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?

A.L.

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%.

Brian Smith
General Manager - Electronic Assembly Americas
DEK International

Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.

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.

Peter Biocca
Senior Market Development Engineer
Kester

Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.

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.

Gary Freedman
President
Colab Engineering

A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.

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%:

The stencil is being held slightly of fthe board. This might happen because of solder particles contaminating the bottom of the stencil near apertures, or by PWB topography (soldermask in particular).
The stencil aperture dimensions are not perfectly in accordance with the design file. Either the aperture X and Y dimensions are off, or the foil thickness is not what you are expecting. There is more variation in aperture size in the X-Y plane than is normally assumed. It's entirely possible that the apertures are a little larger than you expect. Remember that 10% growth in linear dimensions corresponds to a21% increase in area, and thus in volume. It's also not unreasonable to assume that the foil thickness may be uniform, but slightly off nominal.
The squeegee is not cleaning the stencil. If the squeegee glides on a single layer of solder paste particles, you are adding nearly two mils of additional "phantom" stencil thickness.
You have squeeze-out under the stencil. Also see (1); if the stencil-to-board gap is too large,you can get paste squeezing out under the stencil, which will create extra volume. Also, if the design is such that the aperture and pad are the same size,any offset will create a gap through which paste can squeeze. This is normally a small volume contribution, but in some situations could become significant.

Now let's take a look at a few reasons why >100% transfer efficiency might be reported when it really wasn't.

Reference plane error. The measurement system has to calculate where the surface of the pad is in the Z(height) direction, even though it can't see the pad. The method that the machine uses to do this may have some inherent offset. Some machines use relatively sophisticated algorithms, and some are more, well, simple. The characteristics of the PWB itself may affect the direction and/or magnitude of this offset. If the machine thinks the pad is lower than it is, the volume reported will be greater than the actual volume.
Shadowing.This error results when a directional bias exists in a sensor system. Some systems do not truly see vertical edges well, and so they may add "phantom"volume as if the deposit were a pyramid instead of a brick. This type of offset is not as common or pronounced in modern systems. It can be assessed by carefully measuring the same sample in different orientations.
Inadequate X-Y sampling frequency. A system that has a low X-Y resolution may look at a deposit that has a large raised (dog-eared) edge as having an erroneously high height. This type of error is much less common with modern systems.

As you can see, there is ample room for variation, both real and artificial, and it is crucial that we understand what is real and what is not. It is therefore extremely important that we understand the measurement system we are using, including the "error band" on any reported values.

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.

Fritz Byle
Process Engineer
Astronautics

Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

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.

Clive Ashmore
Global Process Manager
ASM

Mr. Ashmore is responsible for the Global Applied Process Engineering group for DEK. Clive specializes in all aspects of manufacturing engineering, with special emphasis on mass imaging technologies.

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