Circuits Assembly - September 2008 - (Page 34)

Soldering Reflow Soldering with a SnCu Eutectic Pb-Free Alloy By Gerjan Diepstraten The performance matches SAC, but with shinier joints. nCuNi is an established no-silver Pbfree alloy for wave soldering applications. The alloy is beginning to be applied to reflow applications. This alloy is more popular in wave soldering for numerous reasons, including lower cost (no silver), minor copper leaching and reduced dross formation. SnCuNi alloys were not of interest for reflow soldering when Pb-free was introduced because of their high melting temperature (227ºC). Since then, process engineers have learned melting point is not the sole factor in Pb-free reflow soldering. There are other issues: • Copper leaching is a critical issue with SAC 305. After multiple soldering processes, the copper pad may have disappeared when it comes time to rework. • Reliability of SAC 305 is acceptable for component leads; it can accommodate strain – e.g., from SOICs and QFPs – but for leadless components, where the solder Table 1. Parameters of 4 Alloy Samples Parameters Solder paste Reflow profile Reflow cycles A SAC 305* 3 min. 1 B SAC 405 5 min. 2 C SAC 305 type 4 D SnCuNi * S has to absorb all the strain (BGAs, LCCCs), SnCuNi is more reliable. 1 • SnCuNi shows less voiding in the majority of applications thus far. • SAC 305 joints are dull and grainy, with the surface showing a microstructure of coarse intermetallic crystals. While simple SnCu eutectic exhibits a dull cracked surface when cooled slowly, SnCu with the nickel addition solidifies with a generally smooth bright finish under the same conditions. 2 Reflow on Copper OSP The goal of this design of experiment is to identify differences in soldering between SAC alloys and SnCuNi. Solder paste was printed on a Cu-plated board with an Ethone Entek Plus (Cu-106HT) passivation on it. Three different SAC alloys were compared with SnCuNi. One of the SAC 305 alloys (A) has the same flux chemistry as the SnCuNi (D) (Figure 1). *Same flux chemistry Figure 1. The two heating profiles monitored in the process window of the solder paste flux chemistry. circuitsassembly.com 34 Circuits Assembly SEPTEMBER 2008 http://circuitsassembly.com

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Circuits Assembly - September 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Reflow Soldering with a SnCu Eutectic Pb-Free Alloy Improving OEE in High Mix Facilities Effectively Managing RF Design in Utility Metering Applications Solder Joint Reliability of Different BGAs Reworked Using Low Melting Point Pb-Free Alloys Tech Tips Wave Soldering Pb-Free Lessons Learned Materials World Process Doctor Equipment Advances Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - September 2008

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