1. Stickiness and Thixotropy
In the printed circuit board (PCB) manufacturing process, screen printing is a critical step that cannot be overlooked. To ensure accurate image reproduction, the ink must possess both appropriate viscosity and suitable thixotropy. Viscosity refers to the internal resistance of a liquid, or the frictional force experienced when one layer of liquid slides over another under external pressure. Thicker liquids encounter higher resistance to flow, while thinner liquids offer less resistance. Viscosity is measured in poise. It is important to note that temperature plays a significant role in affecting viscosity.
Thixotropy is a unique property of certain liquids, where their viscosity decreases when stirred and quickly returns to its original level once the agitation stops. When stirred, the effect of thixotropy can persist long enough to allow the liquid to rebuild its internal structure. Achieving high-quality screen printing requires careful control of ink thixotropy. In particular, during the squeegee process, the ink is agitated, making it more fluid. This effect speeds up the ink’s passage through the mesh and helps to form a uniform layer of ink across the mesh openings. Once the squeegee movement halts, the ink returns to a static state, and its viscosity promptly returns to the desired level.
2. Fineness
**Pigments and mineral fillers** are generally solid substances, and after fine grinding, their particle size does not exceed 4-5 microns, forming a homogeneous fluid state while still remaining solid. Therefore, achieving the required **ink fineness** is crucial.
**2. Precautions for Using PCB Ink**
Based on the practical experience of most PCB manufacturers, the following guidelines should be followed when using ink:
1. Under all circumstances, the temperature of the ink should be maintained between 20-25°C, and temperature fluctuations should be minimized. Significant temperature changes can affect the viscosity of the ink, compromising both the quality and effectiveness of the screen printing. In particular, when the ink is stored outdoors or exposed to varying temperatures, it must be allowed to acclimate to room temperature for a few days before use, or the ink container should be brought to the appropriate working temperature. Using cold ink can lead to screen printing failures and cause unnecessary problems. Therefore, to preserve ink quality, it is best to store or maintain it at standard process conditions.
2. The ink must be thoroughly and carefully mixed, either manually or mechanically, before use. If air has entered the ink, it should be allowed to stand for a while before use. If dilution is required, the ink should be mixed thoroughly first, and its viscosity should be checked afterward. The ink container must be sealed immediately after use. Additionally, never return ink that has been used on the screen back into the ink container to mix with unused ink.
3. It is recommended to use compatible cleaning agents when cleaning the screen, ensuring that the screen is thoroughly cleaned. For additional cleaning, it is best to use a clean solvent.
4. When the ink dries, it must be processed in a device with a good exhaust system.
5. To maintain optimal working conditions, screen printing should take place in an area that meets the technical requirements of the PCB manufacturing process.
**3. Causes and Countermeasures for Common PCB Ink Issues**
1. **Uneven Ink Application**
The ink fails to adhere evenly to the board surface, resulting in dot-like strips or flake-like white spots (areas without ink).
**Possible Causes**:
– Insufficient ink mixing time
– Incorrect ink mixing procedure
– Oil or water contamination on the board surface (inadequate pre-treatment)
– Ink impurities (e.g., oil stains from the tape that affect surface tension)
– Poor-quality stencil material
– Incomplete screen cleaning
– Expired ink after mixing
**Countermeasures**:
– Check the pre-treatment process and ensure the drying section meets operational standards.
– Confirm that each pre-treatment step adheres to process standards (e.g., water breakage, wear marks).
– Verify the ink mixing parameters.
– Clean the screen thoroughly and replace worn-out tools, such as the scraper.
2. **Copper Surface Cavitation**
(1) **Ink separation from the copper surface** over large areas.
**Possible Causes**:
– Poor pre-treatment
– Adhesion of impurities to the board
– Copper surface depression
– Inadequate ink mixing
– Uneven ink thickness on the copper surface
– Impact damage to the ink surface
– Uneven oven temperature distribution, leading to underbaking or overbaking
– Repeated tin spraying or excessively high tin spraying temperatures
**Countermeasures**:
– Inspect the pre-treatment line to ensure each step meets quality standards.
– Verify the oven baking temperature and its heat distribution curve.
– Confirm ink mixing parameters.
– Review the production process to minimize external impacts.
– Check the tin spraying process parameters and conditions.
(2) **Ink separation from the copper surface** on large copper areas or at the corners of the circuit.
**Possible Causes**:
– Ink layer too thin
– Poor pre-treatment at line corners
– Insufficient baking
– Repeated tin spraying or excessively high tin spraying temperatures
– Extended flux soaking time
– Overly aggressive flux attack
– Ink damage at the corners
**Countermeasures**:
– Adjust the PCB solder mask printing thickness to ensure proper coverage.
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