I've always had problems with soldering until recent times, as Ripley says solder has a lot to say same with soldering iron. I use this kind of solder.
https://rover.ebay.com/rover/0/0/0?mpre=...2320000928
Blueberry'S'
https://rover.ebay.com/rover/0/0/0?mpre=...2320000928
Blueberry'S'
Keep on collecting 
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