‘Yea’ is a very old-fashioned formal way of saying ‘yes’, used mainly in voting. It’s the opposite of—and rhymes with—’nay’. When you want to write the common casual version of ‘yes’, the correct spelling is ‘yeah’ (sounds like ‘yeh’).
[Examples of yay:] When the third grade teacher announced a class trip to the zoo, we all yelled ‘yay!’ (the opposite of ‘boo’!). That was back when I was only yay high. from Common Errors in English.
No way! Learn something everyday, eh?