I have been in the US Air force for the last 11+ years and am an Avionics guy for the F-15 and am currently working in a special shop that does micro/micro-miniature electronics repair, allot of the time under a stereo scope (6 weeks just for soldering training 8hrs a doy 5 days a week) We actually dont have ISO rating but our rating is the exact equivilant to those type of specs thats why I say NASA standards. As far as I know they adopted our 2M program for their uses. Like I said I can do anything from plated through hole to SMT to multilayer repair. I currently have an AAS in avionics technology but am looking forward to retirement as hopefully a fisheries biologist in Idaho.
BTW the only thing we dont do is BGA assy. and fiber optics as they arent used really in the fighter world on our equipment at least not yet still using PQFP etc
the solder we use is RMA type flux (rosin mildly actvated) as it will not corode even if left on, which btw is a defect condition.
in the % of 63/37 the reason we use 63/37 is that it has a low 361 deg melting point and it goes straight from a solid to a liquid at that temp with no plastic state as 60/40 or some silver solders do. (this is the only solder that does this)
What this does is removes the chance of a fractured solder joint because a slight movement of the wire in a plastic state will cause that problem. thereby giving you a joint that wont fail.
this is the main reason I dont recomend silver solder unless you have silver components on a criticle piece of electronics