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[转载]Influence of N2 on the Contamination Effects of LF [复制链接]

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2007-04-30
只看楼主 倒序阅读 使用道具 楼主  发表于: 2007-11-29
Influence of N2 Atmosphere on the Contamination Effects of Lead-free Solder Paste During Reflow Soldering Processes

Abstract
“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 N2 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:
• 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
• 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.
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