Summary of common problems in PCB dry area (circuit, solder mask, text) (2)

11. Why does the exposure machine need to suck a vacuum when exposing?

Answer: In non-parallel light exposure operations (exposure machines with "points" as the light source), the degree of vacuum is a major factor affecting the quality of exposure. Air is also a medium layer. , There is air between the air extraction film, then it will produce light refraction, which will affect the effect of exposure. Vacuum is not only to prevent light refraction, but also to prevent the gap between the film and the board from expanding and to ensure alignment /The quality of the exposure.

12. What are the advantages of using volcanic ash grinding board for pretreatment? Disadvantages?

Answer: Advantages: a. The combination of abrasive pumice powder particles and nylon brushes is tangentially rubbed with cotton cloth, which can remove all dirt and expose fresh and pure copper; b. It can form a completely grit, rough and uniform The surface and the multi-peak surface, there is no arable land groove; c. Due to the relaxation of the nylon brush, the connection between the surface and the hole will not be damaged; d. The flexibility of the relatively soft nylon brush can compensate for the The problem of uneven plate surface caused by brush wear; e. Since the plate surface is uniform and without grooves, the scattering of the exposure light is reduced, thereby improving the imaging resolution. Disadvantages: The disadvantages are that the pumice powder is easy to damage the mechanical parts of the equipment, the control of the particle size distribution of the pumice powder, and the removal of the pumice powder residue on the surface of the substrate (especially in the holes).

13. What effect will the developing point be too large or too small?

Answer: The correct development time is determined by the development point (the point where the unexposed dry film is revealed from the printed board). The development point must be maintained at a constant percentage of the total length of the development section. If the developing point is too close to the outlet of the developing section, the unpolymerized resist film will not be sufficiently cleaned and developed, and the resist residue may remain on the board surface and cause unclean development. If the developing point is too close to the entrance of the developing section, the polymerized dry film may be etched by Na2C03 and become hairy due to prolonged contact with the developing solution.

14. Why do we need to pre-bake the board before printing?

Answer: The pre-baking of the board before the character printing a is to enhance the bonding force between the board and the characters, and b to enhance the hardness of the solder mask ink on the surface of the board to prevent the solder mask from being easily crossed during the printing of the characters or the subsequent processing.

15. Why do we need to swing the brush of the pre-treatment plate grinding machine?

Answer: There is a certain distance between the brush pin reels. If you don’t use the rocking plate directly, there will be many places that can’t be worn, resulting in uneven cleaning of the plate surface. If you don’t need to rock it, a straight groove will be formed on the plate surface. Causes wire breakage, easy to break holes and cause tailing phenomenon without swinging the hole edge.

16. What effect does the squeegee play on printing?

Answer: The angle of the squeegee directly controls the amount of oil, and the uniformity of the blade to the surface directly affects the surface quality of the printing.

17. What effect does the temperature and humidity in the solder mask and circuit darkroom have on PCB production?

Answer: When the temperature and humidity in the darkroom are too high or too low: 1. It will increase the garbage in the air, 2. The film sticking phenomenon is easy to appear in the alignment, 3. It is easy to cause the film to deform, 4. It is easy to cause the board surface to oxidize.

18. Why don't PCB board solder resist be used as a developing point?

Answer "Because there are many variable factors in solder mask inks, first of all, there are many and mixed ink types. The properties of each ink are different. When printing, the thickness of each board ink will cause uniformity due to the influence of pressure, speed and viscosity. It’s not the same as the dry film. The single thickness is more uniform. At the same time, the solder mask is also affected by different baking time, temperature, and exposure energy during the production process. Relatively speaking, it is difficult to ensure that the solder mask is used as a development point. The effect of the board is the same. So the practical significance of solder mask as a development point is not great.

19. How are ghosts produced? How to prevent it?

Answer: The ghost image of the circuit board usually appears on the white material board, and it is usually on the position of the single side window, because the white material does not have the UV light blocking function, when the PCB solder mask is exposed, there will be part of the unopened window surface Ultraviolet light penetrates the substrate to the edge of the PAD position of the window, causing the green oil at the bottom of the window opening to be exposed and cannot be completely developed, forming a ghost image. The ghost image of the yellow plate is usually caused by the refraction or diffraction of ultraviolet light, and it is more common in the gold finger position.

 Prevention: 1. The windows on the white board should be designed as large windows on both sides as much as possible. 2. If the window is opened on one side, the back of the window opening should be designed as a large copper surface as much as possible. 3. When the window is single-sided and the back of the window position is the substrate, the exposure energy of the substrate surface during exposure is 1-2 blocks lower than the window surface.

20. What effect does solder mask ink play on PCB?

Answer: Solder mask ink: It is a kind of protective layer that is coated on the substrate and circuit of the printed board that does not need to be soldered. The purpose is to prevent bridging between circuits during soldering, while providing a permanent electrical environment and a chemical-resistant, heat-resistant, insulating protective layer, and at the same time bringing beautiful appearance to the PCB. Solder mask ink is divided into two major systems: 1. Thermosetting epoxy ink, 2. Liquid photosensitive imaging ink.

To be continued...........

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