Copper Pillar: why, when & where

Mass reflow w/CUF (CuP) or Thermo compression bond (TCB/TCFC) -> TC-CUF, TC-NCP, TC-NCF
Electrical performance (current carrying capabilities, EM, fine pitch requirements) are driving CuP.
Mass reflow w/ Cu-pillar (CuP) maximizes UPH and therefore minimizes assembly cost. However, for large die packages, MR generates warpage concerns that can result in ELK/CPI issues. To counter this and lower package stresses, TCB or TCFC are process alternatives (but are low UPH options that end up increasing assembly cost/complexity):
TC-CUF: relatively lower cost alternative, but capillary UF flow may need vacuum underfill or pressure curing
TC-NCP: relatively medium cost alternative
TC-NCF: highest cost, application limited to thin die/ smaller FF packages/ ultra fine pitch
Shrinking pitch & die size: MR + Pb-free solder, MR + CuP/CuBOL, TC-CUF, TCNCP, TCNCF