Ethylene glycol ether free solder paste development

There are growing concerns in the electronics industry for not only finding alternatives to lead but also other potentially hazardous materials as well. This paper summarizes the development of ethylene glycol ether (EGE)-free solder flux for the formulation of lead-free solder pastes. Replacing the toxic components in the flux was only the first challenge, the criteria of commercially proven pastes also had to be met. Both commercial and in-house solder paste formulations were evaluated for printability, reflow, wetting, flux residue removal, and solder void characteristics. Two critical issues, solder bump boids and flux residue removal, were identified and associated with the high temperature reflow of Pb-free pastes. These issues were not effectively improved by the existing commercial EGE-free solder pastes. New solder paste formulations were developed utilizing alternative chemistry than those found in traditional solder paste fluxes. These pastes, some, of which are also water soluble, reduced void frequency and size by more than 4x as compared to vendors' pastes. Solder bump height uniformity of 135 4 m within each die was consistently achieved. Thermal-mechanical reliability tests were performed on various lead-free solder alloys using the new flux formulations. The reliability of flip chip assembled DCA on organic boards with both OSP/Cu and Cu/Ni/au pad metallizations were comparable to eutectic Sn63Pb37 bumped assemblies using commerical pastes.