The ALMA Frontier Fields survey V: ALMA Stacking of Lyman-Break Galaxies
  in Abell 2744, Abell 370, Abell S1063, MACSJ0416.1-2403 and MACSJ1149.5+2223
  The Hubble Frontier Fields offer an exceptionally deep window into the
high-redshift universe, covering a substantially larger area than the Hubble
Ultra-Deep field at low magnification and probing 1--2 mags deeper in
exceptional high-magnification regions. We aim to leverage Atacama Large
Millimetre Array (ALMA) band 6 ($\approx$263\,GHz) mosaics in the central
portions of five Frontier Fields to characterize the infrared (IR) properties
of $1582$ ultraviolet (UV)-selected Lyman-Break Galaxies (LBGs) at redshifts of
$z {\sim}$2--8. We investigated individual and stacked fluxes and IR excess
(IRX) values of the LBG sample as functions of stellar mass
($\mathrm{M}_{\bigstar}$), redshift, UV luminosity and slope $\beta$, and
lensing magnification. Two (2) LBG candidates were individually detected above
a significance of $4.1{-}\sigma$, while stacked samples of the remaining LBG
candidates yielded no significant detections. We investigated our detections
and upper limits in the context of the IRX-$\mathrm{M}_{\bigstar}$ and
IRX-$\beta$ relations, probing at least one dex lower in stellar mass than past
studies have done. Our upper limits exclude substantial portions of parameter
space and they are sufficiently deep in a handful of cases to create mild
tension with the typically assumed attenuation and consensus relations. We
observe a clear and smooth trend between $\mathrm{M}_{\bigstar}$ and $\beta$,
which extends to low masses and blue (low) $\beta$ values, consistent with
expectations from previous works.
