In the PCB system, the screen, a fundamental component in screen printing, is pivotal. Screen printing, integral to PCB technology, relies on these screens, primarily composed of silk fabrics (though non-silk fabrics exist). The prevalent screen type in the PCB industry is the t-type net; s and hd types are rarely used, except for specialized requirements. This distinction underscores the high-tech and specialized nature of PCB manufacturing, distinct from simpler manual and artistic endeavors. Unlike aesthetic pursuits, PCBs necessitate precise electrical performance, requiring exact geometrical dimensions for current conduction and signal transmission. Hence, a thorough grasp of key technical parameters related to wire mesh is imperative.
1. **The thickness of the screen:**
Thickness refers to the measurement of the screen’s thickness without tension. It is the statistical average derived from measured data and is expressed in micrometers (μm). Screen thickness is determined by the wire diameter constituting the screen and affects ink penetration.
2. **The opening area ratio of the screen:**
This ratio indicates the screen’s open area relative to its total area, expressed as a percentage. A higher ratio denotes a larger screen opening.
3. **The ink penetration of the screen:**
This refers to the theoretical ink penetration. In actual screen printing, ink penetration depends on factors such as ink material, viscosity, pigment fineness, and ink thixotropy. Board hardness, printing pressure, speed, touch screen distance, and other factors also influence it.
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2. **Ink:**
Ink refers to colored gelatinous substances used in printed boards. It typically consists of synthetic resins, volatile solvents, oils, fillers, desiccants, pigments, and diluents.
(1) **Composition of ink:**
1. **Resin:**
Resin is crucial for forming the ink film and determines its performance. Well-known PCB ink brands have patented formulations that highlight resin’s role in operability, gloss, adhesion, hardness, water and solvent resistance, acid and alkali resistance, and temperature resistance.
2. **Solvent:**
Solvents serve several functions:
– Dissolve resin to form a coherent material.
– Dissolve pigments and additives.
– Adjust ink viscosity and drying speed.
– Enhance substrate adhesion through penetration.
3. **Pigment:**
Pigments, mainly organic or inorganic, provide color. Organic pigments offer vivid hues but lower hiding power, while inorganic pigments like titanium white and carbon black offer strong hiding power and lightfastness, making them common in PCB inks.
4. **Additives:**
Additives improve ink’s physical properties, printability, and overall printing effects.
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Three. **Several important technical properties of PCB ink:**
The quality of PCB ink hinges on its constituent components. Quality ink reflects scientific formulation, technological advancement, and environmental compatibility, demonstrated through:
(1) **Viscosity:**
Also known as dynamic viscosity, it measures ink flow under stress gradients, typically in Pa·s or mPa·s. In PCB production, viscosity affects ink flow dynamics.
(2) **Plasticity:**
Ink’s ability to retain its properties after deformation enhances printing accuracy.
(3) **Thixotropy:**
Thixotropic ink exhibits gel-like consistency at rest but thins under stress, aiding screen printing by improving ink penetration and maintaining print quality.
(4) **Fluidity:**
Indicates how ink spreads under external force, inversely related to viscosity. High fluidity prevents network formation but promotes even coating.
(5) **Viscoelasticity:**
Ink’s ability to quickly rebound after shear stress promotes efficient printing.
(6) **Dryness:**
Optimal ink drying speed balances screen print quality with substrate adhesion.
(7) **Fineness:**
Particle size of pigments and solids, ideally less than 10μm in PCB inks, ensuring smooth printing.
(8) **Stringiness:**
Measures ink’s tendency to form threads when stretched, impacting print cleanliness.
(9) **Transparency and hiding power:**
Essential for different PCB ink types (e.g., circuit, conductive, character) based on specific application needs.
(10) **Chemical resistance:**
Adheres to stringent standards for acid, alkali, salt, and solvent resistance.
(11) **Physical resistance:**
Meets requirements for scratch, thermal shock, and mechanical peel resistance in varied environmental conditions.
(12) **Safety and environmental protection:**
Emphasizes low toxicity, odorlessness, and environmental friendliness.
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4. **Thixotropy:**
Viscosity and thixotropy are distinct physical properties of ink. Thixotropy indicates viscosity changes under stress conditions.
5. **Reaction mechanism and control of ink thixotropy:**
Achieving desired thixotropic properties involves resin viscosity and precise ratios of fillers and pigment particles.
6. **Practical application of thixotropy:**
Thixotropic ink facilitates screen printing by adjusting viscosity during printing, enhancing ink flow and print quality. Proper thixotropy ensures satisfactory graphic quality on PCB substrates.
