Application of solder mask on PCB board surface

First, the application of solder mask on the surface of PCB board

The solder mask of the printed circuit board is a permanent protective layer. It not only has the functions of solder resist, protection, and insulation resistance, but also has a great influence on the appearance quality of the board. Early solder mask printing was done by using a solder mask to make a screen pattern and then printing a UV-curable solder resist ink. After each printing, due to the deformation of the screen, inaccurate positioning, etc., the excess solder mask remains on the pad, which takes a long time to scrape, which consumes a lot of manpower and time. The liquid photosensitive solder resist ink does not need to make a screen pattern, and uses an empty net printing and a contact exposure. This process has high alignment accuracy, strong adhesion to solder mask, good solder resistance and high production efficiency, and has gradually replaced photo-solid inks.

1. Process flow

Solder mask film backsheet → Punching film positioning hole → Cleaning printed circuit board → Formulating ink → Double-sided printing → Pre-baking → Exposure → Development → Thermosetting

2. Key process analysis

(1) Pre-bake

The purpose of prebaking is to evaporate the solvent contained in the ink to make the solder resist film in a non-stick state. The pre-baking temperature and time are different for the ink. If the pre-baking temperature is too high, or the drying time is too long, the development will be poor and the resolution will be lowered. If the pre-bake time is too short or the temperature is too low, the film will stick to the film during exposure. During development, the solder mask will be subjected to sodium carbonate solution. The erosion causes the surface to lose tarnish or the solder mask to swell.

(2) exposure

Exposure is the key to the entire process. For a positive image, when overexposed, due to scattering of light, the solder resist film on the edge of the pattern or line reacts with the light (mainly the photosensitive polymer contained in the solder resist film reacts with light) to form a residual film, which reduces the resolution. , causing the developed image to become smaller and the lines to be thinner; if exposed

When it is insufficient, the result is opposite to the above case, and the developed pattern becomes large and the lines become thick. This situation can be reflected by the test: when the exposure time is long, the measured line width is a 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 used to determine the optimal exposure time.

(3) ink viscosity adjustment

The viscosity of the liquid photosensitive solder resist ink is mainly controlled by the ratio of the hardener to the main agent and the amount of the diluent added. If the amount of hardener added is not sufficient, an imbalance in ink characteristics may occur. After the hardener is mixed, the reaction is carried out at normal temperature, and the viscosity changes as follows.

Within 30min: The ink main agent and hardener are not fully fused, the fluidity is insufficient, and the screen will be blocked during printing.

30 min to 10 h: The ink main agent and hardener are fully fused and the fluidity is appropriate.

After 10h: the reaction between the materials of the ink itself has been actively carried out, resulting in a large fluidity, which is not easy to print. The longer the time after the hardener is mixed, the more the reaction of the resin and the hardener is, and the gloss of the ink also changes. it is good. In order to make the ink gloss uniform and printability, it is preferable to start printing after the hardener is mixed for 30 minutes.

If the diluent is added too much, it will affect the heat resistance and hardenability of the ink. In short, the viscosity adjustment of liquid photosensitive solder resist ink is very important: the viscosity is too thick, and screen printing is difficult. The screen is easy to stick to the web; the viscosity is too thin, and the amount of volatile solvent in the ink is large, which brings difficulties to pre-curing.

The viscosity of the ink was measured using a rotary viscometer. In production, the optimum value of viscosity is also adjusted according to different inks and solvents.

Second, the application of anti-corrosion coating in PCB pattern transfer process

In the manufacturing process of printed circuit boards, pattern transfer is a key process. Previously, dry film processes were commonly used to transfer printed circuit patterns. Now, the wet film is mainly used for the fabrication of the inner layer pattern of the multilayer printed circuit board and the production of the outer layer pattern of the double-sided and multi-layer board.

1. crafting process

Pre-treatment→screen printing→baking→exposure→developing→anti-plating or anti-corrosion→de-filming→lower process

2. Key process analysis

(1) Selection of coating method

Wet film coating methods include screen printing, roll coating, curtain coating, and dip coating.

Among these methods, the wet film surface film layer produced by the roll coating method is not uniform, and is not suitable for producing a high-precision printed circuit board; the wet film surface film layer produced by the curtain coating type method is uniform and uniform, and the thickness can be precisely controlled. However, the curtain coating apparatus is expensive and suitable for mass production; the wet film surface layer produced by the dip coating method has a thin thickness and poor plating resistance. According to the current PCB production requirements, the screen printing method is generally used for coating.

(2) pre-processing

The bonding of the wet film to the printed circuit board is accomplished by chemical bonding. Usually the wet film is a polymer based on acrylate, which is freely mobile with unpolymerized acrylate groups. Copper bonding. The process uses chemical cleaning and mechanical cleaning to ensure the above-mentioned bonding, so that the surface is free from oxidation, oil, and water.

(3) Control of viscosity and thickness

At the 5% point, the wetness of the wet film is 150 PS, which is lower than the thickness of this viscosity printing, which does not meet the requirements. In principle, wet film printing does not use a thinner, and if it is to be added, it should be controlled within 5%.

The thickness of the wet film is calculated by the following formula:

Hw=[hs- (S + hs)]+P%

Where hw is the wet film thickness; hs is the wire mesh thickness; S is the filling area; P is the ink solid content.

Take the 100 mesh screen as an example:

Screen thickness: 60 μm; opening area: 30%; ink solid content: 50%.

Thickness of wet film = [60-(60 × 70%)] × 50% = 9 μm

When the wet film is used for corrosion resistance, the film thickness is generally required to be 15 to 20 μm; when used for plating resistance, the film thickness is generally required to be 20 to 30 μm. Therefore, when the wet film is used for corrosion resistance, it should be printed 2 times. At this time, the thickness is about 18 μm, which meets the anti-corrosion requirements. When used for anti-plating, it should be printed 3 times. At this time, the thickness is about 27 μm, which is in line with the thickness of the anti-plating film. Claim. When the wet film is too thick, it is easy to produce defects such as insufficient exposure, poor development, and poor etching resistance. When it is resistant to electroplating, it will be etched by the syrup, causing the phenomenon of film detachment, and the pressure resistance is high, and it is easy to produce a sticky film when the film is bonded. When the film is too thin, there are disadvantages such as overexposure, poor plating insulation, release film, and phenomenon of plating metal on the film layer. In addition, when the film is overexposed, the film removal speed is also slow.