The credit crunch has limited the availability of jumbo mortgages, or loans that are too large to be backed by Freddie Mac or Fannie Mae. But jumbo loans are now starting to be more accessible. Bank of America is offering 30-year fixed-rate jumbo mortgages with rates in the 5 percent range, and rates on jumbo loans averaged 6.5 percent for the week ending March 27. Jumbo loans also offer a fairly safe rate of return for banks. For potential jumbo borrowers, however, requirements are still high: Bank of America asks prospective borrowers for a 20 percent down payment, a credit score of at least 720, and at least a six-month reserve fund.