Dielectric constant: 4.0-4.6 at 1 MHz, 4.0-4.5 at 10 GHz. Loss tangent: 0.003-0.006 at 10 GHz. This makes borosilicate moderately transparent at microwave frequencies. For half-wave window design, the thickness equals lambda/(2*sqrt(epsilon_r)) at the operating frequency. The low loss tangent ensures minimal insertion loss (typically 0.1-0.3 dB per window).

Why borosilicate vs other glasses?

Borosilicate has a CTE of 3.3 ppm/K (vs 9 ppm/K for soda-lime glass), providing excellent thermal shock resistance. It withstands temperature differentials of 160 degrees C without cracking. Combined with good chemical resistance and RF transparency, it is the standard for microwave oven windows, hermetic RF packages, and pressure windows in waveguide systems.

Hermetic glass-to-metal seals in RF connectors and packages (matched CTE to Kovar alloy). Microwave oven door windows. Waveguide pressure windows (gas-filled waveguide systems). Radome windows for high-temperature environments. Substrate for thin-film RF circuits (smooth surface, stable dielectric). Test chamber observation windows.

Materials & Substrates

Borosilicate Glass

Borosilicate Glass (e.g., Pyrex, Schott Borofloat) is a low-expansion glass containing 12-15% B₂O₃ that provides excellent thermal shock resistance (CTE 3.3 ppm/K), moderate RF transparency (ε_r = 4.0-4.5, tanδ = 0.003-0.006 at 10 GHz), and chemical durability. Used in hermetic glass-to-metal seals (matched to Kovar), microwave oven windows, waveguide pressure windows, and thin-film RF substrates.

Category: Materials & Substrates

ε_r: 4.0-4.5

Understanding Borosilicate Glass

The boron oxide content replaces some of the sodium oxide in soda-lime glass, dramatically reducing the thermal expansion coefficient. This makes borosilicate ideal for environments with rapid temperature changes (thermal cycling in outdoor radomes, microwave heating applications).

For hermetic RF packages, borosilicate glass forms the insulating seal between the metal housing and the conductor pins. The glass CTE must closely match the metal (Kovar at 5.1 ppm/K) to prevent cracking during cooling after the glass-to-metal sealing process. The RF signal passes through the glass with minimal loss.

Half-Wave Window Design

t = n·λ₀ / (2·√ε_r)
n = 1, 2, 3... (integer multiples)
ε_r = 4.5 for borosilicate

At 10 GHz (λ=30 mm):
t = 30 / (2×2.12) = 7.08 mm
Insertion loss: ~0.1-0.3 dB

RF Window Material Comparison

Material	ε_r	tanδ	CTE (ppm/K)	Use
Borosilicate	4.0-4.5	0.004	3.3	Seals, windows
Fused silica	3.8	0.0001	0.55	Low-loss windows
Alumina	9.8	0.0001	7.1	High-power
Soda-lime	7.0	0.01	9.0	Low cost

Common Questions

Frequently Asked Questions

RF properties?

ε_r=4.0-4.5, tanδ=0.003-0.006 at 10 GHz. Half-wave window: 0.1-0.3 dB loss. Moderately transparent.

Why borosilicate?

CTE 3.3 ppm/K (vs 9 for soda-lime). Thermal shock: 160°C differential. Matches Kovar for hermetic seals.

RF uses?

Hermetic seals, microwave windows, waveguide pressure windows, thin-film substrates, test chamber windows.

Related Terms

← Born Approximation Bosonic Code →

RF Materials

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