For prototyping and evaluation of development, lead-free (tin, silver, copper) thread solder is necessary and purchased immediately. It was quite expensive to have other companies' sites, but Monotaro was saved at an affordable price.
It may be slightly more soluble than traditional solder .... It is unavoidable because it is lead-free, but it is saved if it is a little more manageable.
While I was trying various kinds of solder, it entered in and it was very wet and there was no problem with strength. I was able to cope with the pulling out of the side sill part so I am very satisfied.