We are having a lot of tomb stoning with this no clean solder paste. It's limited to larger, complex boards, affecting 0603 ferrite beads, capacitors and SOT23s. It was suggested that paste removed from a stencil should not be put back in it's original jar, mixing it with the new paste.
Paste control is a subject that has a lot of unfounded claims surrounding it. What is the difference in having a "working paste jar" and a "new paste jar" Either way new and old are getting mixed at some point. We just don't buy onto the whole thing about deformed solder spheres and oxidized powder, and throwing solder paste away each shift. That is a needless waste of money IMO. We have tried several different methods of paste control. NONE of them...I repeat NONE of them showed any significant changes in yields when looking at no-clean formulations. In addition, let me be clear that I am reffering to main line paste manufactuers.Not the ones you buy on ebay or at radio shack.
Let me go further to say that what I mentioned above may not be the case when printing "micro devices", but 0603's to 20mil should work fine. Of course common sense comes into play here too...if you see chunks or the paste seems thick or dry then chuck it.
Dave is correct. Tombstoning is usually caused by incorrect pad geometries or thermal mis-match rather than paste.
Thank very much for the feedback. I was asked why mixing the paste was not a good idea. Could you provide the answer please? It's always better to hear from an outside expert:)
The SOT23 - some of them have actually flipped upside down ; plus we have had a few cases of a bottom terminated DC-DC Stepdown IC blowing off - which means missing or totally off pad. I suspect we have other issues besides using combined paste , but to date, we are haven't figured it out.
On mixing paste: There could be a lot of hand wringing about the need to minimize solvent evaporation, flux separation, and chemical activity. [True alarmists will throw in oxidation of the alloy.] But I think the real concern is a change in viscosity when mixing used paste with fresh paste. If this is the case, a drop in print quality should be fairly easy to spot.
Flipped SOT23: Probably occurring prior to soldering during the placement process [eg, component drops, blown off by release air set too high, etc]
In testing 63/37 SnPb NC paste I have seen HUGE differences in performance. Have you tried a different paste? I prefer AIM (NC257-2), they have performed well in each round of testing.
My observations regarding mixing old and new paste: The paste has volatile compounds in the mix that eventually evaporate. Mixing old and new will not bring the mix back to new condition regarding these volatile compounds, and printing will suffer. Even new paste has a shelf life. A better question may be to define "old" paste. I have had no problem mixing one-day old paste with new paste, but I do not keep doing this day after day either.