PCB solder mask (SM), PCB circuit is usually printed after soldering, because the ink color of the circuit board is usually green, accounting for more than 90% of the PCB industry, so solder mask is also called green oil. So what are the effects of solder masking? As follows: Prevent line pad bridging from shorting during soldering. Reduce solder loss in non-welded areas. Provides a permanent electrical environment and chemical resistant layer to protect the board from moisture and external damage. Another point is that from the aesthetic point of view, let the PCB wear gorgeous clothes to make the PCB more beautiful and beautiful. Solder resist inks are classified into photosensitive inks and non-photosensitive inks when they are distinguished from photosensitivity. We commonly use photosensitive inks. The following is the approximate process of solder mask fabrication:
The usual solder mask inks are green, and other colors are white, black, blue, red, purple, and yellow. According to the difficulty level of production, we divide it into: the common level is green, and the black oil is the most difficult to operate. The two ink colors are called variegated inks. (Neutral level) Before starting the DFM case of solder mask, we need to understand a few concepts, as follows: Open the window: that is, the part of the exposed copper on the PCB after the finished product, that is, the part that does not cover the ink.
Cover line: The cover line refers to the size and how much the solder mask ink covers the line portion. The solder mask opening window is too large, and the cover line distance is too small. In the production process, the exposed copper is bright.
Opening window: There is no solder mask ink printed between the two pads.
Solder soldering bridge: Also called green oil bridge, the green oil between the opening of one component and the opening of another component is a soldering bridge, generally referring to a relatively dense IC. The role of the solder mask is to prevent solder flow during soldering and to prevent short-circuiting of the device.
In order to prevent the solder from being short-circuited, we must ensure the solder bridge of the pad. What is our conventional green oil solder mask? At present, our process capacity is 3mil (the finished copper thickness is 1OZ). The window opening of the pad is usually 2mil. According to the above data, the pad pitch of our design manuscript can be calculated as 2mil+3mil (resistance of solder bridge width) +2mil=7mil on both sides of the pad.
Because the ink has different energy requirements for exposure, the solder bridge of the ink other than green oil and black oil is 2mil+3.5mil (resistance of solder bridge width) +2mil=7.5mil for both sides of the pad. The original pad spacing is 7.5 mils during design.
Some people say that black ink is cool, black ink is cool, no matter whether this kind of swearing is correct or not, but usually the PCB prototype factory wants to cry black ink. Because the black ink is particularly strong in energy absorption of ultraviolet light during the exposure process. (I believe that people who pass through black clothes in the summer feel a little deeper.) Usually the black ink soldering bridge needs to be 4mil, and even some factories are still limited by equipment and process capabilities. The process capability of 5-6mil is up. Therefore, the pad spacing of the black oil bridge is 2mil + 4mil (the soldering bridge width) + 2mil = 8mil on both sides of the pad.