What is PCB Solder Mask (S/M)?
Solder mask located on the top layer and bottom layer copper foil of the circuit boards. It is used to protect the copper foil from being oxidized and polluted to affect the function of the PCBs. Solder mask generally uses insulate resin as the main material and gets printed upon the areas not required to be soldered on the circuit board, in order to protect it from moisture, soldering, high temperature, and physical damage.
PCB board manufacturing has a high degree of customization, one fo which is the selection of solder mask. We have too much to say about the solder mask, but today we only discuss the choice of colors.
More than 99% of the circuit boards employe green solder mask, so the solder mask refers to "green" most of the time. Of course, there are also red, black, blue, white, etc, which are used in different applications. For example, at the NPI stage, some RDs specifically use red to mark the boards; some front panel boards are in black to match the color of the chassis; LED lighting boards often use white ink to enhance the lighting effect...
But which one is for you? What are the pros or cons of each color? Please clarify these simply as following.
How to Choose PCB Solder Mask Color?
Green PCB Solder Mask
A standard PCB board, isn't it? You might think it is dull, but undeniable that green is the best of all available colors, even today. The green solder mask has high recognizability, you can easily inspect the boards with the naked eye to check for manufacturing defects, while, the green solder mask is matched with the white silkscreen for high contras, the flux residue can be removed well to form a professional-looking circuit board.
If you've no idea in the selection of solder mask colors, or there is no limitation for this, please include "green" for the solder mask layer.
For the question "Why PCB solder mask is almost green?", you can refer to this answer.https://qr.ae/TanPGI
Red PCB Solder Mask
Red PCB solder mask looks bold and looks professional. The good contrast between traces, planes and no-copper areas but it is significantly lower than that of the green PCB board. So it might require magnification to check whether there are defects on the fine traces on the board. Screen printing performed well on a red solder mask, and residual flux was removed well, just like on a green circuit board.
Red is always so compelling, if the printed board is visible from the outside of your devices, while it is not used for an HDI application, the red PCB solder mask will make it a star.
Blue PCB Solder Mask
Dark blue, another commonly used. The contrast between traces, planes, and no-copper areas is very low, so you must zoom in to check for manufacturing defects. From the front, the contrast between screen printing and solder mask is very high, so if your circuit board has many part-numbers, the blue PCB solder mask might be for you.
Blue is not so clear as well as red and green. It is a dark color, any dirt on the solder mask is easily seen, and the flux stains probably are difficult to remove.
If you don't want to show the route on the boards or use the printed boards for Arduino open-source projects, the blue PCB solder mask may be a good choice.
Black PCB Solder Mask
The contrast between traces, planes and no-copper areas is pretty low. It is hard to distinguish the route, printed circuit board inspection does not only require a powerful magnification, but it must also tilt the light so that the light casts a shadow and makes the trace slightly higher than the board. You will not love it when the routing is relatively fine.
At least screen printing has good contrast. In fact, screen printing and pads are almost what you can see on the board without lighting.
Another danger of black solder mask is the way it absorbs heat during reflow. You must scale down the configuration file or make sure the temperature sensor is always placed on the board. Screen printing also tends to turn light brown during the reflow process, which may be because the circuit board underneath absorbs a lot of heat.
The dirty on a black surface is easily seen, and difficult to clean up, you must be careful, so sometimes the black solder mask requires an extra fee by PCB manufacturer.
On the positive side, black is really cool color, it is good for LCD backpanel. When the background color is black, no one looks at distracting details on the screen.
The black PCB solder mask can be divided into gloss and matte, matte black has a little bit better contrast than gloss black, but it is more easily soiled, solder mask repairing is difficult.
White PCB Solder Mask
Well, the white solder mask is the only one that is harder to get along with than black. The contrast is the lowest of all solder mask colors, and marks cannot be displayed correctly even under lighting.
Cleaning is as difficult as you would expect, you must work so hard to remove the stains to make the panel look good.
On the other side, the contrast of black printing on the white solder mask is pretty good, so any artwork you get in the silkscreen layer will stand out.
If the white PCB solder mask is necessary somewhere, it should be the LEDs or the demand for a white front panel, which greatly enhances the brightness, so please include white solder mask for LED applications.
Yellow PCB Solder Mask
Why not yellow? It's really good, the contrast between traces, planes, and no-copper areas is as high as the green solder mask, so if you have some beautiful routes to show off, then dark yellow might be your right choice.
The only drawback is that the white silkscreen does not have high contrast with the board, therefore, most of the time, black silk printing is the better choice, even if sometimes it is not satisfactory.
Yellow solder mask is easy to clean, any light-colored residue that does not stand out.
In short, yellow is a surprising choice sometimes, if this lively color will not embarrass you...
Purple PCB Solder Mask
You will rarely see purple printed circuit board, it is an advanced solder mask choice, relatively good contrast between traces and planes on the surface, but not so good to show the white silk printing.
If you prefer a more beautiful color to match with the immersion gold surface, purple is worth a try.
All of the solder mask colors above are available in PS Electronics, please feel free to contact us if any questions or advice.