PCB Tech - PCB liquid photosensitive and dry film solder mask process

1. Curtain Coating method

It is an automatic construction method for the photosensitive green paint coating on the PCB surface. The paint is a diluted non-water-soluble green paint ink. The green paint will flow down continuously from a long strip of opening during construction., It intersects with the automatic conveying board surface perpendicularly, and coats a layer of uniform paint film on the board surface. After the solvent escapes and semi-hardened, the board body is turned over and the other side is coated. When both sides are completed After that, the image transfer of the photosensitive method can be carried out. This "coating method" is not a new creation in the circuit board industry. In the early years, it was also used for automatic coating of wooden furniture, but now it has moved to another use.

2. Encroachment contamination, violation

In the PCB industry, when the board is processed with green paint, it will be serious when the green paint traces accidentally appear on the surface of the soldering pad that should not be painted (referring to the hole ring hole wall of the socket or the board surface soldering pad of SMT, etc.) Affect the solderability of downstream assembly, especially called Encroachment.

3. Liquid Photoimagible Solder Mask, LPSM liquid photosensitive solder resist green paint

It is a kind of solder resist green paint used for the board surface. Due to the increasing number of thin-line boards, the early screen printing baking type epoxy resin green paint can no longer be used. Instead, the "empty board" (or just leave it Screen printing of ink-blocking dots) Full screen printing and construction of photosensitive green paint. After scratching and semi-hardening, you can directly use the negative film for precise alignment and exposure, and then develop and harden the green paint with an accurate position. This kind of active LPSM has been tested in mass production for several years, and its quality has been very good, and it has become the mainstream of all kinds of solder masks.

4. Post Cure is hardened and baked afterwards

In the circuit board industry, liquid photosensitive green paint or solder mask dry film needs to be further hardened after the development is completed to enhance its physical properties and solder resistance. This kind of supplementary work is "follow-up hardening". In addition, after the multi-layer board made of polyimide material is laminated, in order to make it have a more complete polymerization reaction, it must be put back in the oven for 2 to 4 hours post-bake, also known as Postcure.

5. Roller Coating method

Use rollers to coat green paint or "photosensitive circuit ink" on the surface of the board, and then perform semi-hardening exposure and imaging. This method is very beneficial for low-priced and large-volume boards.

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

Although Solder Mask is more general in the original terminology, Solder Resist is still a more formal term. The so-called solder mask refers to the part of the conductor on the surface of the circuit board that does not need to be soldered to be covered with a permanent resin film. This film is called S/M. In addition to the anti-welding function, the green paint can also protect and insulate the covered circuit.

7. Spray coating, spray coating

Use compressed air to spray liquid paint from a small port and spray it on the surface of the object to be treated with fine atomized particles. Similar to "painting", it is called "Spray Coating". It is also possible to apply an electrostatic device at the nozzle to make the sprayed mist point carry static electricity, and apply the opposite static electricity to the treatment part itself to make it directly adsorb. It can not only save paint, reduce pollution, but also make the dead corners evenly distributed, which is called "electrostatic spraying method". The new green paint processing of circuit boards also adopts this method.

8. Tackiness, stickiness

After PCB liquid photosensitive green paint (LPSM) (such as air-screen printing, vertical flow, spraying, vertical scraping, and roller coating, etc.), it must be pre-baked for exposure. Whether this kind of pre-baked paint surface will still stick to the negative film under strong light is called Tackiness. After the solder paste is printed on each SMD pad and the parts are placed on the downstream SMD soldering pads, the solder paste must temporarily exhibit the function of sticking and positioning before waiting for infrared and hot air welding, also known as Tackiness.