NEW YORK (Standard&Poor's) March 12, 2015--Standard&Poor's Ratings Services today said it assigned its 'BBB+f' fund credit quality and 'S2' volatility ratings on both the iShares iBonds Dec 2017 Corporate ETF and the iShares iBonds Dec 2019 Corporate ETF. We also assigned our 'BBB+f' fund credit quality and 'S3' volatility ratings on the iShares iBonds Dec 2021 Corporate ETF, iShares iBonds Dec 2022 Corporate ETF, and the iShares iBonds Dec 2023 Corporate ETF. In addition, we assigned our 'BBBf' fund credit quality and 'S3' volatility ratings to the iShares iBonds Dec 2024 Corporate ETF and the iShares iBonds Dec 2025 Corporate ETF. The funds will track indices composed of U.S. dollar-denominated, investment-grade securities publicly issued by U.S.