1.1 Properties of printing materials: viscosity, fineness and fluidity
1.2 Screen state: mesh, tension and photosensitive adhesive
1.3 Screen printing conditions: printing pressure, blade hardness, angle, screen printing speed
1.4 Screen printing environment: indoor temperature, humidity, cleanliness, if liquid photosensitive printing material is used, (it should also be operated in the yellow light area).
The printing material is composed of synthetic resins, pigments, fillers, solvents, etc. Its viscosity will be affected by temperature, and the viscosity will decrease when the temperature rises.
If the temperature and humidity of the screen printing room are kept constant day and night (usually 21±1℃, 55±%RH), and the screen plate and PCB screen printing conditions are kept standardized, the printed quality will be stable. When the room temperature is low, the viscosity is high. In order to reduce the viscosity, diluent is added. As a result, although the viscosity is reduced, the coating performance is also affected. If diluent must be added, it should be fully mixed and stirred. It is better to start screen printing again.
The ideal printing material is a moderate decrease in viscosity when the temperature rises. Under the room temperature, how much is the most suitable viscosity of the printing material? This number is difficult to determine, because it is related to the screen mesh, printing pressure, screen distance and other settings.
The particle size of the solids in the printing material and the size of the agglomerates determine the mesh number of the selected mesh, so that the printing material has good fluidity. For good screen printing, the opening of the screen should be 3 to 5 times the average size of the particles in the printing material. For polyester mesh, if it is printed with a 305-mesh mesh, the mesh opening is 48.3 μm, the average particle size of the printed material should be below 10 μm, and the particle size of the printed material can be checked with a fineness meter.
After the printing material is stored for a period of time, the pigment particles will agglomerate, or the auxiliary such as leveling agent will float on the surface. Therefore, the printing material must be fully stirred before use, but most printing materials are thixotropic fluids. When the molecular attraction of the thixotropic fluid is cut off after stirring, it must take a period of time to recover, so after the printing material is stirred, it is best to stand for 30 minutes before starting to use.
The so-called thixotropy is a characteristic of non-Newtonian fluids, and its physical meaning is that under a certain shear rate, the viscosity decreases with the increase of time.
The definition of viscosity represents the fluidity of the printing material, which is usually measured by a viscometer, and the thixotropic index can be expressed by TI, which means the ratio of the viscosity value at low speed to the viscosity value at high speed.
The larger the TI value, the greater the thixotropy of the printing material, and the smaller the TI value, the smaller the thixotropy.
The TI value of the printed material is large, the printing pressure is light, and the wire resolution is good, but the surface leveling is poor, and there are easy to have mesh marks left on it. The TI value is small, the wire resolution is poor, but the leveling performance is good, so you should choose a suitable TI range.
In fact, the industrial TI value only refers to the viscosity index, and does not show the essence of thixotropy, so the time factor is not taken into account. Slowly flowing, when it reaches equilibrium and stability, the edge of the printed wire will maintain the sag better than the printing material with small thixotropy, but when the thixotropy is too large, the time required to restore the original viscosity is too long but reduced resolution, and even cause the phenomenon of bleed.
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