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90 SMT007 MAGAZINE I SEPTEMBER 2020 excellent wetting properties on the as-coated OSP. All these combinations have a solder score above 5.93, a requirement for the sample to be called as AA-class per IPC J-STD 003C. However, when tested on 2x reflowed OSP coupons, solder scores for only a few combi- nations remained above 5.93, indicating that only these combinations were able to remove the pre-reflowed OSP coating effectively. Solder score data obtained from such simple wetting experiments helps flux formulators to shortlist ingredients to develop new products with best- in-class wetting on pre-reflowed OSP. Another interesting observation from this experiment is that whenever a secondary sol- vent is used, solder scores of those combina- tions have declined. Combinations 7 and 8 contain the same activator as combination 1 with the addition of two different secondary solvents, and combination 11 has the same activator as 10 with a secondary solvent. We have seen in the earlier section that these sol- vents help to remove thermally treated OSP. The author assumes this anomaly of results between two sets of experiments is due to the partial drying of the coupons due absence of a preheat system in wet- ting balance. Whenever a partially dry coupon is dipped in the sol- der bath, volatiles present in cou- pons dilute the activator system and also cause spattering, disturbing and delaying the wetting process. Wet- ting balance is an excellent tool to screen the efficiency of activators, but it may not be the most reliable method to compare flux perfor- mance, especially for fluxes contain- ing secondary solvents to preserve their activity at higher temperatures. They require more complex evalu- ation processes like wave or selec- tive soldering to compare the activ- ity and other properties. To verify this, we selected few commercially available liquid fluxes containing some of these activator- solvent combinations. Details of the solvents and activators present in the fluxes are described in Table 2. The wave soldering performance of these fluxes was benchmarked on a 2.4-mm 6-layer PCB having ENTEK Plus HT finish. PCBs were subjected to 2x reflow before the wave process. Wave soldering was carried out on an ERSA Power- Wave machine using an SAC305 bath at 265°C. Flux loading for all the fluxes was controlled in such a way that activator amounts on the board remain constant. Wetting performance of the fluxes was evaluated by comparing the x-ray images of the PCI connector barrel filling (Figure 9). Table 2: Details of activators and solvents present in commercial fluxes taken for wave soldering performance. Figure 9: X-ray images of PCI connectors showing the barrel filling for different fluxes.

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