Any board of finance may, whenever in its opinion such a course is advisable and the public money under its control, or any part thereof, will not be needed immediately for public purposes, place such funds on time deposit with a bank, savings and loan association or credit union whose deposits are insured by an agency of the United States, taking the certificate of deposit or other evidence of indebtedness of the bank, savings and loan association or credit union receiving the deposit; provided, however, that all such deposits shall be secured as provided by law. No county or municipal board of finance shall make any deposits outside of its county.