Issue link: https://iconnect007.uberflip.com/i/1493797
MARCH 2023 I SMT007 MAGAZINE 71 "Good start, Einstein!" Sue said, laughing. "It helps to have a solder alloy that has a 'pasty range,' meaning that it doesn't completely melt at one temperature. Since we use SAC305, which has a reasonable pasty range, we can't improve too much in that regard." She continued, "Beyond the pasty range, we can try ensuring that the solder paste deposits are the same amount on each side of the passive and that the components are placed squarely with no skewing. I'm going to ask Miguel to check those things out first." "So, if addressing those t h i n g s d o e s n ' t s o l v e tombstoning, we'll have to reduce the surface ten- sion forces some other way?" Andy asked. "Yes, we could mod- ify the reflow profile so that the melting is slower, but that might negatively affect graping," Sue said. "However, we should look into it." Aer thinking a while, she continued, "Modifying the stencil would help a lot, but that would be an added e x p ens e and we should only do that if all else fails." She then showed him an image of a proposed sten- cil. (Figure 3) " It's quite eas y to see how the stencil would work ," Andy said, excite - m e n t b u i l d i n g i n h i s voice. " There is almo st no paste on the edges of the passive, so there is almost no paste to cause the tombstoning effect." ey chatted for a few more minute s ab out Sue's efforts, then she questioned, "How is the training going on your end?" "We're just about finished," he replied with confidence. "I've covered all the material in the Handbook of Electronic Assembly 2 . I think they Figure 2: The mechanism of tombstoning of a passive component. Figure 3: A stencil designed to reduce tombstoning of passive components.