Hi
Not to overly quibble, but in the context of "every solder alloy on the planet" you do see a lot of them go through a plastic stage In this case I guess I was not very clear. My reference was to plastic in a more general sense.
Back to the point, even with 63/37, the joint is *much* weaker at say 175 than an 125. It's even weaker still at 200C. The solder has not yet turned to a liquid, but it has gotten pretty soft. This is one of the reasons for specifying a cool down profile even when using 63/37. You need some margin between melting point and operating temperature to ensure a solid mechanical situation.
Yes, most alloys are non-eutectic, but in electronics, eutectic solder is (or at least
was before RoHS) a really significant portion of the solders sold. It's a very significant error to omit them.
Yes, eutectic solder's strength and elasticity changes with temperature, but its melting point is just that — a singular point. If you're going to use the technical term, it's best to stick with its established meaning. The plastic phase of a non-eutectic solder has absolutely nothing to do with the weakening you're talking about. At 182C, 63/37 solder is a solid. Not a paste, or a gel, or whatever you want to describe a plastic phase as. A solid. One degree higher, at 183C, it's a liquid. Not a paste or gel, a liquid. (At 200C, 63/37 has been liquid for 17 degrees already and thus is indeed "weaker still".)
Indeed, mechanical strength is part of the reason for a cooldown profile, but I thought the main reason was to reduce mechanical stress because of different temperature coefficients of shrinking.