Why is AuSn the preferred braze for RF die attach?

The 80Au-20Sn eutectic alloy melts at 280 degrees Celsius, low enough to avoid damaging GaAs and InP MMIC dies while high enough to withstand subsequent assembly steps and operating temperatures up to 200 degrees Celsius. It forms a fluxless joint (critical for hermetic packages where flux residues cause reliability failures), has high thermal conductivity (57 W/mK, ten times better than epoxy die attach), and produces a void-free bond line when processed correctly. The high gold content also provides excellent corrosion resistance inside hermetically sealed cavities.

How is CTE matching important for braze alloy selection?

When a braze joint connects materials with different coefficients of thermal expansion (for example, a GaAs die at 5.7 ppm/K to a CuW carrier at 6.5 ppm/K), thermal cycling creates shear stress in the bond line. If the CTE mismatch stress exceeds the braze alloy's yield strength, cracks initiate and propagate until the joint fails. AuSn braze has high yield strength (275 MPa) and reasonable ductility, accommodating moderate CTE mismatches. For large dies (greater than 5 mm) on mismatched substrates, a compliant interlayer or sintered silver (which has a porous microstructure that absorbs strain) may be preferred.

Materials & Manufacturing

Braze Alloy

Q: What is the difference between brazing and soldering?

The distinction is defined by melting temperature: brazing uses filler metals that melt above 450 degrees Celsius, while soldering uses filler metals below 450 degrees Celsius. In RF packaging, this distinction matters because higher-temperature brazes create joints that can withstand subsequent lower-temperature assembly steps (the step-soldering hierarchy). A package might be assembled in three steps: first braze the ceramic feedthrough to the Kovar housing at 780 degrees Celsius (CuAg braze), then attach the MMIC die at 280 degrees Celsius (AuSn), then seal the lid at 280 degrees Celsius (AuSn) or 183 degrees Celsius (PbSn). Each step must not remelt previous joints.

/brayz al-oy/

A metallic filler material with a melting point above 450 °C that joins metal or ceramic surfaces by capillary flow to create high-strength, hermetically sealed joints. In RF packaging, braze alloys bond lids to cavity packages, attach ceramic feedthroughs to metal housings, and serve as die-attach material for MMIC chips where high thermal conductivity and long-term reliability under thermal cycling are required.

Category: Materials & Manufacturing

Key Alloy: 80Au-20Sn (280 °C)

Standard: MIL-STD-883

Understanding Braze Alloys

Hermetic RF packages require joints that maintain a leak rate below 1 times 10 to the negative 8th atm-cc/sec of helium over a 20-year service life. Organic adhesives cannot achieve this; only metallic joints formed by brazing or soldering provide the gas-tight seal needed to protect sensitive GaAs and GaN MMIC dies from moisture ingress. The braze alloy must wet both surfaces (metal housing and ceramic feedthrough, or die metallization and carrier), flow by capillary action to fill the joint gap completely, and solidify into a void-free bond line.

RF package assembly uses a step-soldering hierarchy: higher-temperature brazes are applied first, and each subsequent step uses a lower-melting alloy so that earlier joints do not remelt. A typical sequence might be: ceramic-to-Kovar feedthrough braze at 780 °C (CuAg), die attach at 280 °C (AuSn eutectic), and lid seal at 280 °C (AuSn from the opposite side) or 183 °C (PbSn). This hierarchy allows complex multi-step assembly without reworking previously completed joints.

Thermal and Mechanical Properties

Thermal Resistance of Die Attach:
R_th = t / (k × A) (°C/W)

CTE Mismatch Stress:
σ = E_braze × Δα × ΔT × (L / 2t)

Example: AuSn Die Attach, 3 mm × 3 mm die, 25 µm thick:
R_th = 25e-6 / (57 × 9e-6) = 0.049 °C/W

Where t is bond line thickness, k is thermal conductivity, A is die area, Δα is CTE difference, and L is die edge length.

RF Braze Alloy Comparison

Alloy	Composition	T_melt	k (W/mK)	Yield (MPa)	Application
AuSn	80Au-20Sn	280 °C	57	275	MMIC die attach, lid seal
AuGe	88Au-12Ge	356 °C	44	190	Higher-temp die attach
CuAg (BAg-8)	72Ag-28Cu	780 °C	70	250	Feedthrough-to-housing
PbSn (63/37)	63Sn-37Pb	183 °C	50	40	Low-temp lid seal
Sintered Ag	>95% Ag nano	250 °C (sinter)	200+	50	High-power GaN attach

Common Questions

Frequently Asked Questions

Why is AuSn preferred for RF die attach?

Its 280 °C eutectic is low enough to protect GaAs/InP dies but high enough for 200 °C operating temperatures. It bonds fluxlessly (critical for hermetic packages), has 57 W/mK thermal conductivity (10x better than epoxy), and produces void-free joints when processed correctly. The gold content provides excellent corrosion resistance inside sealed cavities.

What is the difference between brazing and soldering?

Brazing uses filler metals melting above 450 °C; soldering uses below 450 °C. RF packages use step-soldering hierarchy: high-temp brazes first (feedthrough at 780 °C), then die attach (280 °C AuSn), then lid seal (280 °C or 183 °C). Each step must not remelt previous joints.

How does CTE matching affect braze selection?

CTE mismatch between die and carrier creates shear stress during thermal cycling. If stress exceeds braze yield strength, cracks propagate to failure. AuSn's high yield (275 MPa) handles moderate mismatches. For large dies (> 5 mm) on mismatched substrates, compliant sintered silver (porous microstructure absorbs strain) is preferred.

Related Terms

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