Fundamentals of fluxless soldering technology

Fluxless (flux-free) soldering technology deals with investigating and developing techniques and methods that can eliminate the use of fluxes in the soldering process. The fluxless feature in soldering processes has become increasing more important and received more attention from industries because there are more and more devices and products that cannot take fluxes in the soldering process. Examples are MEMS devices, sensor devices, biomedical devices, and photonic devices. In addition, in flip-chip soldering processes with very small gap between chips and substrates, flux residues are hard to clean out or are embedded in the underfills. The residues may reduce the reliability of the resulting flip-chip devices. There are two basic fluxless approaches that have been reported. The first is to use chemicals or RF plasma to convert or to remove the oxide layer that already exists. The existence of oxide layer is the reason why the flux is needed in nearly all soldering operations. The second approach is to remove the root cause, which is solder oxidation. This is accomplished by producing the solder materials in a non-oxidizing environment, followed immediately by capping the solder with a barrier layer that would prevent oxygen from penetrating into the solder layer. In this paper, we first present the root cause of needing fluxes in the soldering process. The fluxless processes dealing with oxides are summarized. The four fundamental steps of the oxidation prevention approach are reported. A fluxless process based on Sn-rich Sn-Au alloys is described as an example to illustrate the fluxless fundamentals. Results show that strong and nearly void-free joints can indeed be produced using this new technology.