Ceramic Package
Understanding Ceramic Package
Co-Fired Ceramic Technologies: HTCC and LTCC
RF and microwave semiconductor dies are extremely fragile and must be protected from environmental factors like moisture, oxygen, and dust, which cause corrosion and performance degradation. While low-cost plastic packaging is common in digital chips, it fails at high microwave frequencies due to its high dielectric losses, parasitic capacitances, and poor thermal management. Ceramic packaging is the gold standard for high-reliability military, space, and telecom RF modules.
These packages are manufactured using co-fired ceramic technologies, which are divided into High-Temperature Co-Fired Ceramic (HTCC) and Low-Temperature Co-Fired Ceramic (LTCC). HTCC is co-fired with refractory metals like tungsten or molybdenum at temperatures around 1600°C, providing excellent mechanical strength and thermal conductivity. LTCC is co-fired with high-conductivity metals like gold, silver, or copper at around 900°C, yielding lower electrical losses which is ideal for high-frequency signal routing.
Hermetic Sealing, Thermal Vias, and RF Launches
To achieve true hermeticity, ceramic packages are sealed using a metal lid soldered or seam-welded to a metallized ring on the package perimeter, or a ceramic lid bonded with glass frits. This seal prevents gaseous contaminants from penetrating, maintaining a controlled dry nitrogen atmosphere inside. Electrical connections are routed through the ceramic walls using co-planar waveguide transitions or via pins, designed to match the system impedance (50 ohms) to prevent reflections.
High-power RF devices, such as gallium nitride (GaN) power amplifiers, generate intense localized heat. Ceramic packages integrate a metal base (flange) made of copper-tungsten (CuW) or copper-molybdenum (CuMo), which has a high thermal conductivity and matches the coefficient of thermal expansion of both the silicon/GaAs die and the alumina ceramic, preventing mechanical fracturing from thermal cycling.
Key Mathematical Relations
Technical Specifications Comparison
| Package Class | Materials | Hermeticity | Thermal Conductivity | Parasitics (RF Loss) | Frequency Limit |
|---|---|---|---|---|---|
| Plastic QFN | Epoxy Mold Compound | No (moisture ingress) | Low (0.5 W/m·K) | Moderate-High | < 20 GHz |
| HTCC Ceramic | Alumina ($Al_2O_3$), Tungsten | Yes (Hermetic) | High (20 - 30 W/m·K) | Moderate | < 40 GHz |
| LTCC Ceramic | Glass-Ceramic, Gold/Silver | Yes (Hermetic) | Moderate (2 - 5 W/m·K) | Very Low | < 100+ GHz |
| Metal-Base Ceramic | Alumina, CuW/CuMo Flange | Yes (Hermetic) | Extremely High (150 - 200 W/m·K) | Low | < 30 GHz (High Power) |
Frequently Asked Questions
What is the difference between LTCC and HTCC packaging?
The primary difference lies in the firing temperature and conductor metals. HTCC is fired at high temperatures (1600°C) and requires refractory metals (tungsten/molybdenum) which have higher electrical resistance. LTCC is fired at lower temperatures (900°C) allowing the use of highly conductive metals like gold and silver, which reduces RF insertion losses at high frequencies.
Why do RF amplifiers require hermetic packages?
RF semiconductors made of GaAs or GaN can fail if exposed to atmospheric moisture, which causes electrochemical corrosion of the sub-micron gate structures. Hermetic packages are sealed in a dry nitrogen environment, creating a barrier that prevents moisture and other corrosive contaminants from reaching the die.
How does cavity resonance affect ceramic package performance?
If the physical dimensions of the internal cavity of a ceramic package approach a half-wavelength of the operating frequency, the package behaves as a resonant cavity. This resonance can cause severe insertion loss spikes, input-output coupling, and instability. Designers must size the cavity to ensure its resonant cutoff frequency lies well above the operating band.