Circuits Assembly - April 2008 - (Page 41)

Cover Story Table 2. Design of Experiments Process Conditions Reflow 25 500 1200 2500 AIR Wave Flux Atmosphere VOC-free Nitrogen Alcohol Air Parameter Settings Reflow Only ✓ ✓ ✓ ✓ ✓ Full Tunnel ✓ ✓ ✓ ✓ Table 4. Best Settings Flux Types, Pin Connectors Factors Layers Pad size Drill size Ppm levels RO/FT Wave ambient Orientation 93 -A Any Any 0.035”, 0.039” 25-2500 RO Any Parallel 93 – VOC-Free 2, 4 and 6 70 0.035”, 0.039” 25-21% Any Any Parallel Table 5. Best Settings Flux Types, Axial Resistors Factors Layers 93 – A Any Any 25-2500 RO Any Perpendicular 93 – VOC-Free 1 0.035” 25-2500 Anyone N2 Perpendicular Table 3. Profile Parameters Parameters Peak Temp. (ºC) Delta T (ºC) Ramp rate (ºC/sec) TAL (s) Heating Time (min) Cooling Rate (ºC/sec) 0.093” thick 245-249 3.9 1 – 1.20 70 – 75 4 -3.5 0.125” thick 244 - 248 3.9 1 – 1.20 65-75 4 -2.8 Drill size Ppm levels RO/FT Wave ambient Orientation on the board in two orientations (parallel and perpendicular to the board direction), with four drill sizes (0.035", 0.039", 0.043" and 0.047"), two pad sizes (0.060" and 0.070"), and connected to 2, 4, 6, or 8 layers. The 0.125"-thick boards reflowed in nitrogen showed that drill sizes and component orientation have a significant effect on through-hole penetration for the pin connector (Figure 11). Nitrogen supply method or oxygen ppm levels were not significantly different. Note all joints showed acceptable through-hole penetration per IPC guidelines.13 The 0.125"-thick boards reflowed in air were characterized by poor through-hole penetration, as observed in (Figure 12). To obtain an acceptable through-hole penetration (>75%), a drill hole of 0.035" on a 0.070" pad attached to maximum two layers is required (based on 12 data points). The component needs to be located parallel to the direction of the board and can be waved soldered under ambient conditions. For the VOC-free based, no-clean flux and 0.093”-thick boards, the factors that affect through-hole penetration were layers, pad sizes, drill sizes, oxygen ppm levels, and orientation, while for the 0.093"-thick boards assembled with the alcohol-based low rosin, no-clean flux, the factors were drill sizes, oxygen ppm levels, nitrogen supply methods, and orientation. Note all through-hole joints are acceptable. Table 4 shows a summary of the best settings for each flux. In summary, a nitrogen environment with as high as 2500 ppm of oxygen supplied only in the reflow zone provides acceptable throughhole penetration on 0.125"-thick boards. A minimum and maximum pin-to-hole ratio of 0.64 (0.025"/0.039") and 0.71 (0.025"/0.035"), respectively, are recommended. This type of component should be located parallel to the direction of the board for better results. For 0.093"-thick boards, nitrogen in reflow benefits were not observed unless 100% through-hole penetration is required. Axial resistors. The resistors were located at various positions on the test vehicle and oriented in two directions, with two drill sizes (0.035" and 0.039"), one pad size (0.060") and connected to one, two and four layers. Good solder joint penetration was observed on throughhole penetration for 0.125"-thick boards when reflowed in nitrogen. When boards were reflowed in air, data analysis suggested use of a 0.039" drill size, connected to four layers, and ambient air in the wave in order to obtain good throughhole penetration (Figure 13). For 0.093"-thick boards, acceptable through-hole penetration was observed for all joints. The best settings for each flux can be found in Table 5. For this type of component and for both board thicknesses, it is recommended the components be oriented perpendicular to the board direction; and that nitrogen in reflow using the reflow-only supply method and up to 2500 oxygen ppm level be used. DIPs. The DIPs were connected to up to six layers, while the drill and pad sizes were kept constant at 0.035" and 0.060", respectively. It has to be noted that no lead protrusion was circuitsassembly.com Circuits Assembly APRIL 2008 41 http://www.seho.de http://circuitsassembly.com http://www.seho.de

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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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