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October 25, 2018

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 toactually have greater than 100% transfer efficiency, but the most likelyscenario to cause this is when the stencil is not sitting flat on the boardduring the print stroke. Any gap between the board and the stencil canproduce a greater paste height than would be theoretically expected based onthe stencil thickness. However, I would say that if the printing processis set up properly (meaning that there is no print gap), it is nearlyimpossible to transfer more than 100% of the expected volume. The most typical wayto read a transfer efficiency greater than 100% is related to the softwarealgorithms in the SPI. As an example of how this may happen, if the pasteexhibits any "dog-earing" behavior, the SPI equipment may read the heightof the paste deposit along the edges instead of the center of the deposit. When this happens, the overall volume will be miscalculated and theresult may exceed 100% transfer efficiency. The volume is probably notactually greater than 100% transfer efficiency in this case, but the SPI isgiving an artificially high reading that exceeds 100%.

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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 thanthe stencil thickness can occur due the compression of the powder/flux mixture. The hydro-dynamic property of the solder paste will be impactedby the squeegee angle and squeegee pressure. So in some cases, a higher volume of solder paste will be shownafter print. A sharper angle of attack at the squeegee combined with highersqueegee pressure can result in higher volumes. This is alsodependent to some extent on the solder paste rheology and the chemicalconstituents in the flux system. Print uniformity and the vision system usedcan also at times show higher volumes, peaks in the print definition or brickmay contribute to slightly higher numbers being recorded.

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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.

Surfacetension effects can cause solder paste to stick to the stencil aperture and/orbottom of the stencil. This can cause a deposit profile somewhat akin toa "Hershey Kiss" with a prominent peak that may slump. If measuring onlypaste height, the deposit may be higher than the stencil thickness. Thisis especially the case with small stencil apertures or apertures with roughsurface finish. For these reasons it is prudent to use a system thatmeasures integrated solder paste deposit volume rather than solder paste heightalone. Ensure that the system is calibrated. Create a calibrationstandard that can be characterized by physical measurement and compare it tothe solder paste inspection system reading. Also understand measurementerror, tolerances and performance characteristics of the paste depositinspection machine.

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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 greaterthan 100%, and yes, it is possible that systematic error in the solder pastemeasurement system can also create a false indication of >100% transferefficiency. First, let's look at some reasons why transfer efficiency might actuallyexceed 100%:
  1. The stencil is being held slightly offthe board. This might happen because of solder particles contaminating thebottom of the stencil near apertures, or by PWB topography (soldermask inparticular).
  2. The stencil aperture dimensions are notperfectly in accordance with the design file. Either the apertureX and Y dimensions are off, or the foil thickness is not what you areexpecting. There is more variation in aperture size in the X-Y plane than isnormally assumed. It's entirely possible that the apertures are a little largerthan 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 assumethat the foil thickness may be uniform, but slightly off nominal.
  3. The squeegee is not cleaning the stencil. Ifthe squeegee glides on a single layer of solder paste particles, you are addingnearly two mils of additional "phantom" stencil thickness.
  4. You have squeeze-outunder 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 extravolume. 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 normallya small volume contribution, but in some situations could become significant.
Now let's take a look at a few reasons why >100% transferefficiency might be reported when it really wasn't.
  1. Reference plane error. Themeasurement 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 themachine uses to do this may have some inherent offset. Some machines userelatively sophisticated algorithms, and some are more, well, simple. Thecharacteristics of the PWB itself may affect the direction and/or magnitude ofthis offset. If the machine thinks the pad is lower than it is, the volumereported will be greater than the actual volume.
  2. Shadowing.This error results when a directional bias exists in a sensor system. Somesystems 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 offsetis not as common or pronounced in modern systems. It can be assessed bycarefully measuring the same sample in different orientations.
  3. Inadequate X-Y sampling frequency. Asystem that has a low X-Y resolution may look at a deposit that has a largeraised (dog-eared) edge as having an erroneously high height. This type oferror is much less common with modern systems.
As you can see, thereis ample room for variation, both real and artificial, and it is crucial thatwe understand what is real and what is not. It is therefore extremely importantthat we understand the measurement system we are using, including the "errorband" on any reported values. This is usually accomplished by performingstructured testing and analysis on the system. Such testing can includemeasuring the same sample multiple times, including in different orientations.It may also include sample measurement on special substrates to reduce oreliminate the height offset problem.

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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.

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Clive Ashmore
Global Process Manager
Dek Printing Machine
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.