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32 SMT007 MAGAZINE I AUGUST 2018 selection of IPC classes as representative of the two test limits is arbitrary. The Class 2/3 acceptability limit is a measured current below 250 µA for a mini- mum of 120 seconds. The Class 1 acceptabil- ity limit is a measured current below 500 µA for at least 60 seconds. For both conditions, test cycles that meet the acceptance require- ments are reported as CLEAN and test cycles that do not meet the acceptance requirements are reported as DIRTY. In addition, the localized cleanliness test system reports a measure that is called the Corrosivity Index (CI). The index is calculated by dividing the maximum current seen during the test by the elapsed time of the test, or by dividing the maximum test current limit by the time required to reach the limit. [3] The calculation for determining the CI for any test indicates that a lower CI is prefera- ble. For reference, the Class 2/3 limit of 250 µA at 120 seconds can be converted to a CI of 2.08; this implies that a CI higher than 2.08 indicates a DIRTY test result for the Class 2/3 limit. The CI at the Class 1 limit of 500 µA at 60 seconds is 8.33. Experimental Method for This Study Test Coupon Preparation - Comb Down The comb down orientation is designed to reproduce the wave soldering process as directly as possible prior to testing. The test coupons have 1–2 mL of liquid flux applied to each comb pattern, and the coupons are tilted vertically to allow the excess flux to drain from the board surface. The coupons are then processed comb-down through a wave solder system using a 5 ft/min (152.4 cm/min) conveyor speed to a topside preheat temper- ature of 220 ± 10°F (104.5 ± 5.5°C). The solder wave contained a tin-silver-copper alloy heated to 510°F (265.5°C) and each coupon contacted the wave for 4 seconds. Test Coupon Preparation—Comb Up The comb-up orientation is designed to simu- late irregular conditions that can be encoun- tered in a modern wave soldering process, such as topside flux overspray. In addition, assem- blies that are processed in masking pallets can contain areas that contact flux, are heated during preheat, but do not directly contact the wave during the process. This results in flux residues that have a limited exposure to thermal energy and can have different electrochemical proper - ties in the service environment of an assembly. The comb-up coupons were prepared identi- cally to the comb-down coupons with respect to flux application, conveyor speed, and preheat temperatures. These coupons are processed with the comb pattern facing up and without direct contact to the solder wave. Test Quantities For this study, three coupons soldered in the comb up orientation and three coupons in the comb down orientation were submit- ted to J-STD-004B SIR testing for each flux. For each test coupon there are a total of four comb patterns present but each coupon is consid- ered a single test replicate (i.e., all four combs must meet the test acceptability requirements for a coupon to be considered to have passed the test. One coupon soldered in the comb up orientation and one coupon soldered in the comb down orientation was submitted to local- ized cleanliness testing for each flux. Each of the four comb patterns is considered an indi- vidual test for each coupon. This provides four replicates of the localized cleanliness test for each flux and soldering condition. Fluxes Tested VOC-Free Fluxes Flux 1 is a legacy VOC-free formulation, orig- inally characterized under the requirements of J-STD-004A. The flux is classified as ORL0 and contains 4.0% solids content. Flux 2 is a VOC-free formulation that meets the requirements of J-STD-004B for a no-clean flux. The flux is classified as ORL0 and contains 4.0% solids content. Flux 3 is a VOC-free formulation that meets the requirements of J-STD-004B for a no-clean flux. The flux is classified as ORL0 and contains 4.0% solids content.