Okay, I know it's an old thread, but, since it's been "bumped" already, I'll just state this:
The Lumia 640 is compatible with Bluetooth keyboards. You can pair a Bluetooth keyboard and use it to type with.
If the phone detects that one of your paired devices has "keyboard abilities", then the on-screen keyboard will not show up. After all, it would be aggravating for people who are using a Bluetooth keyboard for entry if the on-screen keyboard kept getting in the way. So, it's by design.
That being said, there are probably some bugs in how it hides, etc. This generally is a problem only on phones that have ever had a keyboard paired, or something that "reports" to have keyboard functionality.
I never had the issue myself, but have read about it, so thought I would pass on what knowledge I gained.