It should not be necessary
to check solder paste viscosity with the most modern formulations. Most will
now last over several days at high heats and will require little refrigeration,
if any at all. If you are finding to the contrary it could be a good idea to
look around for a new paste supplier.
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.
are some issues that make checking viscosity less useful than it might be:
Manufacturers' control of viscosity is typically much better than it was
years ago (more modern flux formulations, better process control, lower powder
oxide content), so there is less risk of out-of-spec material.
- Differences in the equipment used (manufacturers usually use
more sophisticated equipment than end users have available)
- Differences in the test conditions (very tight control of all
conditions is required)
- Differences in test methods (even with the same equipment, minor
differences in test protocol can create significant differences in results)
- Viscosity changes during shelf life
prefer performance-related tests, e.g. implementing in-line solder paste
inspection (SPI), to monitor the paste printing process. If a problem is
detected with a paste lot, lab techniques can then be used to investigate. SPI
can also be used with a test stencil and substrate to do incoming validation of
paste performance, but the paste sample size need to do so is rather large, and
may not make sense for some users.
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.
If the solder paste
has been correctly shipped and stored then there should be no requirement to
measure viscosity. If you wish to do so as part of incoming inspection then
this would be the time to do so, rather than before use. Make sure that the
storage and handling of your paste meets the manufacturers recommendations and
you should have no problems.
If your paste has expired then it is best to scrap
it as other things can change as well as viscosity. If you do decide to measure
viscosity make sure you have the same equipment and test conditions as the
manufacturer uses otherwise your data will not be comparable and you will have
wasted your time.
Senior Applications Chemist
Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.
No, typically it is not necessary to check viscosity
before each use. We check the viscosity of the paste (and other
parameters) before it leaves our facility to ensure it meets our
specifications. Since our pastes are formulated to operate over a wide
viscosity range, it should not be necessary to check the viscosity provided it
is stored/used per our guidelines.
These guidelines include cold storage,
a sealed container and allowing the paste to reach room temperature before
using it. It is also important to note that viscosity measurements are
highly dependent on the type of equipment used and the sample so if you choose
to measure viscosity, keep that in mind. Of course, if the paste looks
'different' than normal, it is always prudent to ensure the material is still
acceptable before starting a production run.
Product Support Specialist
Brook is Product Support Specialist for PCB Assembly Materials at Indium Corp. With a background in Chemical Engineering and previous experience with conductive adhesive new product development, she provides expertise in choosing the right materials and processes to optimize PCB assembly operations.
Frequent checks of solder paste viscosity is not generally
useful. Most pastes are thixotropic meaning that they paste shear-thin
under the action of the print squeegee pressure and number of cycles.
Checking at the onset of the print run will yield a different viscosity value
than after the paste is worked a while. Other factors that are critical
to paste viscosity are dry-out (evaporation of solvents within the paste
system), absorption of atmospheric moisture, plant temperature and relative
If a paste formulation gets too loose during printing with
constant and appropriate environmental conditions, then I would suggest:
- Reviewing idle time of the paste on the
stencil (paste dry-out being a common issue)
- Reviewing the volume of the paste on the
stencil (apply too much and it will be more prone to dry-out)
- Reviewing re-use of solder paste (paste
scraped off of stencil at the end of a run to be used as again for a future
run... not a great practice)
- Paste storage conditions prior to use
(proper storage temperature and proper warming)
- Proper stirring of paste prior to use
(avoid excessive ultrasonic mixing (known to cause thinning))
- Contacting the paste manufacturer to find
out why the paste is not printing properly
Finding a suitable
replacement paste that maintains a workable viscosity under typical plant
conditions and line volumes
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.
The solder paste users do not need to check the viscosity once it is qualified for their process. It is the responsibility of solder paste maker to make sure the viscosity is within specified range. Solder pastes are designed to have thixotropic property, which means solder paste will be sheer thinned (decrease in viscosity) during the printing process and the viscosity will increase after the printing process.
The thixotropic property of the solder paste will insure a good release from stencil aperture and prevent the printed paste from cold slumping. A freshly opened jar of solder paste should be mixed lightly for several minutes prior to putting on stencil to ensure a good rolling under the squeegee.
Director New Product Development
Metallic Resources, Inc
David Bao has more than fifteen years of experience in developing new solder paste, wave soldering fluxes and other SMT consumables. He currently serves as the Director of New Product Development at Metallic Resources Inc. He received a Ph.D. in Chemistry at Oklahoma State University.