Discussion on Correct Exposure of Screen Stencil (I)

First, foreword Screen printing is a practical technology, its application is more and more extensive, printed circuit is the most demanding one in screen printing. Screen printing includes two parts: plate making and printing. Printing high-quality, high-precision, and high-density printed circuit boards depends on high-quality screen templates. Photolithography process in photolithography is one of the key links. Many of the problems with web templates are associated with the exposure of templates. The purpose of stencil exposure is to generate an image or copy a document on a screen plate. It is important to use ultraviolet light to excite chemical reactions during exposure to convert the polymer compound of the emulsion into a large polymer, change its properties, convert from a water-soluble substance into a water- and solvent-resistant substance, and harden the emulsion. Printing adhesive wear. When the exposure is insufficient, the photochemical effect of the emulsion is incomplete, the adhesive layer is thin, the sharpness of the edge is poor, and jaggies are easily generated. In severe cases, the photosensitive emulsion is detached during the process of developing and rinsing. Excessive exposure may cause difficulty in development and decrease the resolution.
The correct exposure should be a screen stencil sufficient to achieve the required cross-linking strength. The quality of the image when exposed to a screen stencil: its reproducibility, resolution, and stencil printing rate depend on the following independent variables:
* Sensitive emulsion: type of emulsion, coating thickness, degree of drying (temperature and time);
* Wire mesh: the number of meshes, thickness, and the color of the screen;
* Light source and exposure equipment: the type of light source, geometry and photofinish, printing distance and exposure time;
* Matching, the degree of coincidence between the spectral sensitivity of the emulsion and the splitting energy distribution of the light source;
* Glass, polyester film base transmittance of ultraviolet light, exposure table placement.
Second, sensitising emulsions have certain requirements for photographic emulsions, and their plate making performance is good. For example, they are easy to apply and have a suitable range of photosensitivity. The wavelength is between 330 and 420 nm. (The operation and storage of materials with long exposure wavelengths are strictly required. In the darkroom conditions, but the wavelength is too short is difficult to select and protect the light source), high sensitivity, fast energy saving, tolerance, good performance.
The type of sensitizer (ie, photoinitiator) in the emulsion directly affects the photosensitizing time. At present, there are two major classes of sensitizers in the emulsion; diazo resins and non-diazoresins, and dichromate. Sensitizers have been eliminated for environmental reasons.
Diazo resin emulsions have high photosensitivity, good resolution, water resistance, solvent resistance and abrasion resistance. They are currently the most widely used emulsions. The peak of photosensitivity wavelength is near 365nm, and there is a large exposure tolerance. Degree; the other is non-diazoresin, which refers to photographic emulsions that are different from diazo resins. They are a newly developed class of pure photosensitizing compounds. Photosensitivity is faster than that of nitrogen resins, but the exposure latitude is more dilute. Resins are narrow, and such activated emulsions have long storage time, short exposure time, and strong resistance to heat and moisture. Generally, all good emulsions have a wide exposure latitude. A lower exposure latitude means a maximum resolution, and a higher point means the best intensity of a template. In fact, you should choose between these two extremes to find the most appropriate application and required exposure time or exposure energy.
The coating thickness of the emulsion affects the exposure time, and the exposure time is proportional to the coating thickness. Generally, the thicker the emulsion, the longer the exposure time. Uniformity of the thickness of the emulsion coating is also important, and it is difficult to determine the exposure time for a coating of uneven thickness. Semi-automatic or automatic version of the same emulsion coating machine for high-precision large-area emulsion coating can eliminate film thickness uneven phenomenon, set the coating pressure and the number of times to determine the thickness of the coating layer Thickness accuracy can be within 3μm. However, the uniformity of the thickness of the indirect method film film can be guaranteed. The thickness of the film can be measured with a thickness tester. Magnetic induction or eddy current test can achieve very simple and accurate non-destructive testing in various sizes of stencils. First, test the mesh thickness of the mesh and test the coating layer. The thickness of the emulsion can be used to obtain the correct data for the thickness of the emulsion film using the difference between the two tested values.
The degree of drying of the emulsion, that is, the drying temperature and time, also has an effect on the exposure time. Since sensitizing emulsions contain a trace amount of moisture for a long period of time, poorly dried screen stencils have the effect of reducing the photosensitivity of the emulsion emulsion coating due to residual moisture. Although the exposure time or energy is correct, it will still cause defects. In general, the drying temperature should be controlled below 40°C. If the temperature is too high, a dark reaction will occur to the emulsion, which will result in difficulty in developing and flushing. Insufficient dryness of the photographic emulsion coating mainly causes poor adhesion of the film to the screen, and in severe cases, the stencil image partially falls off during the rinsing. It is not a simple matter to determine the moisture content in the process of applying the emulsion. Generally, it is judged according to the air temperature, humidity, and drying time in the operation room. The use of the contact humidity tester can eliminate unreliable judgments and provide the correct template. The necessary basis for exposure. The moisture tester probe is placed on the emulsion coating under test, and the residual moisture can be determined on the indicator so that it can be known whether the moisture in the emulsion coating is completely eliminated.
Third, wire mesh screen is the carrier of emulsion coating. There are two kinds of screens for fine printed circuit board making: monofilament woven polyester mesh and stainless steel mesh. Different specifications and types of screens have different mesh numbers, mesh yarn diameters, opening degrees and thicknesses. This can affect the thickness of the screen stencil coating (in order to obtain the ideal screen printing effect, the coating thickness is usually 1/4 of the screen thickness), and it also affects the light reflection in the mesh. The lower the number of screen mesh, the larger the mesh, the thicker the mesh, the thicker the emulsion coating, the longer the required exposure time; the higher the mesh size, the smaller the mesh, and the finer the mesh. The thinner the emulsion coating, the shorter the required exposure time. However, from another point of view, due to the increase in the surface area of ​​the fine screen, the degree of light reflection during exposure is stronger than that of the thick screen. Therefore, comprehensive consideration must be given to precision plate making.
When the sensitizer in the emulsion is activated by ultraviolet light and reacts with the film forming agent in the emulsion and crosslinks with each other, the film forming agent will be completely broken to form a photosensitive image. When white polyacetal wire mesh is used, the reflection of UV light by the mesh yarn will cause the diffuse reflection of the inverted wall to infiltrate into the underside of the image edge of the photographic film and cause the phenomenon of overexposure, so that the edge definition of the image is reduced, and the sawtooth appears, and the reproducibility is poor; Sensitization time to reduce this side effect, but it will hinder the full hardening of the emulsion, reduce the printing rate of the template, so the fine plate is not appropriate to use white screen.
When using a mesh of more than 125 mesh/inch, dyeing meshes such as orange and warm yellow should be used. Dyeing screens absorb UV light from 330 to 420 nm very well, which eliminates diffuse reflections of the screen mesh yarn and is beneficial to the definition, resolution, and chemical solvent resistance of the template, thereby increasing the accuracy of the screen template and reproducing the original image. The ability and the exposure latitude of dyed silk screens is much larger than that of white screens, so the use of dyed screens has to extend the exposure time by 30% to 50%.
When using a stainless steel screen, the exposure time is longer than that of the vinegar screen because the irradiation light is blocked by the screen mesh.