Some banks can borrow some of their funding requirements at negative rates for short periods of time.
Banks fund themselves using a variety of sources - bonds and money market instruments (of various types), equity, deposits, past profits (which is really the same as equity) and various central bank mechanisms. Of those the only ones where there is a decent chance of funding at negative rates are bonds/money markets and central banks.
Bank profits have been squeezed by negative rates precisely because they've been forced to pass on more of the rate reductions to borrowers than they've been able to recapture for themselves.
But modern banks don't stay with a mortgage loan for long periods of time. Thanks to the "miracle" of derivatives, mortgage loans are repackaged and sold to other institutions, so the risk they carry is very small compared to the old times.
Banks fund themselves using a variety of sources - bonds and money market instruments (of various types), equity, deposits, past profits (which is really the same as equity) and various central bank mechanisms. Of those the only ones where there is a decent chance of funding at negative rates are bonds/money markets and central banks.
Bank profits have been squeezed by negative rates precisely because they've been forced to pass on more of the rate reductions to borrowers than they've been able to recapture for themselves.