Most lenders have a published 620 minimum FICO for conventional mortgages. However, files are typically underwritten through an automated underwriting system like Desktop Underwriter (Fannie Mae) or Loan Prospector (Freddie Mac). Although the 620 score is the ultimate minimum it does not mean that a 620 score qualifies. The automated underwriting systems use proprietary algorithms to rate risk based on debt-to-income ratios, available reserves, credit history, loan-to-value, etc. If the system calculates that the risk level is acceptable then an approval is issued. This only means that the loan officer can proceed and move the loan to underwriting, it does not necessarily mean the loan is approved but most of the time (barring any new information) the loan will be approved.
This answer is probably more than you had bargained for but unfortunately more goes into a qualification then just the credit score. I hope this helps.... more