Vishay EFI offers an 80/20 eutectic AuSn solder deposition. This AuSn is a sputtered alloy and is photo patterned and etched with tight dimensional tolerances allowing precise component placement. This integrated AuSn pad replaces the need for a preform and the thinner thickness provides better thermal transfer than traditional preforms.
Vishay offers details on how to incorporate AuSn on a thin film part in AuSn Design Guidelines.
Thin Film Resistor Parameters Thickness [um] Reflow Temperature [°C] Solder Bleed 4.5 to 7 ± 1 280 - 285 None