Issue link: https://iconnect007.uberflip.com/i/339176
July 2014 • SMT Magazine 85 MEETING FuTuRE STENCIl PRINTING CHAllENGES continues ARTiCLE figure 12: Harsh profile. ture, resulting in a smaller paste deposit. Figure 10 illustrates the relationship of increased sur- face area to available flux as the paste deposit decreases. Note that the ratio of flux available to the surface area decreases as aperture width decreases. This situation will result in less avail- able flux to protect against surface oxidation when aperture widths are small. Profile Types Two reflow profiles were selected. One is for optimal conditions the other for harsh environ- ments. These profiles were chosen to compare powder types and flux chemistry. An optimal reflow profile is shown in Figure 11. This profile is a ramp to peak with a peak temperature of 240–245˚C, a time above liq- uidus of 50–70 seconds, and an average slope from 25–217˚C of 1–1.5˚C/s. This profile works well for these smaller paste deposits and has a gradual ramp, which is also best for the compo- nents and the PCB. A more demanding profile (Figure 12) was also used to challenge the different powder types and flux chemistries. It had a peak tem- perature of 250–255˚C, time above liquidus near two minutes (120 seconds), and a soak from 150–200˚C between 160–180 seconds. Results Particle size Figures 13 and 14 compare both ends of the particle size spectrum, which were tested to ob- serve any trends when progressing from type 3 to type 6 solder pastes. In addition, the effect of the different flux chemistries on solder joint appearance could be determined. The samples shown were reflowed with the ramp to peak profile. The solder joint appearance was slightly affected. Type 3 solder paste was not as bright and shiny and was a bit grainy compared to the finer type 6 solder paste, but overall the appear- ance was fairly good, especially with the no- clean formulation.