Exposure means that under the irradiation of ultraviolet light, the photoinitiator absorbs the light energy and decomposes into free radicals, and the free radicals then initiate the photopolymerization monomer to carry out the polymerization and crosslinking reaction. Exposure is generally carried out in an automatic double-sided exposure machine. Now the exposure machine can be divided into air-cooled and water-cooled according to the cooling method of the light source.

Factors Affecting Exposure Image Quality

In addition to the performance of the film photoresist, the factors that affect the quality of exposure imaging are the selection of light sources, the control of exposure time (exposure amount), and the quality of photographic plates.

1) The choice of light source

Any kind of film has its own unique spectral absorption curve, and any kind of light source also has its own emission spectral curve. If the main spectral absorption peak of a certain type of film can overlap or mostly overlap with the spectral emission main peak of a certain light source, the two are well matched and the exposure effect is the best.

The spectral absorption curve of the domestic dry film shows that the spectral absorption region is 310-440 nm (nanometer). From the spectral energy distribution of several light sources, it can be seen that the pick lamp, high pressure mercury lamp, and iodine gallium lamp have relatively large relative radiation intensity in the wavelength range of 310-440nm, which is an ideal light source for film exposure. Xenon lamps are not suitable for exposure of dry films.

After the light source type is selected, the light source with high power should also be considered. Because of the high light intensity, high resolution, and short exposure time, the degree of thermal deformation of the photographic plate is also small. In addition, the design of lamps is also very important. It is necessary to try to make the incident light uniform and parallel, so as to avoid or reduce the poor effect after exposure.

2) Control of exposure time (exposure amount)

During the exposure process, the photopolymerization of the film is not “one-shot” or “one-exposure”, but generally goes through three stages.

Due to the obstruction of oxygen or other harmful impurities in the membrane, an induction process is required, in which the free radicals generated by the decomposition of the initiator are consumed by oxygen and impurities, and the polymerization of the monomer is minimal. However, when the induction period is over, the photopolymerization of the monomer proceeds rapidly, and the viscosity of the film increases rapidly, approaching the level of sudden change. This is the stage of rapid consumption of the photosensitive monomer, and this stage accounts for the majority of the exposure during the exposure process. The time scale is very small. When most of the photosensitive monomer is consumed, it enters the monomer depletion zone, and the photopolymerization reaction has been completed at this time.

Correct control of exposure time is a very important factor in obtaining good dry film resist images. When the exposure is insufficient, due to the incomplete polymerization of the monomers, during the development process, the adhesive film swells and becomes soft, the lines are not clear, the color is dull, and even degummed, and the film warps during the pre-plating or electroplating process. , seepage, or even fall off. When the exposure is too high, it will cause problems such as difficulty in development, brittle film, and residual glue. What is more serious is that incorrect exposure will cause deviation of image line width. Excessive exposure will thin the lines of pattern plating and make the lines of printing and etching thicker. On the contrary, insufficient exposure will make the lines of pattern plating become thinner. Coarse to make the printed etched lines thinner.