The successor to the Lumia 635 has a bigger, HD screen, better camera and slightly slimmer body. All of which puts it head and shoulders above its predecessor.
Whether it's enough to leapfrog the competition, however, is a different matter…
First impressions and design
The 640 is a full millimetre slimmer than the 635. That might not sound a lot, but it makes a big difference.
It’s not exactly iPhone-slim, but at this price – £120 SIM free – it doesn’t have to be.
It’s well put together and inconspicuous in the pocket. And that’s good enough for us.
Our test unit had the matte black finish. While not as shiny as the glossy cyan, white or orange versions, it’s a lot less slippery.
The back pops off easily too, so you can swap colours as often as you change your socks.
Windows 8.1 Denim comes out of the box. This will be upgradeable for free to Windows 10 when that launches – the latest we’ve heard is the end of July, but that could change.
This OS packs Cortana – Microsoft’s personal assistant – and Office 365, which usually costs £60 per year.
You get Live Tiles on the home page too, so you can see info from apps at-a-glance without opening them (like the weather, say).
It also always shows the time on the screen in dimly-lit numbers; this is known as Glance Screen.
The upshot is that you don’t have to unlock the phone, saving you time and battery power.
The Lumia 640 is powered by a quad-core Qualcomm Snapdragon 400 chip clocked at 1.2GHz, along with 1GB of RAM.
That’s the same processor as in its predecessor (and in the lower-specced Lumia 435), albeit with double the RAM.
Lumia Camera is the default camera app, used for both the 8-megapixel rear snapper and 0.9-megapixel front-facer.
The 5-inch screen is half an inch bigger than the Lumia 635’s, and has a resolution of 1,280x720 pixels.
It makes a big difference – coupled with the slimmer profile, it makes the handset appear much pricier than it is. Which is always welcome.
The Lumia 64 is pretty nippy through menus, and videos and games played without any issues.
But be warned – it won’t handle the most graphically demanding games as well as higher-specced handsets.
Camera-wise, there’s little to grumble about. There’s enough detail for a phone at this price, and colours look natural.
Contrast – the difference between dark parts and light ones in a photo – could be better, but that’s asking a bit much from a phone this cheap.
And the front-facer is perfectly fine for selfies and video calls.
The battery lasted until lunchtime on the second day with fairly intense usage. So no complaints there.
There’s nothing really wrong with the Lumia 640. It’s just that the competition at the low end is so strong nowadays.
If you’re not fussed about which operating system you use, or want more apps, the Moto E and highly regarded Moto G are better value for money.
But if it has to be Windows, the Lumia 640 comes highly recommended.