PCB Tech - Causes and elimination of PCB process defects-negatives

Negative film is one of the important raw materials used by PCBprocessing plants. It has the same working principle as the photographic film, so it has high requirements for storage and use conditions. A little improper handling can cause many problems. In this issue, we will talk about the causes of some defects in the negatives and how to eliminate them.

A. Light drawing as a negative

1. Problem: The negative film is foggy and the contrast is not good

(1) Old developer, the development time is too long. Solution: Use new developer, short developing time and good film contrast (that is, good blackness).

(2) The developing time is too long. Solution: shorten the development time.

2. Problem: The edge of the film wire has a large halo

The developer temperature is too high to cause over-display. Solution: Control the temperature of the developer within the process range.

3. Problem: The transparency of the film is not enough and foggy

(1) The silver powder deposits in the old fixing solution aggravate the fogging of the film. Solution: Replace with new fixer.

(2) Insufficient fixing time, resulting in insufficient transparency of the background. Solution: Keep the fixing time for more than 60 seconds.

4. Problem: The photographic film is discolored

Insufficient cleaning after fixing. Solution: After fixing, use a large amount of running water to clean, it is best to keep it for more than 20 minutes.

B. Copy the original film

1. Problem: The pattern of the reproduced diazo film is deformed, that is, all the wires become thin and irregular

(1) Improper selection of exposure parameters. Solution: According to the state of the film, optimize the exposure time.

(2) The optical density of the original film did not reach the process data. Solution: Measure the optical density to make the bright part reach Dmax4.0 or more; the optical density of the transparent part is not required to be below Dmin0.2.

2. Problem: The width of the wire on the edge of the reproduced diazo film becomes thin and irregular

(1) The process parameters of the light source of the exposure machine are incorrect. Solution: Use an instrument to measure the attenuation of the energy of the UV light source lamp, and replace it if it exceeds the service life.

(2) The area of the diazo film to be turned is beyond the optimal range of the exposure frame. Solution: Reduce the imposition area according to the production situation or because the light source is too close, increase the light source to a proper distance from the exposure table to ensure that the large-size film is in a good photosensitive area.

3. Problem: The resolution of all or part of the reproduced diazo film is poor

(1) The original film quality is poor. Solution: Check the imaging state of the edge of the original film circuit, and take technological measures to improve it.

(2) The vacuuming system of the exposure machine table is malfunctioning. Solution: Check carefully whether there are pores or damage in the air duct.

(3) There are bubbles in the film during exposure. Solution: Check whether there are dust particles on the surface of the exposure machine; check whether the black paper on the sub-film and the surface of the exposure machine has dents or creases.

4. Problem: The wire of the reproduced diazo film becomes wider and the transparent area is insufficient (that is, the Dmin data is too large)

The selected exposure process parameters are inappropriate. Solution: A. Choose an appropriate exposure time. B. It is possible that the storage environment of diazo film is close to ammonia water or ammonia gas is present, causing different degrees of development.

5. Problem: The shading area of the reproduced diazo film is insufficient (Dmax data is too low)

(1) When the diazo film is reproduced, the development is not correct. Solution: A. Check whether the developing machine is malfunctioning. B. Check the ammonia water supply system to determine whether the concentration is above Be'26 (that is, the specific gravity is 1.22).

(2) The material of the original diazonium sheet is poor. Solution: Determine whether the optical density Dmax of the original film material is above 4.0.

6. Problem: The dark area shading performance of the reproduced diazo film is low and occasionally insufficient.

(1) The development of the reproduced diazo film is not correct. Solution: Check the fault status of the ammonia developing machine and make adjustments.

(2) The storage environment of the original film material is poor. Solution: Store in accordance with the requirements of the negative material instructions, especially avoid direct sunlight or close to the ammonia storage place.

(3) Improper operation of the developing machine. Solution: In particular, check the temperature of the conveyor belt of the developing machine, and use a special sticker that senses temperature and discoloration. It should meet the process requirements (non-ammonia tank temperature controller).

7. Problem: There are pinholes or holes in the pattern area of the reproduced diazo film

(1) There is dust or dust particles in the exposure area. Solution: In particular, carefully check the cleanliness of the exposure table, the original film and the surface of the new diazo film, and perform a wipe test.

(2) The quality of the original film is poor. Solution: Check on the tabletop of the transparencies and make careful repairs (if you need to prove the quality of the original film, you can reproduce the second one, and check that it is the same, you can prove it).

(3) There is a problem with the quality of the diazo tablets used. Solution: Take the unexposed original diazo film to develop directly with ammonia to make the whole film appear dark brown with shading, and then carefully check whether there are pinholes and holes. If there are, it can be proved.

8. Problem: The reshaped diazo film is deformed and out of shape

(1) The environmental temperature and humidity are not strictly controlled. Solution: A. Install a temperature and humidity controller to adjust the room to meet the process requirements. B. Operating environment temperature and humidity control: temperature is 20-270C; humidity is 40-70%. For negatives with high precision requirements, the humidity should be controlled at 55-60%RH.

(2) After developing and fixing, the drying process is not properly controlled. Solution: According to the process requirements, place the negative film horizontally for blowing and drying. It is not suitable to be hung to dry, because it is easy to deform.

(3) Improper stabilization of diazo tablets before remanufacturing. Solution: Store in the negative film storage room for 24 hours for stable processing.

C. Black and white negative film reproduction process

1. Problem: The width of all wires of the reproduced black and white negative film becomes thin and uneven

(1) Improper selection of exposure process parameters. Solution: First check whether the positive or negative flip is overexposed, and it should be corrected according to the actual situation.

(2) The quality of the original film is poor. Solution: Check whether the optical density of the original film, especially the "shading density" is too low.

(3) There is a problem with the development control during the reworking process. Solution: Check the developer concentration and device.

2. Problem: The width of the outer wire of the reversed film becomes thin and irregular

(1) The calibration of the exposure equipment has expired. Solution: Re-check according to the process requirements to check whether the energy of the light source is within the technical requirements.

(2) The light source is too close to the larger size film. Solution: re-adjust the distance of the light source or switch to a large exposure machine.

(3) The distance and angle of the light source reflector are out of adjustment. Solution: readjust the distance and angle of the "reflective cover".

3. Problem: The resolution of the reproduced film is not ideal, and the wire edges of the whole film are not sharp

(1) The quality of the original film is poor. Solution: Check the edge condition of the original film wire.

(2) The function of the vacuuming system of the exposure machine is poor. Solution: A. In particular, check whether the tightly connected part is sealed and the part that is tightly connected to the film. B. If the suction is insufficient, check whether the suction hose is damaged.

4. Problem: The local resolution of the reproduced film is poor

(1) The quality of the original film is poor. Solution: Check the bad condition of the lead edge of the original film.

(2) The function of the vacuuming system of the exposure machine is poor. Solution: A. Check the tight joints of the vacuum system and the tight joints of the flipped film. B. Check if there is any damaged part of the air hose.

(3) There are bubbles between the negatives during the exposure process. Solution: There are dust particles on the surface of the exposure machine, and the air extraction system must be strengthened.

5. Problem: The optical density of the reproduced film is insufficient (mainly refers to the insufficient degree of shading in the dark area)

(1) The development process of the reproduced film is incorrect. Solution: Check the developing process conditions and developer concentration.

(2) The storage condition of the original film is poor. Solution: It needs to be stored in a room that meets the process requirements, especially to avoid exposure to light.

(3) The function of the developing device is deteriorated. Solution: check and repair, especially the temperature and time control system.

6. Problem: pinholes or holes appear on the graphic surface of the reproduced film

(1) There is dust or particles on the surface of the exposure machine. Solution: The original film and the exposure table should be cleaned carefully.

(2) The quality of the original film is poor. Solution: Check the surface condition of the original film, if necessary, you can try to turn over the second film for comparison inspection.

(3) The quality of the original film base material is poor. Solution: Perform a trial inspection to observe whether there are pinholes or voids on the black surface of the dark area after the entire film is exposed and developed.

7. Problem: The circuit pattern of the reproduced film is deformed

(1) The temperature and humidity of the working environment are not correct. Solution: The temperature and humidity control of the working environment: temperature 20-270C; humidity 40-70%RH, the working humidity should be controlled at 55-60%RH for the film with high precision.

(2) The drying process is incorrect. Solution: Place the negative film horizontally and blow dry, the drying time is thick (100μm), the negative film is dried for 1-2 hours; the thickness is 175 microns, and the substrate is dried for 6-8 hours.

(3) The pre-processing of the negative film to be reproduced is inappropriate. Solution: It needs to be placed in the film room environment for at least 24 hours for stability treatment.

8. Problem: The transparent area of the film is insufficient or the film base appears cloudy

(1) There are inclusions in the original film base material. Solution: Use high-resolution quality original film.

(2) The surface of the original film base is poor. Solution: Ensure the temperature and humidity control of the storage environment.

(3) The original film is of poor quality. Solution: First, check the performance and quality of the original film.

(4) There are problems in the exposure and development process. Solution: Check and adjust the condition of the equipment and the developer, fixer and process conditions.