So far I had 5 pcs Lumia 640. All had the same problem: they did not charge anymore. It was not the connector. The internal battery charger circuit was defective. It is repairable. The first 3 phones were simply replaced within warranty; the fourth one was repaired within warranty. I keep it as spare now.
In my opinion, the 640 has problems with overvoltage. When I used the original 230V charger, there was no problem. But I often use a car charger: Ansmann USB2DRIVE 5V 1000mA. It gives 5V, so this should be OK. But I suspect there are higher voltage spikes than 5V, and they kill the 640 charger. Now I added a 5.1V zener diode (limiting diode) inside my car charger, and the problem is solved. Well, so far, so good.
I destroyed my 4th phone internal charger circuit by connecting a 5V power bank that I received as a gift when I bought the phone....
So now I stick to the car charger and the original 230V charger, and never connect something else. I even suspect PC's. When I remember it correctly, one of my phones did not charge anymore after connecting it to my home PC.