Development And Performance Characterization Of Low Residual Lead-free Solder Paste For Low Temperature Soldering In Electronic Packaging

Sn-Bi based solders have attracted increasing attention in electronic and electrical industries recently due to their obvious advantages such as low melting temperature,low cost and excellent spreading performance.With the rapid development of surface mount technology(SMT),there is an increasing demand for low temperature lead-free solder pastes.Heretofore,however,the majority of the commercially available fluxes are designed and targeted for use in Sn-Pb and Sn-Ag-Cu solder pastes,which show low activity and easily cause large amount of residues when used in low temperature soldering processes.Further,most of the currently available Sn-Bi solder pastes exhibit poor storability and are more likely to produce black residual oxides around solder joints after soldering.These drawbacks have limited the further applications of Sn-Bi solder pastes in the field of SMT.This thesis study aims to develop a Sn-Bi solder paste suitable for low temperature soldering in electronic packaging,and to evaluate the properties of the as-fabricated solder paste.Firstly,the solderability was improved by eliminating the black residual oxides through adding selected organic acids and activity enhancers.Upon determining the components of inhibitor,the corrosive effect of activator on solder alloy powders was reduced and the storability of the Sn58 Bi solder paste was improved.Different components of rosin,solvent,thixotropic agent were then added into the flux,and qualitative and quantitative analyses were done to improve the performance of the soldering flux used for the Sn58 Bi solder paste.The Sn-Bi solder paste was developed successfully by mixing the alloy powder and flux at an optimized ratio.Finally,overall performance of the so-developed solder paste was tested according to technical standards with a comparison to two commercial Sn-Bi solder pastes.Results show that the addition of organic acid A,organic acid B can reduce the wetting time and increase the spreading area of Sn-Bi solder paste.The residual black oxides are eliminated effectively by adding liquid carboxylic acid A.Imidazole B inhibitor can prevent solder powders from being corroded by the flux.The flux for Sn58 Bi solder paste prepared in the present work possesses excellent physical and viscosity stability,and results in lessresidues after soldering,as well as few verdigris and corrosion spots being formed on the Cu substrate.The viscosity of the solder paste increases with increasing the percentage of solder powder in the paste,and the solder paste with a solder powder to flux ratio of 89 to 11(wt.%)exhibits an appropriate viscosity.By comparing the Sn-Bi solder paste developed by this work with two commercial Sn-Bi solder paste products,clearly the self-developed Sn-Bi solder paste possesses excellent solderability,suitable viscosity,high slump performance and less residue dryness during soldering.