|Ask the Experts|
January 22, 2020
Transfer Efficiency Greater Than 100%
While I understand the concept of transfer efficiency and its relationship to solder paste volume, is it possible to produce transfer efficiency greater than 100% of the theoretical volume.
Can the volume we're measuring with our solder paste inspection system to be believed, or is this measurement an anomaly of the inspection measurement process?
|Expert Panel Responses|
It is possible to actually have greater than 100% transfer efficiency, but the most likely scenario to cause this is when the stencil is not sitting flat on the board during the print stroke.
Any gap between the board and the stencil can produce a greater paste height than would be theoretically expected based on the stencil thickness. However, I would say that if the printing process is set up properly (meaning that there is no print gap), it is nearly impossible to transfer more than 100% of the expected volume.
The most typical way to read a transfer efficiency greater than 100% is related to the software algorithms in the SPI. As an example of how this may happen, if the paste exhibits any "dog-earing" behavior, the SPI equipment may read the height of the paste deposit along the edges instead of the center of the deposit. When this happens, the overall volume will be miscalculated and the result may exceed 100% transfer efficiency.
The volume is probably not actually greater than 100% transfer efficiency in this case, but the SPI is giving an artificially high reading that exceeds 100%.
General Manager - Electronic Assembly Americas
Getting a higher height and larger volume of solder paste than the stencil thickness can occur due the compression of the powder/flux mixture.
The hydro-dynamic property of the solder paste will be impacted by the squeegee angle and squeegee pressure.
So in some cases, a higher volume of solder paste will be shown after print. A sharper angle of attack at the squeegee combined with higher squeegee pressure can result in higher volumes.
This is also dependent to some extent on the solder paste rheology and the chemical constituents in the flux system. Print uniformity and the vision system used can also at times show higher volumes, peaks in the print definition or brick may contribute to slightly higher numbers being recorded.
Senior Market Development Engineer
Surface tension effects can cause solder paste to stick to the stencil aperture and/or bottom of the stencil. This can cause a deposit profile somewhat akin to a "Hershey Kiss" with a prominent peak that may slump. If measuring only paste height, the deposit may be higher than the stencil thickness. This is especially the case with small stencil apertures or apertures with rough surface finish.
For these reasons it is prudent to use a system that measures integrated solder paste deposit volume rather than solder paste height alone. Ensure that the system is calibrated. Create a calibration standard that can be characterized by physical measurement and compare it to the solder paste inspection system reading. Also understand measurement error, tolerances and performance characteristics of the paste deposit inspection machine.
Yes, it is possible for transfer efficiency actually be greater than 100%, and yes, it is possible that systematic error in the solder paste measurement system can also create a false indication of >100% transfer efficiency. First, let's look at some reasons why transfer efficiency might actually exceed 100%:
This is usually accomplished by performing structured testing and analysis on the system. Such testing can include measuring the same sample multiple times, including in different orientations.It may also include sample measurement on special substrates to reduce or eliminate the height offset problem.
Yep this greater than 100% is a strange one. When I've experienced this observation it's been caused by excessive solder paste height. Therefore how do we get a higher paste height than the metal stencil?
One theory is the filling pressure created by the squeegee not only fills the aperture but causes some of the material to flow around the back of the blade thus creating a "wake" which is slightly higher than the stencil thickness. I've found that this effect is more prevalent in apertures larger than 250 microns therefore transfer efficiency of larger apertures can be reported as greater than 100%.
Of course there are caveats to this statement - SPI machines can produce random results if the programming is not fully understood or the machine is not capable therefore basic checks need to be carried out to ensure you have confidence in the inspection tool; your supplier would be able to advise on these procedures.
Global Process Manager
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