There are two main systems for photosensitive adhesives, one is diazo photosensitive adhesive, and the other is pre-sensitized photosensitive adhesive, such as SBQ photosensitive adhesive. Different systems require different methods to measure the exposure time.

The advantage of diazo is to change the color of the emulsion from light yellow to neutral yellow. The yellow color comes from the sensitizer. Once the diazo reacts, the cross-linking is completed, and the color disappears. You can usually see the obvious difference in color between the exposures at all levels until the cross-linking is completed. From the exposure, if the color does not show any change, in general, this is the correct exposure. At this exposure, the amount of resolution or subtle levels that can be seen depends on many variables:

(1) The quality of the exposure lamp.

(2) The color of the wire mesh.

(3) The thickness of the coating.

(4) The color of the emulsion.

(5) Resolution of emulsion.

(6) The expiration date of the emulsion.

(7) Exposure latitude of emulsion.

In any case, the color change may be a good way to determine the correct cross-linking of the diazo photoresist.

It is a little more difficult to judge over-sensitization, because the color of the white silk screen is different from the dyed silk screen, and the color of the photoresist on it is also different. A bad exposure lamp may show overexposure despite underexposure.

In order to distinguish between overexposure and bad exposure light, you should check the previous exposure for color changes. If the correct exposure looks like overexposure, it is likely that the exposure light is not good.