Application of Screen Printing for Alumina Metallized Ceramics

In view of the above characteristics, screen printing is widely used in different fields, and the metallization of alumina ceramics in the electronics industry is a typical application field. With the miniaturization and powerization of electronic devices, the requirements for Ceramic metallization are getting higher and higher. It is necessary to have a deep understanding of the application of screen printing technology in ceramic metallization. The following will introduce the practical experience in the production of aluminum oxide metallized heat dissipation ceramic substrate in semiconductor refrigeration devices in recent years.

1 Screen printing plate making
1.1 Materials for screen printing
1) Silk screen is one of the main materials. The mesh number is an important indicator, which refers to the number of meshes per square centimeter of the mesh. The higher mesh means the denser the silk screen. When the mesh is large, the slurry is deposited more and the film thickness increases. According to different printing needs, different meshes can be selected, generally between 120 and 300 meshes. In chip semiconductor refrigeration devices, silk screen with 250 to 300 meshes are mostly used.
At present, the commonly used silk screen is nylon one, which has the advantages of smooth surface, good slurry permeability, long service life, great acid resistance and ideal organic solvent resistance.
2) Stretching the net. Generally, the screen frame is made of aluminum. The stretching process directly affects the quality of the plate making. The critical point of stretching the screen is to control the tension.
3) Screen photosensitive glue. The main requirements of screen printing platemaking for photosensitive gule materials are good platemaking performance, easy to coat, a certain photosensitive spectral range (ultraviolet), good development performance, and high resolution. The main components of photoresist are film formers, sensitizers and auxiliary agents.
4) Scraper material. Scrapers are commonly made of polyester compound, rubber, PVC, etc. When the mechanical device is stable, the quality and use of the scraper is one of the factors that affect the printing quality. .
5) Bottom plate. It is the basis for screen printing plate making, and comes from the original design. At present, the design of the original manuscript is designed by CAD method, and the material of the bottom plate is generally made of film.

1.2 Plate making operation
With the above basic conditions, the operation of screen printing plate making can be carried out. The main points are as follows:
(1) Surface treatment of nets and frame, cleaning and decontamination. (2) Coating with mesh glue. (3) Stretching net production. (4) Preparation of photosensitive adhesive. According to different photoresist, different methods are used. (5) Silk screen glue. (6) Making a mask. (7) The service life of the screen template.

2 screen printing

The current methods of screen printing include manual or mechanical flat printing, mainly involving manual or flat printing machines, and its working principle is shown in Figure 1.

Figure 1

The slurry is transferred through the mesh and deposited on the Ceramic Substrate; at this time, during the screen printing process, the Alumina Ceramic plate is adsorbed on the platform, the squeegee squeezes the slurry and the screen, and the printing plate makes the screen printing plate and the substrate The sheet forms a pressing line, the screen has tension N1 and N2, which generates F2 force on the squeegee, and the rebound force makes the screen printing not contact with the substrate except the pressing line, and the extrusion force of the paste on the squeegee is F1. Under the action, the desired pattern is produced by transferring and depositing from the moving embossing line to the substrate through the mesh. During the printing process, the screen printing plate and the squeegee move relative to each other, and the squeezing force F1 and the elastic force F2 also move. During the printing process, it is constantly deformed and rebounded. During manual printing, the proficiency of the operator directly affects the formation of the embossing line. During printing, the squeegee moves straight forward, cannot shake left and right, cannot be slow forward and fast back, and the inclination angle of the squeegee should be consistent.

3.Printing paste

3∙1 Preparation of printing paste
According to the requirements of the product, the printed paste is W(Mo)-Mn paste. It consists of three parts ① conductive component: W or Mo powder; ② activator: SiO2, MnO and Al2O3; ③ organic carrier. The proper ratio of the three will produce the ideal metallization pattern and metallization film thickness.
Slurry: put the organic carrier, powder and ceramic balls in appropriate proportions, put them in a vibrating mill for a few hours, and then sieve the balls out for use. This mixed liquid is called slurry. The organic carrier in this process must be appropriate, too few organic carriers, and the mixed paste concentration is too high, printing is difficult, resulting in pinholes in the printed graphics. On the contrary, if there are too many organic carriers, the graphics are prone to appear with low resolution; therefore, mastering the ratio of organic carriers, powders and ceramic balls is a key factor in printing.

3.2 Influence factors of paste on printing process
The influence factor of the paste on the printing process is the viscosity, which refers to the resistance generated when one layer of fluid moves relative to the other. The viscosity is high, the fluidity is low, and it is easy to miss printing and stick to the screen during printing, which causes trouble and unnecessary waste to the subsequent inspection process; Therefore, the viscosity of the paste is also an important factor affecting the printing quality, and the printing is generally around 5Pas. When the viscosity is too large, an organic carrier can be added as a diluent to appropriately reduce the viscosity, which is beneficial to printing. In addition, the particle size of the paste has a great influence on printing. For the same paste composition, if the particles are large, the paste viscosity will be low; if the particles are small, the paste viscosity will be high. The powder in the slurry is generally very fine, that is, the finer the particles, the better, generally less than 0∙8 μm, so that it can pass through the mesh. At the same time, it is beneficial to drying, sintering, electroplating and other processes, especially after electroplating. If the paste meets the requirements, the electroplating time is short, and the product surface is smooth, clean and bright.

4 Quality control points of silk screen printing
The quality of the printing plate has a direct impact on the printing quality. In order to ensure the quality of screen printing, the quality control of the screen printing plate mainly checks the following aspects:
(1) Good reproducibility. Reproducibility refers to the reproduction ability of the screen printing plate to the original text, which is mainly reflected in the accuracy, clarity and resolution of the printing plate.
(2) The thickness of the plate film is appropriate. Its thickness determines the thickness of the print and the clarity of the graphics.
(3) High print durability. It refers to the ability of the printing plate to withstand the number of prints.
(4) Good release property. It refers to the ease with which the film is removed from the screen of the printing plate.

5 Factors Affecting Quality Control Points
In actual production, there are many defects after printing. The operators on site monitor the printing quality in real time. When printing defects occur, the production should be stopped immediately and reasonable process parameters should be adjusted. It can be seen from the above analysis that the quality of the whole process of screen printing directly affects the performance of the alumina ceramic metallization layer. The metallization properties of alumina ceramics are more excellent.