How to securely wipe an Android smartphone or tablet

Getting rid of your Android smartphone or tablet, but want to make sure that all your data has been securely deleted. Here's how you go about the task.

• Backup all your data; because once it's gone, it's gone. There's no undo button you can press.
  
• Next, plug the charger into the device so it's getting power for the deletion process. This is important because the process can take hours.
  
• Go to Settings >> Security >> Encrypt phone (or Encrypt tablet) to begin the process on Android 4.4 or lower, or go Settings >> Security >> Screen lock to begin the process on Android 5.0 and higher (this is unnecessary on new devices running Android 6.0 Marshmallow).
  
• You will be guided through the process. Set a strong password. Don't bother encrypting the SD card because you can pop this out and keep it rather than wipe it. This process can take over an hour, so be patient and make sure you have the time.
  
• Go to Settings >> Backup & reset >> Factory data reset and then tap Reset phone (or Reset tablet).
  
• Once this process is complete your data is gone. Technically, it's still written there on the flash memory in what is now reported as free space, and will eventually be overwritten. Because it's encrypted, recovering it would be difficult. However, if you want to completely eradicate it, jump to the next step.
  
• The best way to get rid of the encrypted data is to overwrite it with new data. There are two ways you can do this. You can load some big files onto the device (big video files are ideal for this) until the storage is full up and then delete these files, or you can use a custom app such as iShredder to digitally "shred" the data.
  

That's it! Your data is now gone, and you're safe to pass the smartphone or tablet on.

Source:

http://www.zdnet.com/article/how-to-securely-wipe-an-android-smartphone-or-tablet/#ftag=RSSbaffb68