Since hot slump is a common thread in the formation of both solder bridges and MCSBs, my first thought was that the more heat-resistant properties of Pb-free solder pastes may be providing some benefit.
If the better characteristics are a result of hot slump properties, then the hybrid paste should behave more like Pb-free with respect to the formation of MCSBs. If it's due to the oxide film on the molten alloy, then the hybrid paste should behave more like the SnPb.
* ImSn produced the least amount of MCSBs, followed by ENIG, OSP and ImAg.
* Soak reflow profiles produce more MCSBs than ramp profiles.
* Stencils of 0.006" produced far more MCSBs than 0.005" stencils.
For Pb-free paste there appeared to be more MCSBs for the intermediate size devices (0603) especially for ImAg and ENIG board finishes.
* The radiused inverted home plate aperture design that used the proportions 20/60/20, which was found to minimize MCSBs, produced no tombstones in this study.
Mid-chip solder balls (MCSB) are also a common defect that can be impacted by stencil design.