Influence of N_2 Atmosphere on the Contamination Effects of Lead-free Solder Paste During Reflow Soldering Processes

摘要

"Lead-free" concept introduction in electronics industry presents many challenges for all companies implied in the supply chain, and a key concern remains the non defined nor mastered impact on the soldering process itself. Lead-free alloys melting point is higher compared to the usual Pb based alloys used up to now, and this temperature increase means also a potential increase of the oxides formation, main enemy of stability and reliability of the final joints. One area posing a challenge is the flux performance. In the reflow process, fluxes with higher activity have been developed to offer an attractive alternative to oxides formation. Main drawback is the residue amounts that weaken mechanically the joint and increase corrosion sensibility. The purpose of this study is to evaluate the possible influence of an inerting atmosphere on the residues formation during the reflow process. Five lead-free solder pastes have been selected with various activity levels. Wetting forces and time measurements have been determined with the method developed by Air Liquide in 2002 and based on the Malcom SP2 device. This specific and high performance equipment allows simulating the temperature profile of a reflow process and has been adapted to work under controlled atmospheres. This test is then very representative of reflow soldering processes. Results under air and N_2 are compared. Some solder pastes present similar wettability behaviour between Air and Nitrogen : theses pastes are the most activated ones and by then are supposed to imply a higher residue level on the boards. On this purpose, these measurements have been completed by residues characterization tests which now become an essential point with lead-free processes. The usual SIR test, IPC-TM-650 2.6.3.3, is widely used in industry but offers no challenge and is not representative of actual products, as specified in the SIR Handbook IPC-9201. The following tests have then been chosen: 1. SIR based on standard IPC-TM-650 2.6.3.3 but with lower standoffs and a Ni-Au finish, that are closer to today's boards design 2. and BONO test, used in France to characterize corrosion resistance, which is known to be more selective. Main trend is that higher residues deposition effects on joint resistance can be recovered by inerting, and various effects of nitrogen are underlined, such as the significant decrease of the measurement dispersion. Nitrogen offers an alternative to higher activated fluxes and to contaminants effects.