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Analysis of Silkscreen Printing Process

by: Dec 12,2018 392 Views 0 Comments Posted in PCB Manufacturing Information

Background:

PCB manufacturing is now a very developed high-tech industry, and the rapid development of silkscreen technology has injected new vitality into the industry, brought about major changes to the electronics industry, and it has become an indispensable part of various electronic equipment and instruments. The current screen printing technology has been able to adapt to the high density of PCB production.

silkscreen_副本11.png

Youtube: https://www.youtube.com/watch?v=mwy9Kigv9Mg

Materials:

1)     Stencil:

Stencil is the most important part of this process, this is because it is a key factor in controlling the flow and thickness of the printing ink, and it also determines the durability of silkscreen printing. In addition, stencil and the photosensitive materials have excellent combination is also an important factor in the production of high-quality, high-precision silkscreen printing. In order to ensure a good combination of stencil and photosensitive materials, the traditional approach is to the new stencil coarsening and degreasing treatment, so as to ensure the quality of the stencil, colleagues can also extend the service life of the stencil.

2)     Light-sensitive Materials:

Printing light-sensitive materials commonly used are diazo photosensitive agent, photosensitive film, etc. Diazo emulsion is widely used in silkscreen printing. The film thickness is uniform and controllable. High resolution image force, high resolution, wear resistance, strong adhesion with the screen and other characteristics, in the printed board character printing has been a wide range of the silkscreen printing process.

3)     Stencil Frame:

The material and section shape of the stencil frame is very important, relative to a certain specification of the stencil frame, if the stencil frame strength is not enough, it cannot ensure the consistency of tension. Now the general use of high tension aluminum stencil frame.

Silkscreen Printing:

1)    Silkscreen Printing Process:

Make soldering film negative → Punch position holes for the film negative → Surface cleaning → Configure printing ink → Print both sides of PCB → Preheat the board → Exposure → Develop → Hot-set

2)    The Key Process of Silkscreen:

A.     Preheating:

Preheating is to evaporate the solvent contained in the ink, so that the soldering film becomes a non-sticky state. For the different ink, its preheating temperature, time is not the same. If the preheating temperature is too high or the drying time is too long, it will lead to poor development and reduce the resolution. Of course, if the preheating time is too short or the temperature is too low, the film will stick to when it is on the exposure, and then, in the development stage, the solder resistance film will be eroded by the sodium carbonate solution, which will result in the loss of luster on the surface or expansion and peeling of the solder resistance film.

B.     Exposure:

Exposure is the key to the whole process. In the case of positive images, due to light scattering, soldering film on the edge of the graph or line reacts with light (mainly photosensitive polymer in the soldering film reacts with light) to generate residual film in the case of overexposure, resulting in reduced resolution, smaller graphics and thinner lines. If the exposure is insufficient, the result is contrary to the above situation: the developed figure becomes larger and the line becomes thicker. This situation can be reflected by the test that if the exposure time is long, the measured line width is negative tolerance. On the contrary, if the exposure time is short, the measured line width is positive tolerance. In the actual process, the "light energy integrator" can be used to determine the best exposure time.

C.     Ink Viscosity Regulation:

Liquid photoresist ink viscosity is mainly controlled by the ratio of hardener and main agent and the amount of diluent added. If the amount of hardener added is not enough, may produce ink characteristics of the imbalance. After the hardener is mixed, it will react at room temperature. The viscosity changes are as follows.

Within 30min: the ink main agent and hardener have not fully integrated, liquidity is not enough, which will block the stencil when printing.

30min~10h: ink main agent and hardener have been fully integrated, the flow is appropriate.

After 10h: the ink itself has been the reaction between the material has been active, resulting in greater fluidity, it is not easy to print, the longer the hardener mixture of time, resin and hardener reaction is more adequate, then the ink gloss is also better. In order to make the ink gloss uniform, good printing, it is best to mix the hardener after 30 minutes to start printing.

If the thinner added too much, will affect the heat resistance and hardening of ink. In short, liquid photosensitive soldering ink viscosity regulation is very important. For example, if the viscosity is too thick, silkscreen printing is difficult. If the viscosity is too thin, the amount of volatile solvent in the ink is more, which will be more difficult to precure.

Conlusion:

The whole process of screen printing is not too difficult, but it is not very simple. In order to ensure the quality of silkscreen printing, in operation, each step requires the operator to strictly follow the operating standards, to prevent any quality defects arising which caused by the operator's non-standard operation in silkscreen printing process.


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