Shaken or Stirred?

Does the paste mixing methodology affect print performance?

Solder paste formulation technology has come a long way. What once had to be treated with precise handling – limited exposure time on the stencil, rotations, refrigeration protocols, etc. – even as early as 10 years ago has changed dramatically. Solder paste chemistries are so robust today, in fact, some don’t even require refrigeration.

Given this advancement, a recent conversation about some analysis our team performed was a bit surprising. During our printing evaluations, the solder paste material wasn’t processing as it should, so one of the material supplier’s application engineers came in to assist. We explained the problem and he asked, “Are you manually stirring the material or putting it in a machine to rotate?” When we responded that we were doing the latter, he said, “Don’t.”

Given how far paste material stability has progressed, this response had our team wondering if solder paste mixing was even necessary. Does it introduce variations into the stencil printing inputs that are best left alone? And, if mixing is required, is hand stirring superior to machine blending? Although we thought we knew the answers, we decided to test our hypotheses. Using an industrial Type 4 standard SAC paste most often sourced for our lab work (not from the supplier mentioned above), printing tests were run on an unstirred pot of solder paste from three locations: top, center, and bottom of the jar. As expected, the three samples revealed significant print differences. The material from the top had the worst printing results with tremendous variations in the KPIs, which was due to separation (metal particles from flux). With metal content most concentrated at the bottom of the jar, the particle-flux imbalance there also presented issues. The stiffer nature of the material made transfer efficiency inconsistent. One would also assume that reflow soldering and joint intermetallics, which we didn’t test, would also be negatively impacted.

With confirmation that material mixing is (as expected) required for consistent print results, the second part of our analysis set out to compare hand stirring to machine blending. After manually mixing the material for thirty seconds, the same stencil printing analysis was conducted using solder paste from the top, center and bottom of the container. Each sample had consistent printing results, indicating that stirring by hand is an acceptable method.

Next, jars of solder paste were placed into the mixing machine and blended for five, 10, 15 and 30 minutes. Temperature measurements of the solder paste were taken after each five-minute interval, and the pastes mixed for five and 10 minutes were printed for evaluation. Again, paste was taken from each container’s top, center and bottom. Both samples printed without issue and within normal standard deviations. It should be noted, however, that our analysis revealed that every five minutes of solder paste rotation by machine increased the material temperature by 1°C. Certainly, at five minutes, the slight increase in temperature – which also changes the viscosity – is negligible. That’s within a room temperature change that you might see in a factory, so not significant. Leaving the material in the mixer for longer, however, may have an impact on the material’s printability. Considering that solder paste material specifications generally recommend 21°C as the temperature at which to print for optimal results, it seems a bit of a gamble to push the temperature up by another three or four degrees Celsius by over-mixing.

Therefore, if using an industrial mixing approach, five minutes would be our suggested maximum blending time. However – and this is a BIG however – we do know that solder pastes come in a variety of chemical formulations with many composition variations. This, combined with the recommendation from one supplier not to machine mix their paste, might indicate that hand stirring anywhere from 30 to 60 sec. is the safest bet. Hand mixing works, it doesn’t take long, and there is no risk of overmixing and altering the temperature to a non-ideal, amalgamated state.

Naturally, if your organization has had no issues with its solder paste mixing routine and results are high yield, carry on. But, when in doubt, hand stirring is the best default.

Clive Ashmore is global applied process engineering manager at ASMPT (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.