Circuits Assembly - April 2008 - (Page 36)

Cover Story Figure 6. Bridging as a function of SO16 location. U7-U9, U30 are parallel to direction of the board. Figure 7. Location of U3. Figure 8. A 0.125”-thick board, air reflow, alcohol flux. Figure 9. Main effects plot for 0.125”-thick boards. Figure 10. Main effects plot for 0.093”-thick boards. (210,000 ppm O2, or 21%) can be applied only as “full tunnel.” The full tunnel method supplies nitrogen to every zone of the oven, resulting in almost-consistent oxygen concentration throughout the tunnel inclusive of the cooling zones (Figure 2). The reflow-only method selectively supplies nitrogen to specific zones. In this case, the oven was equipped to supply nitrogen only in heating zone 9, which influenced the behavior of zone 8 and cooling zone 1 where the alloy reflow takes place (Figure 3). Ppm measurements were taken using three oxygen analyzers located in heating zones 1, 3, 5, 7, 8 and 9, and cooling zones 10, 11 and 13. Reflow profiling was performed using scrapped test vehicles with attached type K thermocouples. The thermocouples were placed on four locations of a bare board: topside near moveable rail trailing edge, topside center of the board, topside near fixed rail leading edge, and bottom-side center of the board. The reflow profile was a direct ramp-to-peak style profile, which produced a maximum solder joint temperature of approximately 250oC and a total time of above liquidus (TAL) between 65 and 75 sec. Two reflow settings were developed for the two board thicknesses. Reflow profiles were used instead of curing profiles to simulate double-sided assemblies, while simultaneously curing the adhesive. Table 3 contains a summary of the reflow parameters. Wave soldering. A Delta Wave 6622 with a dual head spray fluxer and three heating zones (calrod system, top and bottom heatings with forced convection on the bottom and IR lamps on the top; and double waves – chip and main with smart wave) were used. Wave profiles were developed and optimized for the two board thicknesses and two fluxes based on manufacturer recommendations and observations from initial runs. 36 Circuits Assembly APRIL 2008 Flux amount was optimized by weighing a standard test board. Flux specification or golden boards defined the appropriate amount of flux. Fax paper or pH paper was placed on the topside of the boards, which were then run over the fluxer to ensure good through-hole wetting. For the VOC-free flux, the maximum specified amount of flux was 1500 µg/in. This amount resulted in poor overall soldering on the 0.125"-thick setup boards. The addition of flux showed improvement in through-hole penetration. However, the excessive amount of flux resulted in top- and bottom-side board contamination. Therefore, results for this flux and board thickness were discarded. For the alcohol flux, the amount was based on previous experiments performed in collaboration between Vitronics and Flextronics because the MSD for the flux does not specify the optimal amount. For all assemblies, SAC 305 was used at 265°C solderpot temperature. During this process, the topside temperature was below 217°C in all cases. Pallets were used to wave solder all boards. (Settings and observed values for the wave process are available upon request from the authors.) Results and Discussion Visual inspection. Bottom-side visual inspection of the wave-soldered surface-mount components and through-hole components was performed on all boards. The most prominent defect was solder bridging or shorts. The formation of solder bridging is affected mainly (Figure 4) by board thickness, flux type and wave-soldering atmosphere. At 95% confidence interval one way, ANOVA favors the use of the alcoholbased, no-clean flux, nitrogen over the wave, and 0.093”-thick boards. The reflow process did not influence bridging. 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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