Dry film exposure means that under ultraviolet light, the photoinitiator absorbs the light energy and decomposes into free groups, which then initiate the polymerization cross-linking reaction of the photopolymeric monomer, and form a large molecular structure insoluble in dilute alkali solution. Exposure in general automatic double-sided exposure machine, exposure machine now depending on the cooling system of the light source can be divided into air cooling and water cooling type two kinds, the influencing factors on the quality of the exposure imaging in addition to the dry film to resist performance, the choice of light source, exposure time, exposure volume) of the quality of the control, photoplate etc are important factors affecting the quality of the imaging exposure.

First, the choice of light source

Each kind of dry film has its own characteristic spectral absorption curve, and each kind of light source also has its own emission spectral curve. If the absorption peak of a dry film overlaps or mostly overlaps the emission peak of a light source, the two are well matched and the exposure effect is very good. The spectral absorption curve of domestic dry film shows that the spectral absorption region is 310-440 nm(nanometer). Pick lamp, high pressure mercury lamp and iodide gallium lamp all have high relative radiation intensity in the wavelength range of 310 -- 440nm, so they are ideal light sources for dry film exposure. Xenon lamps are not adapted to dry film exposure. At the same time, we should also consider the selection of high-power light source, because the light intensity is large, the resolution is high, and the exposure time is short, the degree of thermal deformation of the photographic plate is small, in addition, the design of lamps and lanterns is also very important, to make the incident light uniformity is good, high parallelism, in order to avoid or reduce the uneven exposure of the graph.

Two, exposure time (exposure) control

In the process of exposure, the photopolymerization of dry film is not "one initiation" or "one exposure", but generally through three stages.

The dry membrane is hindered by oxygen or other harmful impurities and thus requires an induction process in which the free radicals produced by the decomposition of the initiator are consumed by oxygen and impurities and little polymerization of the monomer is achieved. However, when the induction period is over, the photopolymerization of the monomer takes place quickly, and the viscosity of the film increases rapidly, approaching the degree of mutation. This is the stage of sudden consumption of the photosensitive monomer, and the time proportion of this stage in the exposure process is very small. When most of the photosensitive monomers are consumed, the monomer depletion zone is entered, and the polymerization reaction is completed at this time. The process is similar to that of an atomic bomb.

Correct exposure time control is very important to obtain good dry film anti - corrosion image. When the exposure is insufficient, due to the incomplete monomer polymerization, in the development process, the film melts and becomes soft, the lines are not clear, the color is dim, and even degumming. In the process of electroplating or pretreatment, the film becomes warped, permeated and even falls off. When exposed too will cause difficult to develop, film brittle, leaving gum defects. More serious is the incorrect exposure will produce the deviation of the image line width, excessive exposure will make the graphic electroplating lines thin, make the printed etching lines thick, on the contrary, the lack of exposure to make the graphic electroplating lines thick, make the printed etching lines thin.

How to determine the correct exposure time?

It is difficult for dry film manufacturers to recommend a fixed exposure time due to the different exposure machines used by different film manufacturers, that is, different light sources, lamp power and lamp spacing. Foreign dry film production companies have their own special or recommended use of some kind of optical density scale, dry film factory is marked with the recommended imaging level, domestic dry film manufacturers do not have their own special optical density scale, usually recommend the use of Iston 17 or Stouffer 21 grade optical density scale.

The optical density of the first stage Di was 0.5, and then increased with the difference of optical density AD of 0.05 for each stage until the optical density of the 17th stage was 1.30. The optical density of the first stage Di of the Stuffie 2L scale is 0.05, and then the optical density difference △D is 0.15 for each stage, until the optical density of the 2nd L scale is 3.05. In optical density measurements during exposure, optical density small (that is, the more transparent) level, dry film of ultraviolet energy, aggregation is completely, and the light dense degree of (transparent) with poor grades, dry film with less energy of uv light, incomplete, not occur polymerization or polymerization part is show off or leaving only at the time of development. In this way, different exposure time can be selected to obtain different imaging levels. The usage method of reston grade 17 optical density scale is introduced as follows: A. The film is downward when the exposure is made; B. Put the film on the copper-clad foil board for 15 minutes before exposure; C. After exposure, it was left for 30 minutes to develop. Choose any exposure time as the reference exposure time, denoted by Tn, and the Max series left after development is called the reference series. Compare the recommended use series with the reference series, and calculate according to the coefficient table below. Printing ink dry film manufacturer

Series difference coefficient K

1, 1.122 2.000 6

2, 1.259, 2.239 7

3 1.413 8 2.512

4 September 2.818 1.585

5 10 3.162 1.778

When the use series needs to be increased compared with the reference series, the exposure time of the series T = KTR is used. The exposure time of the series T = TR/K is used when the using series needs to be reduced compared with the reference series. In this way, a very good exposure time can be determined with only one test. In the case of matt density scale can also be observed by experience, with the method of gradually increasing the exposure time, according to the development of dry film brightness, image is clear, image line width is consistent with the original film to determine the appropriate exposure time.

Strictly speaking, measuring exposure by time is unscientific, because the intensity of the light source often changes with fluctuations in external voltage and lamp aging. The formula for the definition of light energy is E = IT, where E represents the total exposure, in millijoule/cm2; I represents the intensity of light in milliwatts per square centimeter; T is the exposure time in seconds. It can be seen from the above equation that the total exposure E varies with light intensity I and exposure time T. When the exposure time T is constant, the light intensity I changes, and the total exposure also changes. Therefore, although the exposure time is strictly controlled, in fact, the total exposure received by the dry film at each exposure is not necessarily the same, so the polymerization degree is different. To make each exposure energy the same, the light energy integrator is used to measure the exposure. Its principle is that when the light intensity I changes, it can automatically adjust the exposure time T, in order to keep the total exposure E unchanged.

Three, the quality of the photographic plate

The quality of photographic plate is mainly manifested in optical density and dimensional stability.

As for the optical density, it is required that the maximum optical density Dmax is greater than 4 and the minimum optical density Dmin is less than 0.2. Max optical density refers to the lower limit of light blocking film on the surface of the film in ultraviolet light, that is to say, the film opaque area of light blocking density Dmax exceeds 4, to achieve good light blocking purpose. The minimum optical density refers to the upper limit of light blocking outside the transparent film of the film in ultraviolet light, that is to say, when the optical density Dmin of the transparent region of the film is less than 0.2, good light transmission can be achieved. The dimensional stability of photographic plate (refers to the change with temperature, humidity and storage time) will directly affect the dimensional accuracy and image coincidence degree of the printed board. The photographic plate size will greatly expand or shrink the photographic plate image will deviate from the drilling of the printed board. The size of domestic rigid film varies greatly under the influence of temperature and humidity, and its temperature coefficient and humidity coefficient are about (50-60)×10-6 / ℃ and (50-60)×10-6 / %. For a film with a length of about 400mm, the size can change up to 0.5-1mm in winter and summer. May deviate the distance from half hole to one hole during imaging on a printed board. Therefore, photographic plate production, use and storage are in constant temperature and humidity environment. In addition, if there is a problem with the cooling system of the exposure machine, the high temperature in the machine will cause the film to expand and shrink, affecting the positioning accuracy.

The use of thick polyester based silver salt sheets (e.g. 0.18mm) and diazo sheets improves the dimensional stability of photographic substrates. In addition to the above three main factors, the choice of vacuum system and vacuum frame material of the exposure machine will also affect the quality of exposure imaging.

Four, exposure positioning (applicable to non-automatic exposure machine)

1) Visual positioning is usually applied to manual positioning exposure using diazo substrate, which is brown or orange translucent; But not ultraviolet light, through the diazo image so that the bottom plate of the pad and the printed board hole coincidence alignment, fixed with tape can be exposed.

2) Off-pin positioning system The off-pin positioning system includes photographic film puncher and double round hole off-pin positioner. The positioning method is: first of all, the front and the back of the two plate drug film relative alignment under the microscope; Will aim at two negatives with film puncher on blank effective image was charged two locating hole, take a rush good positioning hole blank as to drilling program, can be drilled at the same time element data with holes and positioning hole, a one-time holes and positioning hole drilled component, PCB hole metallization and after copper plating technique, used double round hole out of stock locator positioning exposure.

3) Fixed pin positioning, fixed pin is divided into two systems, one fixed photographic plate, the other fixed printed board, by adjusting the position of the two pins, to achieve the photographic plate and printed board overlap alignment.

Generally, the polymerization reaction will continue for a period of time after exposure. In order to ensure the stability of the process, do not immediately remove the polyester film after exposure, so that the polymerization reaction will continue. Generally, after exposure, it is necessary to place the film for 15-30 minutes before developing, and then remove the polyester film before developing.