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60 SMT007 MAGAZINE I OCTOBER 2020 Flux spattering causes problems, such as the generation of a contact failure, which occurs when flux adheres to the contact point of con- nectors, as well as illuminance and recogni- tion errors that happen when flux adheres to light-emitting diodes (LEDs), lenses, and sen- sor components. As a means to prevent splat- tered flux from adhering to components, some customers use protective tapes to reduce such flux attachment. To lessen these defects and reduce the requirement for protective tapes in these growing applications, solder pastes that do not cause the occurrence of flux splattering are increasingly needed. This article reports on tests to determine flux splattering generation timing, conditions of the occurrence of splattering which are dependent on reflow profile, and measures to reduce flux splattering through the development of the flux in the solder paste. Experimental Flux Splattering With Conventional Lead-Free SnAgCu Solder Paste Using Reflow Simulator To investigate the splattering of flux, con- ventional Sn3Ag0.5Cu Type 4 no-clean sol- der Paste A was evaluated during reflow. The test vehicle was a copper plate of dimension 30-mm x 30-mm x 0.3-mm thickness. The sten- cil used had a 6.5-mm diameter aperture with a thickness of 0.2-mm thickness. The timing of the occurrence of splattering and the amount of splattered flux were checked using a reflow simulator (Figure 2). As shown in Figure 3, spacers were placed on the copper plate, on which solder paste had been printed, and then a glass plate was placed on the spacers 2 mm above the solder paste so that the timing of flux splattering and the amount of splattered flux could be counted. The splattered flux adhered to the glass plate during reflow in the reflow simulator with the number of flux splatters adhering to the glass counted. The Occurrence of Spattering by Adjustment of Reflow Profile Using a Reflow Oven The relationship between the reflow profile and the amount of flux splattering was stud- ied using SnAgCu no-clean Type 4 conven- tional Paste A. Three different preheat temper- atures and times were used corresponding to Profiles A, B, and C (Figure 4). For Profile A, the preheat temperature was 100–200°C for 95 seconds. For Profile B, the preheat tempera- Figure 2: Reflow simulator used for flux splattering timing evaluations. Figure 3: Evaluation method for flux splattering onto a glass plate.

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