Well, to tell you the truth, I used to not worry about it either in the
days when I could get true 60/40 solder from Radio Shack, but those days
appear to be long gone.

60/40 solder from any source is wretched stuff. From China or not.

What you want is true eutectic solder, 63/37. Accept no substitutes. Plenty of US sources as well.

Peter Wieck
Melrose Park, PA