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October 12, 2018

Frequency of Checking Solder Paste Viscosity

Is it necessary to check viscosity of solderpaste before each use? How often should we check and verify solder paste viscosity?


Expert Panel Responses

It should not be necessaryto check solder paste viscosity with the most modern formulations. Most willnow 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 tolook around for a new paste supplier.

Greg York
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.

Thereare some issues that make checking viscosity less useful than it might be:
  • Differences in the equipment used (manufacturers usually usemore sophisticated equipment than end users have available)
  • Differences in the test conditions (very tight control of allconditions is required)
  • Differences in test methods (even with the same equipment, minordifferences in test protocol can create significant differences in results)
  • Viscosity changes during shelf life
Manufacturers' control of viscosity is typically much better than it wasyears ago (more modern flux formulations, better process control, lower powderoxide content), so there is less risk of out-of-spec material. I stronglyprefer performance-related tests, e.g. implementing in-line solder pasteinspection (SPI), to monitor the paste printing process. If a problem isdetected with a paste lot, lab techniques can then be used to investigate. SPIcan also be used with a test stencil and substrate to do incoming validation ofpaste performance, but the paste sample size need to do so is rather large, andmay not make sense for some users.

Fritz Byle
Process Engineer
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 pastehas been correctly shipped and stored then there should be no requirement tomeasure viscosity. If you wish to do so as part of incoming inspection thenthis would be the time to do so, rather than before use. Make sure that thestorage and handling of your paste meets the manufacturers recommendations andyou should have no problems. If your paste has expired then it is best to scrapit as other things can change as well as viscosity. If you do decide to measureviscosity make sure you have the same equipment and test conditions as themanufacturer uses otherwise your data will not be comparable and you will havewasted your time.

Neil Poole
Senior Applications Chemist
Henkel Electronics
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 viscositybefore each use. We check the viscosity of the paste (and otherparameters) before it leaves our facility to ensure it meets ourspecifications. Since our pastes are formulated to operate over a wideviscosity range, it should not be necessary to check the viscosity provided itis stored/used per our guidelines. These guidelines include cold storage,a sealed container and allowing the paste to reach room temperature beforeusing it. It is also important to note that viscosity measurements arehighly dependent on the type of equipment used and the sample so if you chooseto measure viscosity, keep that in mind. Of course, if the paste looks'different' than normal, it is always prudent to ensure the material is stillacceptable before starting a production run.

Brook Sandy
Product Support Specialist
Indium Corporation
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 generallyuseful. Most pastes are thixotropic meaning that they paste shear-thinunder the action of the print squeegee pressure and number of cycles. Checking at the onset of the print run will yield a different viscosity valuethan after the paste is worked a while. Other factors that are criticalto paste viscosity are dry-out (evaporation of solvents within the pastesystem), absorption of atmospheric moisture, plant temperature and relativehumidity. If a paste formulation gets too loose during printing withconstant and appropriate environmental conditions, then I would suggest:
  • Reviewing idle time of the paste on thestencil (paste dry-out being a common issue)
  • Reviewing the volume of the paste on thestencil (apply too much and it will be more prone to dry-out)
  • Reviewing re-use of solder paste (pastescraped off of stencil at the end of a run to be used as again for a futurerun... 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 findout why the paste is not printing properly Finding a suitablereplacement paste that maintains a workable viscosity under typical plantconditions and line volumes

Gary Freedman
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.

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.

David Bao
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.