About soldering wire for sophicated instruments
TL;DR - Use 63/37 Tin-Lead solder whenever possible. It has been extensively tested for reliability and is the preferred alloy for aerospace and military applications.



To answer this fully and correctly we would need to know a great deal more about the specific components used, operational conditions, and needs for ROHS compliance. Base metals, solder working temperature, and operational temperature/conditions can all play a part. 

In general terms, you will want to select a eutectic alloy for any high reliability need. Without getting too bogged down in technicals, "eutectic" means it transitions directly between solid and liquid at a single temperature. This improves reliability by dramatically limiting the chances of inconsistencies and micro-fractures in the joint. 

Assuming there is nothing that would exclude the use of leaded solder (e.g. ROHS compliance, high operational temp) or otherwise require special alternative alloys (e.g. silver or gold base metal), 63/37 Sn/Pb is almost always the best choice. It's eutectic, relatively low cost, has a low working temperature, and has extremely low odds of tin whisker formation. 

The field of lead-free solder is a rapidly developing one and too broad to approach recommendations without far more info. Since it is a relatively young field, reliability testing documentation is limited and often contradictory. Suffice to say if you don't need it, don't use it. If you do need it, you will need to do some research as to which of near countless alloy choices will be best suited for your application. 95.6/3.5/0.9 Sn/Ag/Cu is determined by NIST to be truly eutectic, so it may be a good place to start.