The critical oxide thickness for Pb-free reflow soldering on Cu substrate

► Critical oxide thickness for Pb free solder on Cu substrate is 18 ± 5 nm. ► Above the critical oxide, non-wet solder joint increases exponentially. ► A maximum 13-nm oxide thickness is suggested for good solder joint. ► Initial growth of oxide thickness is logarithmic and then parabolic after 12 nm. ► Thick oxide (360-560 nm) is formed as pores shorten the oxidation path.