Many tin-based solders readily dissolve gold, which may either result in embrittlement of the joint through the formation of a tin/gold intermetallic, or removal of the solderable layer leaving a weak joint.
Though expensive, possibly the best option when soldering to gold conductor networks is to use indium-based alloys such as indium50lead50 (melting range 178-210°C), indium60lead40 (melting range 173-181°C), or indium80lead15 silver5 (melting range 142-148°C). The solubility of gold in these alloys is much less than in conventional solders, and hence the scavenging effects are not as damaging.
Another option is the lead-free alloy gold80tin20 (melting point 280°C); however, processing temperatures using this alloy are high because of its high liquidus temperature.
Further information
Some information is only available to TWI Industrial Members. Best practice guides for the electronics and electrical industries:
Soldering - processes
Soldering - process control
Soldering - materials and design