Manufacturers spec sheets often don't list the type of microSD card that you can use with your phone. And so the confusion arrives for some picking up a new phone, like the Lumia 640
The important spec to remember is that the Lumia 640, and all new Lumias for that matter, supports the SDXC standard. And since it's a standard, that's what matters. So you can use any of the microSD cards you find conforming to this, or the lower-spec SDHC standard.
Sandisk announced a 200GB microSDXC card at Mobile World Congress which should also be compatible with these Lumia phones when it goes on sale. But it also costs about 4 times more than a Lumia 640, so there's that to bear in mind as well!
Hopefully that helps clear up any questions you may have had about using a microSD card with your new Lumia 640, Lumia 640 XL or any of the current Windows Phones.
If you've any questions that need answering, be sure to stop by the Windows Central Forums where there are some great people waiting to help!
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Richard Devine is a Managing Editor at Windows Central with over a decade of experience. A former Project Manager and long-term tech addict, he joined Mobile Nations in 2011 and has been found on Android Central and iMore as well as Windows Central. Currently, you'll find him steering the site's coverage of all manner of PC hardware and reviews. Find him on Mastodon at mstdn.social/@richdevine