Circuits Assembly - April 2008 - (Page 38)

Cover Story During visual inspection, a 34% reduction on bridging was observed affects through-hole penetration. Therefore, the data were separated and when using nitrogen on the wave soldering process, 27% reduction analyzed based on board thickness and flux types. Flux exerted a definitive effect, but this was minimized by applying when using 0.093”-thick boards, and 26% when increased amounts of flux as required to optiusing low-rosin, no-clean flux. Reporting the mize results on baseline runs (as described in the number of bridging entails counting the numExperiment section). ber of pins involved in each bridge. Figure 5 Board thickness effects. The 0.125"-thick illustrates a bridge that involves three pins. boards show instances of poor hole penetration 5DX inspection. Data acquired from x-ray when air profiles were used during reflow (Figinspection were used only to inspect bridging of ure 8). IPC-A-610D standard indicates a minisurface-mount components. All bridging defects mum of 75% fill for Classes 1, 2 and 3 is acceptwere found on the SO-16 components (Figure able.12 Other significant factors were nitrogen 6). The most affected location was U3, for which Figure 11. Main effects plot for pin connecthe orientation was perpendicular to the direcsupply methods and oxygen ppm levels (Figure tors. tion of the board and located on a second 9). For boards reflowed with nitrogen, row (Figure 7). the data show applying nitrogen in the Placing the components parallel to the reflow areas results in only slightly better direction of the board showed a major through-hole penetration. For boards improvement. The bridging DPMO reflowed in ambient atmosphere, the (defect per million opportunities) averdata provide guidance in respect to nitroages for parallel versus perpendicular gen use during wave soldering. However, locations were 6,727 and 70,409 for the on average, a through-hole penetration 0.093"-thick boards, respectively. For the of 62% was achieved in this case, versus 0.125"-thick boards, the DPMO averages 59% when using air in wave. were 111,545 and 65,008 for parallel and For 0.093"-thick boards, acceptable perpendicular, respectively. This opposite solder joint penetration for all joints trend was observed mainly when using Figure 12. Main effects plot for pin connector in air. was observed (Figure 10). The reflow alcohol-based flux. It seems the high atmosphere did influence through-hole preheat temperatures (topside temperature penetration, but not to the extent to make 128°C) burn off the flux on the bottom-side the joints unacceptable. Major factors found to affect through-hole penetration were before wave soldering. flux, reflow atmosphere and nitrogen sup5DX for PTH parts. 5DX was used to inspect ply method. Boards reflowed in air showed through-hole penetration. Solder penetraa decrease in solder penetration. At a 95% tion analysis in each via was accomplished by confidence interval, the data favor use of the first taking x-ray measurements at 0, 10, 20, alcohol-based, no-clean flux and reflow-only 30, 40, 50, 60, 70, 80, 90 and 100% intervals supply method. of through-hole. This technique provides It can be concluded that for 0.125"-thick quantitative data by characterizing grayscale Figure 13. Main effects plot for resistors reflowed boards, acceptable through-hole penetration intensity. A threshold value was determined in air. can be achieved when nitrogen environbased on experimentation and calibration ment is used in the reflow process as high performed prior to this experiment. Based as 2500 ppm O2, and nitrogen can be on this threshold value, grayscale values below the threshold value are recorded as applied only on the reflow areas. For the no solder, whereas grayscale values above 0.093"-thick boards, nitrogen’s benefits in the threshold value are recorded as the reflow were not observed unless 100% presence of solder. Through-hole penetrathrough-hole penetration is required. tion was reported as the maximum height Board designs and component types or interval where solder was observed. effects. Board layout included throughWave parameter effects. The purpose hole designs attached to different layof this investigation is to identify the ers (one to eight layers), pad diameters impact of reflow soldering atmosphere (0.060" and 0.070"), and hole diameters on joint formation. Consequently, the Figure 14. Main effects plot for DIP – 21%. (from 0.035" to 0.047"). In addition, three wave soldering process was optimized types of through-hole components were to the extent that it does not exert a significant influence on the joint hand-placed and assembled as described. Each component had varying formation. Attempts to make the wave soldering process neutral were pin counts and pin diameter. moderately successful. The data indicate board thickness significantly Pin connector. The pin connectors were placed on various locations 38 Circuits Assembly APRIL 2008 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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