In the exposure process of PCB manufacturing, parameter control is of vital importance, directly affecting the accuracy and quality of the circuit pattern. The following are the key parameters that need to be controlled in the exposure process and their influences:
Light source selection and intensity:
Light source type: The selection of the exposure light source should match the spectral absorption curve of the dry film. Commonly used light sources include high-pressure mercury lamps, gallium iodide lamps and LED light sources. The main spectral emission peak of the light source should overlap with the main spectral absorption peak of the dry film to ensure the best exposure effect.
Light source intensity: The intensity of the light source directly affects the exposure energy. Insufficient intensity will lead to underexposure and unclear graphics. If the intensity is too high, it may cause overexposure, making development difficult. It is necessary to regularly use a light intensity sensor to measure the output of the light source to ensure its stability and compliance with the process requirements.
Exposure time:
Exposure time control: Exposure time is a key parameter that determines the degree of dry film polymerization. The time is too short, the dry film polymerization is incomplete, and the lines are not clear during development. If the time is too long, it may lead to difficulties in development or residual glue. Usually, a light energy integrator is used to control the exposure energy to ensure that the total energy of each exposure is consistent.
Energy calculation: The exposure energy (E) is determined by the light intensity (I) and the exposure time (T), and the formula is E=IT. The exposure energy should be adjusted according to the type of dry film and the thickness of the PCB, generally ranging from 40 to 100 millijoules per square centimeter.
Alignment accuracy
Alignment system: The alignment accuracy of the exposure machine directly affects the positional accuracy of the line graphics. The high-precision alignment system can ensure that the alignment error between the mask and the PCB board is less than ±0.01mm, reducing the inter-layer deviation.
Positioning methods: Common positioning methods include visual positioning, out-of-stock positioning systems, and fixed pin positioning. For high-precision PCBS, it is recommended to use laser Direct imaging (LDI) equipment, which can enhance the graphic resolution capability and alignment accuracy.
Environmental control
Temperature and humidity control: The temperature and humidity in the exposure environment will affect the adhesion of the dry film and the development effect. It is usually required that the temperature be maintained between 20 and 24°C and the humidity between 60 and 70% to ensure the stability of the process.
Cleanliness: During the exposure process, the environment must be kept clean to prevent dust and impurities from contaminating the dry film surface and affecting the quality of the circuit.
Equipment maintenance and calibration
Light source maintenance: Regularly inspect and maintain the light source, and promptly replace aged tubes to ensure the stability of the light source intensity.
Mechanical calibration: Regularly calibrate the mechanical system of the exposure machine to ensure alignment accuracy and exposure uniformity. It is recommended to conduct calibration once every 1,000 PCB boards produced or after the equipment has been running for 8 hours.
Dry film selection and storage
Dry film type: Select the appropriate dry film thickness and type based on the PCB design requirements. For fine circuits, it is recommended to use thin dry film. For high reliability requirements, dry films with strong adhesion can be selected.
Storage conditions: The dry film should be stored in a low-temperature and dry environment to avoid moisture or expired use, which may affect the exposure effect.
