Isolating Micro-Machined Accelerometer Zero-G Bias Shifts under Static Thermal Cycles
Static thermal zero-g bias shifts in MEMS accelerometers are isolated by enforcing prolonged thermal dwell periods to decouple stress creep from thermal gradients.
AuSn Eutectic Bonding
Metallurgical Joining
Wafer-level packaging processes often utilize gold-tin alloys to establish hermetic, high-strength connections for sensitive optoelectronic and sensor components. This method, termed AuSn eutectic bonding, produces a joint with a melting point of two hundred and eighty degrees Celsius at a specific weight ratio of eighty percent gold to twenty percent tin. The resulting connection possesses high thermal conductivity and resists fluxless soldering, making it suitable for flux-free packaging environments.
These properties prevent contamination of optical surfaces during the sealing process.
Deposition Protocol
Precise control of the alloy composition is achieved by depositing gold and tin layers using physical vapor deposition or electroplating. The thickness of each layer must be closely monitored, since even a small deviation from the eutectic ratio will significantly increase the required melting temperature. Sputtering provides the highest precision for thin-film applications, though electroplating is often used for thicker structures to accommodate surface non-uniformity.
This layer structure is often protected with a thin gold cap layer to prevent oxidation of the tin before the bonding process begins.
Assembly Verification
High-resolution acoustic microscopy represents the primary non-destructive method used to evaluate the density and voiding of the completed joint. Voids in the interface reduce both the thermal performance and mechanical strength, which can lead to premature failure under thermal cycling. Pull testing and shear testing provide destructive means to measure the mechanical yield force of the bond on sample wafers.
This combination of testing methods ensures that the manufacturing process maintains high yield rates and produces stable, long-lasting assemblies.
Thermal Profile
The heating cycle must be carefully controlled to ensure complete melting without causing excessive squeeze-out of the liquid alloy. Heating rates must be fast enough to prevent premature diffusion of the tin into the gold. A typical profile involves heating the assembly to three hundred and ten degrees Celsius before cooling it rapidly.