Electronic industry has been driven toward lead-free by RoHS (Restriction of Hazardous Substances Directive) which is in force since 2006. Recently REACH (Registration, Evaluation and Authorization of Chemicals) further drives the industry toward halogen-free. As a result, solder pastes for PCB assembly are required or desired to be both lead-free and halogen-free. Lead-free solder alloys in general wet poorer than tin-lead due to the higher surface tension of the former alloys. In the mean time, halogen-free fluxes typically also wet poorer than the more powerful halogen-containing fluxes. Consequently, the lead-free and halogen-free solder paste products emerged inevitably suffer a considerably inferior soldering performance than that of conventional halogen-containing tin-lead solder pastes. The deficiencies include poor wetting, solder balling, voiding, graping, head-in-pillow, etc. This gap is particularly significant for fine-pitch applications where the impact of oxidation is more profound. Furthermore, the higher soldering temperature of the higher melting lead-free alloys also aggravates the challenge of in-circuit test for no-clean processes, mainly due to the difficulty for probe to penetrate through the toughened flux residue. Although use of inert reflow atmosphere may alleviate some of the problems, the higher cost of it is prohibitive for most of the manufacturing firms. In this work, a halogen-free lead-free no-clean solder paste system, Indium8.9HF series, has been developed. It exhibits superior oxidation tolerance, thus assures superior resistance against graping, head-in-pillow, solder balling, voiding, and poor wetting for miniaturized electronic applications. In spite of the immense challenge in material science, this system also shows outstanding probe testability, in addition to its very good printability, non-slump, SIR, and ECM performance. The superior performance of this Indium8.9HF system effectively sealed the gap caused by lead-free and halogen-free requirements.
Key words: lead-free, halogen-free, no-clean, solder, solder paste, miniaturization, graping, head-in-pillow, voiding, solder balling, probe testability, ICT, oxidation