What Does Exposure Mean in PCB Production?

In PCB (Printed Circuit Board) production, the main function of exposure is to enable the transfer of the lines on the negative to the laminated substrate. Specifically, the exposure process involves the use of diazo film to cover the pads on the PCB board so that they are protected from UV light during the exposure process. The soldermask protective layer is more strongly attached to the PCB board surface after UV irradiation, and the pads can be exposed to copper soldering disks because they are not exposed to UV irradiation, so that they can be rectified in the hot air leveled on the lead tin.

Exposure is the use of light will mask the graphics on the plate after the optical system projected onto the photoresist, to achieve the graphics transfer, is one of the important processes in the integrated circuit manufacturing lithography process. Poor exposure may lead to post-process mounted components of the pad or the need to solder the place of crosslinking reaction, in the development of this part of the green oil is not dissolved by the solution, failed to reveal the soldering of the pad part of the outside or all of them, called poor exposure. Poor exposure will lead to the emergence of such as the inability to mount the original, poor soldering and serious open-circuit and other quality problems.

Exposure plays a vital role in PCB production.

Through the exposure, the designed circuit pattern can be transferred to the photosensitive adhesive layer on the PCB board, thus realizing the transfer of circuit graphics. The quality of exposure directly affects the quality and performance of the final PCB product.

The exposure process uses light to project the graphics on the mask plate onto the photoresist after passing through an optical system, so that the localized area that is irradiated reacts and forms the circuit graphics after development. Poor exposure can lead to quality problems such as failure to mount the original, poor soldering and severe open circuits.

In addition, the exposure process requires attention to control the exposure time, light source selection, exposure environment control, film negative installation and the first piece of production and other aspects of the problem, in order to ensure that the exposure of quality and effectiveness.

PCB production exposure process is as follows:

Pre-treatment: Before exposure, the PCB surface is first pre-treated to remove surface oils, oxides and other impurities to improve the adhesion of photographic ink and PCB surface.

Coating Photographic Ink: After pretreatment, the PCB surface is coated with a layer of photosensitive ink. Photographic ink is a light-sensitive material, under the action of light will occur chemical changes, so as to realize the transfer of circuit graphics.

Drying: After coating the photographic ink, the PCB needs to be dried to improve the adhesion and corrosion resistance of the photographic ink.

Exposure: The circuit negative is aligned with the circuit board with pressed dry film, and then placed on the exposure machine for exposure. The dry film is fully exposed under the energy of the exposure machine lamps to the areas of the circuit negative where there are no lines. After this step, the line is transferred to the dry film, the state of this time is that the dry film has a line of the place is not exposed (light by the film on the pattern blocking, did not illuminate the board), there is no line of the place is exposed.

Developing: Develop the unexposed part with the developer, the developer does not reflect on the exposed part. Therefore, the final picture is a yellow copper on the part with lines, while the part without lines is still blue (the exposed dry film).

Electro-copper: The board is put into the electro-copper equipment, the part with copper is electrically coated with copper, while the part blocked by the dry film does not react.

In the exposure process of PCB production, you need to pay attention to the following matters:

Exposure Time Control: Exposure time is very important, it determines the degree of curing of photosensitive adhesive on the PCB.Too long exposure time will lead to excessive curing of the adhesive layer, and too short exposure time will lead to the adhesive layer is not cured completely.Therefore, it is necessary to adjust the exposure time according to the thickness of the photosensitive adhesive, the intensity of the light source and the characteristics of the exposure equipment.Selection of light source: the choice of a suitable light source plays a crucial role in the quality of exposure.Common light sources include ultraviolet lamps, laser light sources and so on.Different light sources have different wavelengths and light intensity, you need to choose the right light source according to the requirements of the PCB board.

Exposure Environment Control: The exposure environment should be kept dry and clean to avoid the entry of dust and impurities. Dust and impurities may lead to adverse effects during exposure, such as bubbles or particles on the adhesive layer.

Film Negative Installation:  Before the film on the machine, you need to wipe the upper and lower surfaces of the film with a dust-free cloth and film cleaning solution to check for scratches and ink adhesion, and only after passing the inspection can it be put on the machine. The positive characters face up, the film number in the upper right corner is the upper frame of the film, in the upper left corner is the lower frame of the film.Wipe the glass cover plate and the table with a dust-free cloth moistened with alcohol, and then the PIN holes of the PCB board to be produced are set on the PIN nails on the table for positioning (the upper and lower frames of the PCB board are distinguished from the same as the film). Film placed on the board, manually align the film alignment points with the alignment points on the board surface, click the negative installation, exposure machine automatic cover plate vacuum installation of film, installation is completed automatically open the cover plate. In order to prevent insufficient vacuum, you can put black tape around the film to assist in fixing.First production: to be exposed to the board and film installation is completed according to the different ink models, according to the document requirements to set the corresponding board thickness, alignment accuracy, exposure energy, and put the corresponding digital grid of the exposure ruler.