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.
Experts Comments
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 off
the 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 a
21% 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 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.
