All rad with channels as BM or all BI family (BIPRO, BIX,..) are made with brass (tanks too only fins are copper). Copper is a very mallable metal and cannot be used for rigid support structures in radiators but only for fins to provide good thermal transfer..
You'd need a thick tube (1mm) to make channels in copper as those heat exchangers used in refrigeration units (Rad Cube)
Brass makes it rigid enough to hold the core structure but it is thin enough to efficiently conduct the heat to the copper fins, thickness of these channels is 0.05 to 0.07mm (BI family) so thermal transfer is very very quick between channels and copper fins. And there's no problem at all
