It is mainly coated with a layer of green solder resist to form a solder mask. Like the first picture on the first floor, it was a board that was not coated with solder paste. This should be a hand-etched board. The solder resist is applied on the surface of the PCB board in order to protect the circuit on the board from being short-circuited by foreign matter. The second is to cover the copper wire, so that the copper wire is not easily peeled off (some exposed pads are easy to fall off. Solder mask, it is not easy to fall off). Nowadays, there are not only green solder masks, but also other colors such as red and black.
In addition to the green oil, there is a difference between the commonly used blue oil, butter, black oil, black matt oil, purple oil, red oil, green matt, green sun, etc. I think the main problem is cost. Green ink is the largest ink manufacturer in China, the general price is about 1KG 80, while the price of the same type of green sun and green matte ink between 150 - 250 of course there are cheaper blue matte ink , The price is about 1KG in 60, the question can mainly know, now the high-accuracy PCB board still needs to carry on the alignment operation manually, the other ink color is too easy to cause the flaw to the manual operation. (Alignment is a step of solder resist, which is to wash away the unnecessary solder mask by potion. The accuracy of the washout needs manual alignment operation to complete, and the details are not elaborated.) In this way, the rework is much better. It is a big test for manufacturers.
In summary, the cost is the biggest reason. If you want more interesting information about PCBs, please click the link hina pcb manufacturer.