Is 63/37 better? I read that some people were talking about cold joints with 60/40
Because 60/40 is not eutectic, it has a “slushy” phase where it’s not fully solidified. (Technically, once cooled, 60/40 becomes 63/37 with tiny crystals of lead dispersed throughout.) While in the slushy phase, the joint can still be disturbed.
The upshot is that eutectic alloys like 63/37 go directly from liquid to solid. This reduces the window of time between removing the soldering iron and the joint solidifying, which reduces the chances of a disturbed joint.
And is 0.5 enough for SMD? Where do you have to find 0.3?
”SMD” covers a huge range of pad sizes. There’s no one size of solder that works for everything.
And of course it also depends on technique. For example, if you solder a 0.5mm pitch QFP pin-by-pin under a microscope, you’ll want a really fine solder. If you solder the same chip by drag soldering, the size of the solder is practically irrelevant.
I use 0.8 on a daily basis I don't know if there is a big difference with the 0.5
It’s a huge difference, as ebastler said.
At the bench at work, I have 1mm, 0.7mm, 0.5mm, and 0.3mm, at home I have 0.8mm, 0.5mm, and 0.3mm. (You can always twist together two or three strands of solder to make a bigger one.)
The 1mm I mostly use for tinning the soldering iron tip. I rarely solder joints big enough to need it.