Unless you actually know for sure, buying a microSD card to go with your new Windows Phone is potentially confusing. Thankfully, the Windows Central Forums helps break down the techy talk.
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 only thing listed on the microsoft spec page is that it will take MicroSD cards. Will it accept HD and XC type cards? Are there any capacity limitations?" - Forums member Medwynd
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.
"It will work with SDHC and SDXC. The difference between HC and XC is that the size of HC is between >2 and 32 GB and the size of XC is 32 GB to 2TB. (And from 32 onwards you can not use FAT32 formatting). There is no max size for your phone the 128 GB is "just" put in the spec because that is the current max size for microSD cards and SDXC"
"Side note: A class 4 card may not work because it is to slow. UHS (Ultra High Speed bus) standard is not supported so you will not get 30 or 48 MB/s because the max speed of the HIgh Speed bus in your phone is 25 MB/s." - forums Ambassador Harrie-S
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!