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PCB/PCBA knowledge
PCB solder mask – an ultimate guide for beginners

PCB solder mask is a thin layer of polymer that is placed on a circuit board to protect the copper from oxidation and short circuits during handling, it also protects the PCB from the environment. PCB solder mask can have many kinds of colors, but it is often green. Do you know why and do you know what is PCB solder mask bridge? Let’s get started!

Table of Contents

What is PCB solder mask?

PCB solder mask
PCB solder mask

PCB solder mask, also called PCB solder resist, uses green, yellow, red, black, blue and other photosensitive inks to spray on the surface of PCB circuit boards. It is an essential material in circuit building projects, protecting the PCB from oxidization and environmental influences such as dust and several other contaminants that may lead to shorts.

Why are the PCB solder mask colors different for different circuit boards?

PCB solder mask can have different colors, including green, white, blue, black, red, yellow, matte color, purple, chrysanthemum, bright green, matte black, matte green, etc. Under normal circumstances, the white ones are white PCBs that must be used to make LED lighting products. Other colors are mostly due to the product classification system, and each company is different. Some use red to represent the experimental boards, some use blue to represent the key boards, and some use black to represent the boards used in the computer.

Why green oil is widely used in PCB solder mask ink?

green PCB
green PCB

1. In terms of function, the ingredients added to the green ink have already been fixed. Basically, the equipment potions are all for green, which is easy to develop and not easy to fall off.

2. In terms of appearance inspection, the contrast with the copper surface (yellow) is obvious, and it is easier to detect defects such as scratches and offsets. Other variegated oils are added with some powdery substances that change the color. It is relatively high cost for PCB production. But for the finished product, some colors look more high-end than green.

In addition, there is manual visual inspection in the PCB quality inspection process of the PCB factory. For PCB manufacturers, green is more eye-friendly, and it is also more friendly to visual inspection staff.

3. Green ink can achieve smaller error, smaller area, and higher precision. Compared with other colors, green, red and blue have higher design precision.

4. Compared with inks of other colors, green ink has better characteristics. Compared with other colors, the green color has better characteristics, especially the plugging characteristics.

5. The cost of green ink is relatively lower. Since green is the mainstream in the production process, the process is the most mature and the simplest, and the purchase volume of natural green ink will be larger, so its purchase cost will be lower than other colors.

6. In order to reduce costs, many PCB ink manufacturers also produce green oil in large quantities, which also makes the price of green oil lower.

Does the PCB solder mask color affect the circuit board?

PCB ink in different colors
PCB ink in different colors

In fact, PCB ink has no effect on the finished circuit board. But it has a great impact on semi-finished products. There is matt green, sun green, dark green, light green, etc. in green colors, and they are different. If the color is too light, it is easy to see if the appearance of the board after the plugging process is good. If the color is too dark, the quality of the ink used by some circuit board factories is not good enough, and the ratio of resin and dye is wrong, there will be problems such as bubbles, and serious ink will fall off during final curing.

Why is PCB solder mask necessary?

In the printed circuit board (PCB) manufacturing process, the coating of PCB solder mask ink is a very critical process. The main function of the solder mask on the PCB board is to protect the circuit, prevent conductors from being stained with tin; prevent electrical short circuits between conductors caused by moisture or chemicals; prevent circuit breakage caused by PCB back-end process production and improper handling in electrical equipment; and prevent PCB boards from being invaded by various harsh environments.

Why is PCB solder mask necessary
Why is PCB solder mask necessary

There are copper layers on both sides of the PCB board. The PCB board without PCB solder mask is easily oxidized in the air and becomes a bad product, which also affects the electrical performance of the PCB board.

The surface of the PCB board also needs to be soldered with electronic components, so part of the copper layer needs to be exposed to facilitate soldering of components. This part of the copper layer is the PCB pad. As mentioned above, the exposed copper layer is prone to oxidation reactions, so the pads also need a protective layer to prevent oxidation. Therefore, the pads need to be plated, which is what we often call PCB surface treatment. It can be plated with inert metal gold or silver, or it can be a special chemical film to prevent the copper layer of the pad from being oxidized when exposed to air.

PCB solder mask thickness standard

PCB solder mask must have good film-forming properties, and its thickness is required by specifications. At present, most PCB manufacturers are identified according to the American civil standard IPC-SM-840C specification. In this standard, the thickness of the solder mask for grade 1 products is not limited; the minimum thickness of the solder mask for grade 2 products is 10um; the minimum thickness for grade 3 products should be 18um. The flame resistance of the solder mask is usually based on the specifications of the UL agency in the United States, and must pass the requirements of UL94V-0 (UL94-flammability test of plastics used in devices and equipment).

PCB solder mask thickness standard
PCB solder mask thickness standard

Generally, the thickness of the solder mask in the middle of the line is generally not less than 10um, and the position on both sides of the line is generally not less than 5um.

PCB ink solder mask bridge

1. What is a solder mask bridge

The solder mask bridge is the green oil between two pins of component, which generally refers to the solder mask strip between the relatively dense IC pins corresponding to the pads. The function of the solder mask bridge is to prevent the flow of solder during soldering, prevent the short circuit of the device from connecting tin, etc. Usually, in order to prevent soldering and tin short circuit, it is necessary to ensure the solder mask bridge of the pad.

solder mask bridge
solder mask bridge

2. PCB solder mask bridge process

The manufacturing capability of the solder mask bridge is related to the color of the ink and the thickness of the copper. The solder mask bridge of green oil is easier to control than the variegated ink, and the solder mask bridge can be kept to a minimum. The thicker the copper, the larger the solder mask bridge needs to be, and the solder mask bridge of thin copper is easier to control than thick copper. Here are some factors that affect whether the solder mask bridge can be processed: whether the IC pin spacing is sufficient, the ink color of the board, and the copper thickness.

The manufacturing capability of the solder mask bridge is not only related to the ink color and copper thickness, but also related to the manufacturing capability of the PCB board factory.

What is the reason for the falling off of PCB solder mask green oil?

1. When the PCB is printing ink, the pre-treatment is not done properly. For example, there are stains, dust or impurities on the surface of the PCB, or some areas are oxidized. In fact, it is very simple to solve this problem. Just do the pre-treatment again, but we must clean up the dirt, impurities or oxide layer on the surface of the circuit board.

2. The baking time of the circuit board is short or the temperature is not enough. Because the circuit board must be baked at high temperature after printing the thermosetting ink, and if the baking temperature or time is insufficient, then the ink on the board surface will fall off.

3、Ink quality problems or ink expiration, or purchasing ink of unknown brands, these will also cause the ink on the circuit board to fall when passing through the tin oven.

What is the difference between PCB solder mask layer and solder flux layer?

What is solder flux layer? Solder flux layer, also called paste mask, is used by the SMT placement machine for placement. PCB solder flux layer corresponds to the pads of all placement components. It is the same size as the top layer/bottom Layer layer and is used to open the steel mesh to leak tin.

difference between PCB solder mask layer and solder flux layer
difference between PCB solder mask layer and solder flux layer

The PCB solder mask is mainly to prevent the PCB copper foil from being directly exposed to the air and play a protective role. While the PCB solder flux layer is used to make stencils for the stencil factory, and the stencil can accurately place the solder paste on the patch pads that need to be soldered when tinning.

FAQ

PCB solder mask, also called PCB solder resist, is a thin layer of polymer that is put on a circuit board to protect the copper from oxidation and shorts during operation. It also protects the PCB from environmental influences such as dust and several other contaminants that may lead to shorts in the long run.

In fact, PCB ink has no effect on the finished circuit board. But it has a great impact on semi-finished products.

The main function of the solder mask on the PCB board is to protect the circuit, prevent conductors from being stained with tin; prevent electrical short circuits between conductors caused by moisture or chemicals; prevent circuit breakage caused by PCB back-end process production and improper handling in electrical equipment; and prevent PCB boards from being invaded by various harsh environments.

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