I really like p2p or dead bug design, that way there's only half as many solder joints (think about it) so I only worry half as much. I've always wondered if you can hear different solder. I highly doubt it, but at RF a solder joint can get you into real trouble.
I sleep pretty sound by using solder for SMD with 1 or 2% silver and a really hot (like 380°C) solder iron.
IF you use wire or other components that have a silver content you must use a silver bearing solder, otherwise over time the silver content is leached and the joint has problems... a eutectic solder melts and solidifies at practically the same temperature... making the solder joints much easier to make.
Soon we will have to weld in an inert gas environment... to heck with this solder stuff...
The point here, is it needs to be a eutectic alloy. Apparently there are patents on some of the silver solder compositions, so just because it is a silver containing solder does not mean it may necessarily be a eutectic alloy.
The main reason silver bearing solder exists is that when working with very fine copper wire, enough copper will dissolve in plain molten solder to weaken the wire. It was also specified for the old Tektronix scopes that used ceramic connection strips with U-shaped fired silver areas. Without silver bearing solder, the fired silver would delaminate from the ceramic or dissolve entirely. It makes sense that you'd want it for any other soldering involving silver. It was also recommended to increase the service life of soldering iron tips, the cheap copper ones not-so-slowly dissolving away in use. I like silver bearing solder because it seems to resist oxidation a bit better than plain 63/37. It's gotten rather expensive and there's nothing wrong with the plain 63/37 or 60/40 if that's all you can get. It's a bit inexplicable, as the price of silver isn't that high and the percentage used is low. Like certain other things containing silver, some opportunistic pricing is probably taking place. I'm not aware of any patents on the traditional solder alloys that are still in force, though there are probably some on the lead-free alloys and certainly on many modern fluxes. Mildly activated rosin is still the best, whether you clean it or not. "No clean" is just less rosin and I don't see the point. I also like the organic water washable flux, which makes a great looking board, but you have to be sure to get every trace of it off when you wash the board. Don't use it on switches or things that can't be immersed. The rational behind lead-free escapes me and the risk of tin whiskers and eventual circuit failure shouldn't be discounted. I have a lot of 50 year old test equipment that works perfectly, and when I build for me, I build for the same time-frame, even if I won't be around to listen to it by then.
A eutectuc material will either be liquid or solid. All metallurgical phases solidify at the same temperature. (or melt). It has no "slushy zone". 60/40 0r 63/27 are for the most part the same.
Silver migration even with silver bearing solders will happen when heated. I don't have a good tertiary diagram available to me on Pb/Sn/Ag but what I do have shows that above about 2.5%Ag you start to form a third phase upsetting the single liquidus. It does have better oxidation properties
I use Stannol Type 2630. It has S-SN60PB39CU1. I solder between 270 to 300° Celsius on a Weller PU81 with a 3mm or 0.8mm tipp. I desolder at 350°. The result is shiny and when i cut a joint the amalgam looks like a solid piece of metal. I can not say that it sounds any better or worse than other types but compared to Mundorf, WBT or Eichmann that i use sometimes when a customer wants it i can not hear any problem. It does not sound shrill and i can not hear any loss of information.