Regarding the spacing of PCB solder mask bridges, everyone has been entangled. Let's make a topic like the sun to show everyone's confusion.
Solder mask (SM) of PCB, solder mask is usually printed after the production of PCB circuit, because the color of ink usually used for circuit boards is green, accounting for more than 90% of the PCB industry, so solder mask is also called green oil. So what is the role of solder mask? as follows:
Prevents line pad bridge shorts during soldering. Reduce solder loss in non-soldering areas. Provides a permanent electrical environment and chemical resistance to protect the board from moisture and external damage.
Another point is that from an aesthetic point of view, let the PCB wear gorgeous clothes to make the PCB more beautiful.
Solder mask inks are divided into photosensitive inks and non-photosensitive inks in terms of photosensitive properties. We commonly use photosensitive inks.
The following is the approximate process of solder mask production:
The usual color of solder mask ink is green, and other colors of ink are white, black, blue, red, purple and yellow. According to the difficulty of making, we divide it into:
The commonly used grade is green, and the black oil is the most difficult grade to operate. We call other ink colors other than 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 where the copper is exposed on the board after the PCB is finished, that is, the part that does not cover the ink.
Cover line, cover line refers to the size and how much the solder mask ink covers the circuit part. The solder mask opening is too large, and the cover wire distance is too small, which will result in exposed wire and bright copper during the production process.
Open the window, that is, there is no solder mask ink printed between the two pads.
Solder mask bridge, also known as green oil bridge, is the green oil between one window of the component and the other window is the solder mask bridge, which generally refers to denser ICs.
Hey, does the solder mask bridge sound great, and I am disappointed to see it. The function of the solder mask bridge is to prevent the flow of solder during soldering and prevent the device from being short-circuited with tin.
Usually, in order to prevent soldering and tin short circuit, we must ensure the solder bridge of the pad. What is our regular green oil solder mask bridge? At present, our company's process capacity is 3mil (finished copper thickness is 1OZ). The opening of the pad is usually 2 mil, then according to the above data, we can calculate the pad spacing of our design manuscript as 2 mil + 3 mil (solder mask bridge width) + 2 mil = 7 mil for the opening on both sides of the pad.
Because the ink has different energy requirements for exposure, except for green oil and black oil, the solder mask bridges for other inks are 2mil+3.5mil (solder mask bridge width)+2mil=7.5mil for opening windows on both sides of the pad. The original pad spacing is 7.5mil when designing.
In the PCB industry, some people say that black ink is dazzling and black ink is cool. No matter whether this kind of foolishness is correct or not, but usually PCB factories want to cry when they make black ink. Because the black ink absorbs the energy of ultraviolet light particularly strongly during the exposure process. (I believe that people who wear black clothes in summer have a deep feeling about this.) Usually, the solder mask bridge of black ink needs to be 4mil. Even at present, due to the limitation of equipment and process capabilities in some factories, the current black oil bridge is still there. The process capability of 5-6mil is hovering. Therefore, the spacing between the pads with black oil bridge is 2mil+4mil (solder mask bridge width)+2mil=8mil with windows on both sides of the pad.
So with this as a background, there is the following case.
The customer designed the pad original pitch to be 7mil, and the solder mask color is required to be black. Due to the limitation of the process capacity of the black oil bridge, our company issued the following engineering confirmation to the customer. Because the pitch of the original pads is 7mil and the color is required to be black ink, our company cannot guarantee the solder mask bridge. suggestions below:
a. Do not keep the solder mask bridge, and treat it as the open window. There is a risk of tin short circuit when accepting soldering.
b. Reduce the width of the pad to ensure the solder mask bridge.
Please select one of the above suggestions. The client called shortly after receiving the project confirmation email.
Hello, what is your company's solder mask process capability? Last time I made green ink in other homes, I didn't receive any questions about solder mask bridges. This time, the information has not changed, but the ink color has been changed to Black solder mask, your company cannot guarantee the solder mask bridge, your company's process ability is a bit poor.
It is true that people are sitting in the house, and misfortune falls from the sky. Fortunately, the customer is more open-minded, complaining is a complaint. After listening to our detailed explanation, I am relieved. Considering the reliability and processing ability, I accept the change of the width of the small pad to ensure the solder mask bridge, thus ensuring the smoothness of the board. offline.
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