Rosin-based Flux Research Articles

Wetting characteristics of Sn-5Sb-CuNiAg lead-free solders on the copper substrate

PurposeThe purpose of this paper is to investigate the wetting behaviors of Sn-5Sb-CuNiAg solders on copper substrates in different soldering processes and the effects of alloying elements on the wettability.Design/methodology/approachSn-5Sb-CuNiAg solder balls (750 µm in diameter) were spread and wetted on 40 × 40 × 1 mm copper plates, in different fluxes, soldering temperatures and time. The contact angles were obtained by a home-made measuring instrument. The samples were polished and deep etched before analyzed by scanning electron microscopy. Energy dispersive X-ray spectroscopy was used to identify the composition of the joints.FindingsThe effects of different soldering processes and alloying elements on the wetting behaviors of Sn-5Sb-CuNiAg solders on copper substrates were calculated and expounded. The rosin-based flux could effectively remove oxidation layers and improve the wettability of Sn-5Sb-CuNiAg solders. Then with the increase of soldering temperature and time, the contact angles decreased gradually. The soldering processes suited for Sn-5Sb-CuNiAg solders were RMA218, 280°C and 30 s. Considered the effects of alloying elements, the wettability of Sn-5Sb-0.5Cu-0.1Ni-0.5Ag was relatively favorable on copper substrates. Besides, Ni could accumulate at the solder/Cu interface and form a jagged (Cu,Ni)6Sn5 IMC.Originality/valueThis work was carried out with our handmade experiment equipment and the production of the quinary lead-free solder alloy used in wetting tests belongs to us. The investigated Sn-5Sb-CuNiAg alloys exhibited higher melting point and preferable wettability, that was one of the candidates for high-temperature lead-free solders to replace high-Pb solders, and applied extremely to high temperature and frequency working environments of the third-generation semiconductors components, with a greater potential research and development value.

Reliability of Printed Circuit Boards Processed Using No-Clean Flux Technology in Temperature–Humidity–Bias Conditions

Printed circuit board (PCB) specimens containing three different IPC-B-25 test structures were exposed to temperature-humidity-bias conditions in order to evaluate the effects of no-clean flux chemistry, conductor spacing, voltage bias, and test environment on surface insulation resistance (SIR). Comb patterns on the PCBs were coated with a eutectic (63Sn/37Pb) solder applied by a hot air solder leveling and processed by using no-clean aqueous-based and rosin-based fluxes. The SIR failure rate with rosin-based no-clean flux was observed to be greater than that with aqueous-based no-clean flux. This was explained by the more corrosive nature of the flux residues and the larger concentration of hygroscopic weak organic acids in the rosin-based flux residues. A characteristic of the SIR failures for PCBs processed with rosin-based flux was a series of intermittent SIR drops, which could severely affect the reliability of electronic assemblies. It was hypothesized that flux residues combined with adsorbed moisture from the environment form an acidic medium, occasionally breaking through the tin oxide passivation layer on the electrodes. Penetration of the passivation layer combined with conductive flux residues bridging the electrodes caused the resistance to decrease, and rehealing of the passivation layer resulted in the intermittent behavior. Conductor spacing was observed to represent a factor in the electrochemical migration process that is independent of electric field. Since conductor spacings in electronic products continue to decrease, the experimental results support recommendations to replace 25-mil (0.64-mm) comb structures on industry standard test boards with those having smaller spacings, below 12.5 mil (0.32 mm), that accurately reflect the greater risk for SIR drops of today's higher density assemblies.

Effect of Compatibility between Fluxes and Cyanate Ester Underfill on Lead Free Flip Chip Assembly and Reliability

The quality and reliability of flip-chip assembly is severely impacted by the compatibility between various materials used in the package. Currently, no-clean fluxes are widely used for the assembly of flip chips. Poor compatibility between the flux residue and the underfill can lead to the formation of voids, and consequently, reliability problems. Therefore, a major concern for flip-chip assembly is the compatibility between the flux residues and the underfill. The principal objective of this research was to develop a flux for lead-free packaging, which would be compatible with high performance moisture resistant cyanate ester-based underfills. During this study, commercially available fluxes, along with tailor made epoxy-based flux, were tested for their compatibility with the underfill. The results indicated that the assembly process window for the rosin-based fluxes was much wider than the epoxy-based fluxes. Synthetic flux had poor soldering performance and relatively poor compatibility with the cyanate ester underfill than rosin-based flux. Epoxy flux exhibited narrow process window and the soldering performance was sensitive to flux thickness, reflow profile, substrate pad surface finish and topography. For smaller packages with die size of 10 mm by 10 mm, the compatibility between the cyanate ester underfill and the different fluxes was comparable. However, for the packages with die size of 22 mm by 22 mm, the compatibility between the epoxy-based flux and the cyanate ester underfill was relatively better than that with the other two flux chemistries.

Viscosity transition of ZnO-containing rosin

Eutectic alloy Sn–9Zn is regarded as one of the potential substitutes for Pb-containing solders, although it has some drawbacks yet to overcome. One of the problems involved is drastic viscosity increase of rosin-based flux during soldering or storage. In the present work we investigated the role of ZnO in the viscosity transition of rosin. The results show that the viscosity transition is directly caused by the presence of ZnO in molten rosin and, at a certain temperature, there exists a critical ZnO concentration for its occurrence. The critical ZnO concentration increases with temperature. A graph is presented to show the conditions for the viscosity transition, in a ZnO concentration–temperature space. The transition is found to be reversible when shifting temperature or ZnO concentration across the boundary and to assume the features of a thermally activated phase transformation.

Production of solder and flux with low radioactivity

The problem of radioactivity in solder has been studied. A low-radioactive rosin-core solder has been produced. Low-radioactive alternatives for rosin-based flux have also been considered.