The Lumia 640 XL was supposed to have support for wireless charging, but Microsoft decided at the last minute to remove that hardware feature from the shipping product. Now a Windows Central reader has described and demoed a procedure he carried out to put wireless charging back in the Lumia 640 XL
In his forum post, Yang Hu shows that there are still four contacts on the back of the Lumia 640 XL that are used for wireless charging. The Lumia 920, which does support wireless charging out of the box, also has those same four contacts. Yu studied the service schematics of both phones and found a way to connect the contacts of the Lumia 640 XL so that the wireless charging support will work on that phone as well.
Obviously, this kind of project is not for everyone, and it's also likely that making alterations to the hardware of the Lumia 640 XL will void the smartphone's warranty. However, if you really want to add this feature to the phone, you can check out the forum post by Hu for more information.
You'll need some soldering skills and while not a morning project it would make a fun one for the weekend. So bookmark it for later!
Thanks again to Yang Hu!