Impact of inerting on wetting and solderability: nitrogen offers the same level of wetting performances as low-activated pastes without the effects of residues.

The RoHS directive will ban lead use in new electrical and electronics equipment. Solder pastes will be replaced by lead-free compositions with comparative performances but higher melting points. The challenge is the increased soldering temperature and associated increased oxidation. In this context, inert atmosphere soldering seems justified. But as commercial and technical brochures indicate, solder pastes are available with modified formulations and higher activation rates that give good results in air and nitrogen. Whether highly activated pastes affect end-product quality will be discussed here.

The purpose of this study is twofold: To determine the impact of atmosphere on 1) wetting processes and 2) final product reliability. Wetting tests have been carried out thanks to a specific device representative of a reflow process, which led to a comparison between wetting behaviors in air and nitrogen. In parallel, board reliability has been evaluated using contamination tests: a BONO corrosion test and a SIR test adapted to be representative of current designs. The approach in this study is a way to predict the benefits of inert soldering, which are often process-specific and reliability-based.

New solder paste formulations have been developed to limit oxidation phenomena and to obtain the same behavior in air and under nitrogen. (1) Herein is an evaluation of the wetting performances of several lead-free pastes and analyses of differences in behavior. Besides traditional lab methods for evaluating soldering performance--spread and wetting balance (the latter being more representative of wave processes)--a new commercially available device has been designed to reproduce actual reflow conditions. Improvements have been made by Air Liquide and IFTEC toward an accurate simulation of reflow oven temperature cycles.

This study shows that while the use of activators increases wetting performances under air, it also strongly impacts long-term board reliability. In contrast, nitrogen offers the same level of wetting performances as low-activated pastes but without the effects of activator residues.

The results confirm that better wetting of highly activated pastes under air leads to higher residue deposition levels. On a representative sample of solder pastes, a correlation between wetting performances under air and electromigration phenomena is due to contaminants remaining on the board. As a consequence, the use of highly activated pastes leads to the deposition of harmful residues.

The study also indicates soldering with nitrogen improves wetting and provides better control, and that nitrogen use overcomes the lower wetting performances of non-activated pastes while keeping residue levels low.

Ed.: For the full article, please see circuitsassembly.com/cms/content/view/1526/

Helene Daniel is a research and development engineer for Air Liquide (airliquide.com); [e-mail]. Marc Leturmy, Sylvie Lazure and Thomas Vukelic are also with Air Liquide. Daniel Muller is technical director of The IFTEC Institute (iftec.fr); [e-mail].