Eutectic gold tin alloy is commonly used for
soldering of optoelectronic devices and microsystems.
The main reason is the possibility of fluxless soldering.
Flux, even after removal, would contaminate optical and
other surfaces and would have a strong impact to
performance and reliability of optoelectronic devices.
Most often the solder is used as preform and has to be
placed on the solder pad prior to die bonding. Typically
the preforms are fifty or twentyfive microns in thickness
and have approximately the size of the die. This means a
difficult pick and place step prior to the alignment and
soldering of the device. In contrast, thin film solders are
already deposited on the substrate. The lower thickness
and the possibility of patterning thin film solder pads
make completely different assembly approaches possible,
e. g. area array interconnects using flip chip technology
and AuSn bumps