Effects of Commercial No-Clean Flux on Reliability of Fine Pitch Flip-Chip Package With Solder Bumps and Copper Pillars

In this work, two commercial no clean fluxes, namely, extremely low residue no-clean flux (NCF) (NC-1) and ordinary NCF (NC-2), were used for the preparation of a fine pitch flip-chip package with a
$7.6\,\times \,7.6$
mm
²
Si die on a
$17\,\times \,17$
mm
²
organic substrate. One water-soluble (WS) flux was also used for comparison. Moisture sensitivity of the package was evaluated using JESD22-A113D (30 °C/60% relative humidity (RH), 192 h,
$3\times $
reflow at 260 °C). Thermal cycling tests were performed following JESD22-A104D (−65 °C to 150 °C). The high-temperature storage life (HTSL) of the packages was investigated using JESD22-A103C (175 °C). Underfill delamination or voids in the package were investigated by confocal scanning acoustic microscopy (C-SAM). Failure characteristics of the packages were also studied using scanning electron microscopy (SEM). After moisture sensitivity level (MSL) test, vehicles prepared by NC-1 and NC-2 showed failure. SEM images revealed severe delamination between underfill and solder mask in the presence of amine/amide and carboxylic acid-based NCF residue. Solder bump crack were found at < 500 cycles for samples prepared with NC-1 which contains tertiary amine and long carbon chain (C8:C10:C12) carboxylic acid-based. However, failure occurred at < 1500 cycles for NC-2 containing secondary amide and shorter carbon chain length (C3) carboxylic acid. Samples prepared using WS did not show any failure for up to 2000 cycles. Thus, NCF-treated assemblies performed poorly in moisture sensitivity and temperature cycling tests.