of the board, and then semi-hardened exposure and development work, this method is very advantageous for low price and large production boards.

6, Solder Mask(S/M) green paint, solder mask

Although the Solder Mask is more common in the original term, the Solder Resist is still a more formal term. The so-called solderproof film refers to the circuit board surface to do not need to be welded part of the conductor, with a permanent resin film to cover, this layer is called S/M. In addition to the function of welding protection, green paint can also play a role in protecting and insulating the covered lines.

7, Spray Coating

Compressed air is used to Spray the liquid Coating from the small mouth, and small atomized particles are sprayed on the surface of the object to be treated, similar to "Spray Coating". An electrostatic device can also be applied at the nozzle, so that the spray spray point has static electricity, and the opposite static electricity is applied in the treatment part itself, so that direct adsorption. Not only can save paint, reduce pollution, and can make the dead corner can also be evenly distributed, called "electrostatic spraying method". The new green paint processing of the circuit board has also used this law.

8. Tackiness. 

After the surface of the plate is coated with liquid light-sensitive green paint (LPSM) (such as empty screen printing, vertical flow, spray, vertical scraping, and roll coating methods), it is also pre-baked for exposure. Whether this pre-painted finish will stick to the film under strong light is called Tackiness. After the solder paste is printed on the downstream SMD welding pad and the parts are placed, the solder paste must temporarily show the function of adhesive positioning, also known as Tackiness, before waiting for infrared and hot air fusion welding.

Use an even layer PCB

When an odd number of PCB layers appear in the design, the following methods can be used to balance layering, reduce PCB production costs, and avoid PCB bending. The following methods are listed in order of preference.

1.A signal layer and use. This method can be used if the PCB is designed with an even power layer and an odd signal layer. The added layer does not increase the cost, but it can shorten the lead time and improve the PCB quality.

2. Add an additional power layer. This method can be used if the PCB is designed with an odd power layer and an even signal layer. A simple way to do this is to add a layer in the middle of the stack without changing the other Settings. First lay the PCB in an odd number of layers, then copy the layers in the middle and mark the remaining layers. This is the same electrical property as the coating of thickened layers.

3. Add a blank signal layer near the center of the PCB stack. This approach minimizes layering unbalance and improves PCB quality. First, the odd-numbered layer is routed, and then a blank signal layer is added to mark the remaining layers. It is used in microwave circuits and mixed media (media with different permittivity) circuits.