PCBA Tech - Application of Surface Resistance Layer on PCB Board

Application of Surface Resistance Layer on PCB Board

The welding resistance film of the PCB board is a permanent protective layer. It not only has the functions of anti-welding, protection, and improving the insulation resistance, but also has a great impact on the appearance quality of PCB board. Early printing of solder-resistant film is to use solder-resistant negative to make screen graphics before printing UV-curable solder-resistant ink. After each printing, due to wire mesh distortion, improper positioning, and other reasons, residual resistance film on the welding pad takes a long time to scrape, consuming a lot of manpower and time. Liquid photoresist ink does not need to make screen graphics, it uses empty screen printing, contact exposure. This process has high alignment accuracy, strong adhesion of resistance film, good weldability, and high production efficiency, and has gradually replaced photosetting ink.

1. Process flow for surface resistance layer of PCB board

Making Solder Resistance Film Negative Pushing Negative Positioning Hole Cleaning Plate Preparing Ink Double-sided Printing Prebaking Exposure Development Thermosetting

2. Key Process Analysis of Surface Resistance Layer of PCB Board

1) Pre-baking of PCB board

The purpose of pre-baking PCB boards is to evaporate the solvents contained in the ink and to make the resistance film non-stick. The pre-baking temperature and time are different for different ink. Pre-baking temperature is too high or drying time is too long, which will lead to poor development and reduce the resolution. The pre-baking time is too short, or the temperature is too low, the negative will be adhered to when exposed, and the resistance film will be eroded by the sodium carbonate solution during development, causing the surface to lose gloss or the resistance film to expand and fall off.

2) Exposure of PCB board

Exposure to PCB plates is the key to the whole process. For positive pictures, when overexposed, due to the scattering of light, the photosensitive polymer contained in the resistance film and the light reaction at the edge of the graphic or line, the residual film is generated, which reduces the resolution, resulting in smaller developed graphics and thinner lines. If Exposed

When insufficient, the result is contrary to the above, the graphics become larger and the lines become thicker. This situation can be reflected by the test: if the exposure time is long, the measured line width is negative tolerance; When the exposure time is short, the measured line width is a positive tolerance. In the actual process, the Optical Energy Integrator can be used to determine the optimal exposure time.

3) Ink Viscosity Adjustment for PCB Board

The viscosity of liquid photoresist ink is mainly controlled by the ratio of hardener to main agent and the amount of diluent added. If the amount of hardener is insufficient, the ink characteristics may be unbalanced. When the hardener is mixed, it reacts at normal temperature and its viscosity changes as follows.

Within 30 minutes: Ink principal and hardener have not been fully blended, fluidity is insufficient, screen clogging will occur during printing.

30min~10h: Ink principal and hardener have been fully blended and fluidity is appropriate.

After 10 hours: the reaction between the various materials of the ink itself has been active, resulting in greater fluidity, poor printing, the longer the curing agent mixes, the more reaction between the resin and the curing agent, and the better the gloss of the ink. To make the ink glossy and print well, it is best to place the curing agent 30 minutes after mixing and start printing.

If too many diluents are added, the heat resistance and hardness of the ink will be affected. In conclusion, it is very important to adjust the viscosity of liquid photoresist ink: it is too thick and difficult to screen. Screen plates are easy to stick to. Viscosity is too thin, and the amount of volatile solvents in ink is large, which makes it difficult to precure.

The viscosity of the ink on the PCB board is measured by a rotary viscometer. In production, we also need to adjust the optimal value of viscosity according to different ink and solvents.

Application of Anti-corrosion and Anti-electroplating Coating in Graphic Transfer of PCB Board

Graphics transfer is a key process in the production of PCB boards. Previously, dry film process was commonly used to transfer printed circuit graphics. Now, the wet film is mainly used to make the inner line graphic of the multilayer board and the outer line graphic of the double-sided and multilayer board.

1. Process of PCB board

Pretreatment Screen Printing Baking Exposure Development Anti-plating or anti-corrosion Desmearing Next process

2. Analysis of key process for PCB board

1) Selection of coating method

The methods of wet film coating are screen printing, roll coating, curtain coating and immersion coating.

Among these methods, the wet film surface layer made by roll coating method is not uniform and is not suitable for making high-precision plates. The wet film surface layer made by the curtain coating method is uniform and uniform, and the thickness can be precisely controlled, but the curtain coating equipment is expensive and suitable for mass production. The wet film made by dipping method has thin film thickness and poor electroplating resistance. According to the current requirements of PCB board production, the printing method is generally used for coating.

2) Preprocessing

The bonding of wet membranes to plates is accomplished by chemical bonding. Usually, wet membranes are polymer-based on propionate, which is bound to copper by free-moving unpolymerized propionate groups. This process uses chemical cleaning followed by mechanical cleaning to ensure the above bonding so that the surface is free from oxidation, oil and water.

3) Control of Viscosity and Thickness

The relationship between ink viscosity and thinner is shown in Fig. L.

As can be seen from the diagram, at 5% of the points, the wet film has a low dryness of 150 PS, which is below the thickness of this viscosity print and does not meet the requirements. In principle, no thinner should be added to wet film printing, 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%

In the formula, HW is the thickness of the wet film; HS is wire mesh thickness; S is the filling area; P is the solid content of ink.

Take the 100-purpose wire mesh as an example:

Screen thickness: 60 μ M; Opening area: 30%; Solid content of ink: 50%.

The thickness of wet film=[60-60] * 70%] * 50%=9 μ M

When a wet film is used to resist corrosion, its thickness is generally required to be 15-20 μ M; When used for anti-plating, the film thickness is generally required to be 20-30 μ M. Therefore, when the wet film is used to resist corrosion, it should be printed twice with a thickness of 18 μ M or so, which meets the requirements of corrosion resistance; When used for electroplating resistance, it should be printed 3 times with a thickness of 27 μ M or so, which meets the requirements of anti-plating film thickness. When the wet film is too thick, it is liable to have shortcomings such as underexposure, poor imaging, poor etching resistance, etc. When the film is plated, it will be immersed by medicinal water, causing defilm phenomenon, and high-pressure sensitivity. When the film is bonded, it will be easy to produce the sticky film. Over-exposure, poor electroplating insulation, defilm and electroplating of metal on the film are easy to occur when the film is too thin. In addition, the speed of defilm removal is slower when the film is overexposed.