Now, I consider myself a fairly reasonable solderer.
I have a fine point tip, nice hot iron and some old 40/60 solder.
I have successfully soldered 4 lots of headers to different Pi0/W. (And many Arduino projects on veroboard over the years).
But, I recently purchased a couple of PHATS from Pirmoroni that needed their headers soldering on too.
The PHATS were MUCH, MUCH easier to solder to. The solder flowed very easily and in a very short time.
All the Pi's on the other hand take much longer for the solder to flow. And I'm always fearful of holding the iron on for too long or ending up with dry joints.
I did notice that the PHAT holes were more silvered whereas the Pi's are gold. But I don't think the PHAT's were pre-tinned.
Is it that I need to use a more modern solder mix with the Pi's?