Filing an extension to extend your time to file does not extend your time to pay (if you owe taxes.). You still have to pay by July 15th not matter what, to avoid late payment penalties and interest.
If when you file, what you owe the IRS is more than $1000 or more than 10% of your total tax liability (whichever is higher) then you will be assessed an underpayment or under withholding penalty, regardless of when you actually pay what you owe.
