1. Causes and Impacts of Microbubbles
Microbubbles (1–50 μm voids) formed by trapped gas during lamination degrade:
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Electrical performance: Increased dielectric loss (e.g., +0.005 tanδ at 10 GHz);
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Mechanical strength: 30–50% reduced flexural strength;
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Aesthetics: Surface pits or delamination.
Root Causes:
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Residual solvents/moisture in prepreg;
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Insufficient vacuum (<1 mbar target);
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Poor resin flow due to improper temperature/pressure profiles;
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Layer misalignment-induced voids.
2. Material Pretreatment
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Prepreg pre-baking: 120–140°C for 2–4 hours (moisture <0.1 wt%);
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Copper foil treatment: Micro-etching or plasma activation (Rz=3–5 μm);
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Low-volatility resins: Epoxy with <0.5% volatile content.
3. Process Optimization
(1) Vacuum Phase
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Vacuum control:
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Initial vacuum: 10–100 mbar for 5–10 min;
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High vacuum: <1 mbar for 15–30 min;
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Leak rate <5 mbar/min.
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Ramped heating: 1–2°C/min to 80–100°C for gradual outgassing.
(2) Pressure Phase
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Pressure profiling:
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Low pressure (0.5–1 MPa) for initial resin flow;
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High pressure (2–3 MPa) at 170–180°C for full consolidation;
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Dwell time: 30–60 min.
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Dynamic pressure adjustment: Real-time monitoring via sensors.
(3) Cooling Phase
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Controlled cooling: ≤2°C/min with vacuum maintained until <50°C.
4. Equipment & Tooling
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Vacuum chamber: Multi-stage pumps (rotary + roots) for <1 mbar;
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Caul plates: Silicone pads/graphite plates (Ra<0.1 μm) for pressure uniformity;
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Alignment systems: Optical/X-ray alignment (±10 μm).
5. In-line Monitoring
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Dielectric analysis (DEA): Real-time resin flow/cure monitoring;
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Ultrasonic scanning (SAT): 10–50 MHz for microbubble detection (<0.1% area);
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Thermal imaging: Temperature uniformity control (±5°C).
6. Material-Process Synergy
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Low-viscosity resins: Epoxy with <1000 cP melt viscosity;
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Nano-fillers: 1–3 wt% SiO₂/clay to suppress bubble nucleation;
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Breathable films: Porous PI membranes for gas venting.