How to choose the solder mask in PCB fabrication and assembly?
The role of solder mask
Solder masking oil is a thin lacquer-like layer of polymer applied to the surface of the PCB. On the one hand, it is to protect the circuit on the board from being shorted by foreign objects as much as possible, and on the other hand, it can be used to prevent the copper wire from falling off. Because the exposed pad is easy to fall off. However, with the solder mask, the copper wire does not tend to fall off.
In general, big circuit board manufacturing companiessuch as Huawei and ZTE are generally green, because the green process is mature and simple. However, in special cases, there will be red, white, blue, yellow, and matt colors, and more specifically, black, purple, chrysanthemum, and bright green. Under normal circumstances, white ink is used for light source products. . Because these products must use white PCB boards. Other colors are used to distinguish different product types. Each company is different. Some use red to indicate the experimental board, some blue to indicate the key board, and some black to indicate the board used in the computer.
How does the color of solder mask affect the PCB?
The effect of ink on the finished product is very small, but it has a greater impact on the semi-finished product, mainly on the difficulty of the semi-finished product. For example, there are matt green, sun green, dark green, and light green in green. The color is a little different. If the color is too light, it is easy to see that the appearance of the board is not good. If the color is too dark, when the manufacturer's ink is not good enough (for example, there is a problem with the ratio of resin and dye), there will be problems such as air bubbles. Seriously, the ink will drop in the final curing. There is a slight change in color that cannot be felt without years of experience. (This problem is a bit complicated. Some inks are used in comparison with different machines. There are electrostatic spraying, spraying, and screen printing. The inks are different, the proportions are different, and the colors are a little different. If they are used wrongly, they will be a little small. Problems, but in fact, for the finished board, these are all appearance problems and have no impact on the product itself).
The effect of solder mask thickness on the circuit board manufacturing
Although the ink color has no effect on the plate, the ink thickness has a very large impact on the impedance, especially for water-gold plates. The thickness control of this plate ink is very strict. According to experience, use red ink, bubbles, and thickness. The better control, the first is that the ink is more expensive and the quality is indeed desirable. The second is that this color can cover some defects on the line, and green will be more obvious. The third is the imaging process. The exposure of red and yellow seems to be relatively stable, green is not necessarily, and white is more difficult to control in prototype pcb manufacturing.
Characteristics of different color in PCB fabrication and assembly
Different color solder masks have different viscosities. Green is smaller for cheap pcb manufacturing, while red, yellow, and white are slightly larger. The black solder mask oil has the highest viscosity. On high-density PCBs, the distance between the pads is small and the width of the solder mask is small. In the process of silk screen, the solder resist oil with lower viscosity is easier to print on the PCB. So sometimes the PCB that can be made of green can't guarantee the quality of the solder mask after changing the color. In addition, blue and black are doped with cobalt and carbon, which have a certain conductivity, so there is a risk of short circuit. Green PCBs are more environmentally friendly, and will not release toxic gases when used at high temperatures in circuit board manufacturing.
