Evaluation of the Comparative Solderability of Lead-free Solders in Nitrogen

摘要

Lead-free soldering technology is still in its infancy with technical and cost issues posing major challenges for the industry. It is expected that soldering in a nitrogen atmosphere might overcome some of the technical barriers and provide soldered products comparable to those using conventional lead-containing materials processed in air. But quantitative data regarding the soldering behaviour of lead-free solders under various atmospheres are sparse. Hence this work was undertaken to build on the previous studies of the solderability of SnAgCu alloys under a range of residual oxygen levels between 10 ppm and 21%. The current work extends the study to other lead-free solders, SnCu, SnZn and SnAgBi, in atmospheres containing as little as 10 ppm oxygen at superheats from 20 to 60 degrees. The results clearly demonstrate the benefit of inerting for these solders. Apart from widening the process window, it can reduce oxidation and improve solderability of lead-free systems to a level close to that of SnPb. In many cases the addition of an inert atmosphere during lead-free soldering can allow a ～30°C reduction in soldering temperature AND give the same solderability as using SnPb in air. Inerting is especially beneficial at low soldering temperatures or for challenging assemblies such as multi-layer boards. Whilst in the case of soldering with SnZn alloy inerting to < 100 ppm was essential, for SnCn and SnAgBi solders inerting only to <5,000 ppm oxygen offered significant benefits in wetting. For many of the challenging conditions and poor solderability associated with lead-free solders, nitre gen offers an attractive alternative to stronger fluxes and higher processing temperatures.