It most heavily weightsUS Bancorp ( USB ) at 21% of assets,PNC Financial Services ( PNC ) 12% andBB&T (BBT) 7%. The Federal Reserve has pledged to hold down short-term interest rates near zero until inflation hits 2% and the unemployment falls below 6.5%, which could take years. "(BofA Merrill) data show that mortgage credit for housing purchases or refinancing has improved in the past 18 months, but from a very low base."