Circuits Assembly - April 2008 - (Page 32)

Cover Story Impact of Soldering Atmosphere on Solder Joint Formation By Ursula Marquez de Tino, Dr. Denis Barbini and Wesley Enroth A Flextronics-Vitronics Soltec collaborative investigation into techniques for reducing nitrogen consumption. oldering atmosphere influences final solder interconnect formation. Based on previous studies, SnPb and Pb-free solders perform differently under varying soldering atmospheres.1,2,3,4,5,6 Pb-free pastes’ significant reductive influence on wetting and spreading may mandate an inerted atmosphere.7,8 The question becomes, To what extent should nitrogen be used, if at all? Nitrogen adds cost to the assembly process, the amount dependent on geography.9 As a result, industry has shown wide interest in decreasing consumption. This project identifies atmosphere’s influence on Pb-free solder joint formation by controlling the O2 ppm levels in the oven during assembly of specifically designed PCBs. The O2 ppm levels are quantified by measurements taken at defined locations in the reflow oven tunnel. Characterizing the atmosphere throughout the oven is critical to understanding the solder joint formation-soldering atmosphere relationship. Post-reflow, boards were subjected to secondary wave soldering where through-hole components Figure 1. Bottom-side view of PCB. were soldered. Through-hole 32 Circuits Assembly APRIL 2008 Table 1. Through-Hole Design Dimensions Components Pin connector Pad (mils) 60 60 60 60 70 Hole (mils) 35 39 43 47 35 39 43 47 35 35 39 Layers 2, 4, 6, 8 2, 4, 6, 8 2, 4, 6, 8 2, 4, 6, 8 2, 4, 6, 8 2, 4, 6, 8 2, 4, 6, 8 2, 4, 6, 8 1, 2, 6,8, 1, 2, 4 S 70 70 70 DIP 16 Axial resistors 60 60 60 penetration measurements were performed to compare the different process parameters. Materials in this experiment exert significant influences on the outcome. The project studies Pb-free PTH components assembled on 0.093" and 0.125" copper OSP boards assembled with two fluxes. Wave profiles were optimized per flux type to minimize any adverse influences from flux performance. Characterizations included visual inspection and 5DX. This project aims to identify relationships between various inerted soldering atmospheres and through-hole penetration. Surface mount defect data and through-hole penetration analyses were used to quantify effects of: • Oxygen concentration during reflow. • Nitrogen supply methods during reflow. • Flux on wave soldering. • Ambient on wave soldering. • Board thickness. • Through-hole design and component orientation. circuitsassembly.com http://circuitsassembly.com

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Circuits Assembly - April 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing Design and Modeling of High-Speed, High-Density 3-D CSPs and Memory Modules The ‘Big Brush Off’ Revisited Impact of Soldering Atmosphere on Solder Joint Formation Beyond Moore’s Law ESD Control For Class 0 ESDS Devices Growing Your Brand This Year’s Model Tech Tips Reflow Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Equipment Advances Apex Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - April 2008

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