PCB board "beautiful coat" - solder resistance

一、  What is PCB solder resistance？

PCB solder resistance, also known as PCB solder resistance film in flexible circuit boards, using green ink spray on the surface of the PCB circuit board.

PCB solder resistance can be displayed in different colors, including green, white, blue, black, red, yellow, matte color, purple, matte black, matte green, etc. Under normal circumstances, the production of LED lighting products must use white PCB boards, other colors are in accordance with customer needs. The color is customized printed circuit board, which green circuit board is the most common, because the green process is the most mature, the simplest, and its manufacturing cost is more cost-effective, in addition to green and bright green, light green, matte green and the like.

二、 What is the role of pcb solder resistance

In the process of manufacturing printed circuit board, the coating of solder resistance ink is a very key process. The main function of the solder resistance film on the PCB board is to protect the circuit and prevent the conductor from being stained with tin; prevent electrical short circuits between conductors caused by moisture or chemicals; as well as a variety of harsh environment on the invasion of PCB board.

Both sides of the PCB board are copper layers, and the PCB board without welding resistance is easily oxidized in the air, and becomes a bad product, which also affects the electrical performance of the PCB board. Therefore, there must be a protective coating on the surface of the PCB board that can block the oxidation reaction between the PCB and the air, and this coating is a welding resistance layer covered by welding resistance paint material. Various colors of solder resistance have also emerged, forming a colorful PCB circuit board, such as white pcb, green pcb, yellow pcb, blue circuit board, and solder resistance color has nothing to do with the quality and electrical performance of the PCB board.

PCB surface also need to weld electronic components, it is necessary to have part of the exposed copper layer to facilitate welding components, this part of the copper layer is the welding pad, the exposed copper layer is prone to oxidation, so the pad also needs to have a protective layer to prevent oxidation. Therefore, there is the plating of the pad, which is what we often say, PCB surface treatment. It can be plated with metal gold or silver, or it can be a special chemical film that prevents the copper layer of the pad from being oxidized when exposed to air.

三、 Process interpretation of PCB solder resistance layer

The solder resistance process in the printed circuit board is to cover the solder pad on the printed board with a photographic base plate after the screen printing, so that it is not exposed to ultraviolet radiation in the exposure, and the solder shield is more firmly attached to the printed board surface after ultraviolet light, the solder pad is not exposed to ultraviolet light, so that the brass plate can be exposed in the hot air.

1、 Pre-curing

The purpose of pre-curing is to evaporate the solvent contained in the ink, so that the solder resistance film becomes non-stick status. For different inks, the temperature and time of pre-curing are different. The temperature is too high, or drying time is too long, will lead to poor development, reduce the resolution. If the time is too short, or the temperature is too low, the film will stick to the negative during exposure. During development, the solder resistance film will be eroded by sodium carbonate solution, causing the surface to lose luster or the solder resistance film to expand and fall off.

2、Exposure

Exposure is the key to the entire process. If the exposure is excessive, due to the scattering of light, the solder resistance film on the edge of the graph or line reacts with light (mainly the photosensitive polymer contained in the solder resistance film reacts with light), resulting in residual film, and the resolution is reduced, resulting in the developed graph becoming smaller and the line becoming thinner. If the exposure is underexposed, the result is the opposite of the above situation, and the developed figure becomes larger and the line becomes thicker. This situation can be reflected by the test: the exposure time is long, the measured line width is negative tolerance. If the exposure time is short, the measured line width is a positive tolerance. In the actual process, the "light energy integrator" can be selected to determine the best exposure time.

3、Ink viscosity control

The viscosity of liquid photosensitive solder resistance ink is mainly controlled by the ratio of hardener and main agent and the amount of diluent added. If the amount of hardener is not enough, there may be an imbalance in ink characteristics.

四、 Advantages of PCB solder plug holes

At present, in addition to the parts pin holes, mechanical holes, heat sink holes and test holes in various PCB holes, other via holes do not need to be exposed are required to use solder plug, especially HDI high-density connection technology is becoming more and more dense, and more and more VIP holes and VBP holes are used for packaging PCB boards, so it is necessary to use solder mask via hole.

Solder mask via hole is to plug solder mask through the holes, generally to fill two-thirds of the part, opaque will be better.

1、The plug hole prevents the possibility of short circuits caused by closely spaced devices such as BGA. This is the reason why the BGA has holes in it during the design process. Short circuit may occur if there is no plug hole;

2、The plug hole can prevent short circuit caused by tin penetrating through the component surface through the pilot hole when PCB over-wave soldering. That is to say, within the range of the wave soldering design area (the welding surface is generally 5mm or above), there is no hole or the reason why the hole is plugged;

3、Avoid flux residue in the through hole；

4、Prevent the surface solder paste from flowing into the hole to cause virtual welding and affect the mounting；

5、Prevent the tin bead from popping out during over wave soldering and resulting in short circuit.

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