  We present full-sky coadded maps created by uniformly combining the first
five years of Wide-field Infrared Survey Explorer (WISE) and NEOWISE imaging at
3.4 microns (W1) and 4.6 microns (W2). By incorporating both pre-hibernation
WISE exposures from 2010-2011 and the first four years (2013-2017) of
post-hibernation exposures from the NEOWISE-Reactivation mission, we are able
to provide W1/W2 coadds that span a 15 times longer time baseline and are
substantially deeper than the standard AllWISE data products. Our new five-year
"full-depth" coadds are now the deepest ever all-sky maps at 3-5 microns,
permitting detection of sources ~2 times (~0.7 mag) fainter than AllWISE at 5
sigma significance. We additionally present an updated set of "time-resolved"
W1/W2 coadds, which separately stack each of ~10 sky passes at each inertial
sky location, enabling motion and variability measurements for faint infrared
sources over a long ~7.5 year time baseline. We highlight new processing
improvements relative to our previous "unWISE" coadd releases, focusing on
astrometric calibration and artifact flagging. The deep WISE stacks presented
here are already being used to perform target selection for the Dark Energy
Spectroscopic Instrument (DESI), and our full-sky coadded WISE/NEOWISE products
will be key precursor data sets for upcoming wide-field infrared missions
including SPHEREx and NEOCam.
