Issue link: https://iconnect007.uberflip.com/i/1293772
62 SMT007 MAGAZINE I OCTOBER 2020 after the solder is molten using the reflow sim- ulator (Figure 6). Although components of flux—such as sol- vents—volatilize during preheating, solder is not yet melted, and space exists between the solder powder. This allows generated gas to be discharged easily; therefore, splattering rarely occurs. In contrast, the number of splatters was the greatest immediately after the melting of the solder. When solder paste melts, solder powder merges together, at which time more flux is incorporated. This flux becomes volatile and is pushed out of the solder. This behav- ior causes the occurrence of a number of flux splatters (Figure 6). The Occurrence of Splattering Based on Different Reflow Profiles in the Reflow Oven The occurrence of splattering on the copper plate was determined using reflow profiles with different preheat conditions in the reflow oven (Figure 7). It was confirmed that by using a higher preheat temperature and time and nitrogen atmosphere, flux splattering could be reduced. To suppress the occurrence of splat- tering, it is necessary to increase the volatilization of the volatile sub- stances in solvents during preheat- ing to reduce the remaining volatile substances during proper heating. However, increasing preheating temperature causes the degrada- tion of active flux components dur- ing preheating and the increased reoxidation of solder powders and the component terminations and board surface finishes, which can result in the generation of defects during soldering. Hence, it is nec- essary to have a balance between increasing preheating temperature and the result of increasing solder- ing defects. The splattering of solder paste can be suppressed to some extent by changing the reflow profile. However, changing the reflow profile is an issue for customers who may have many assembly lines. In addition to what was previously men- tioned, increasing the preheating temperature leads to degradation of the component termi- nation, board surface finish, and solder paste as well as potential warpage of substrates and packages. The Occurrence of Flux Splattering Based on the Variation of Stencil Thickness in the Reflow Oven The amount of flux splattering by differ- ent stencil thickness was tested using Profile B in the air atmosphere using conventional Sn3Ag0.5Cu no-clean Type 4 Paste A. It was Figure 6: Timing of occurrence of splattering mainly occurring after the solder is molten. Figure 7: The number of flux splatters based on reflow profile and atmosphere.