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March 16, 2017

Frequency of Checking Solder Paste Viscosity

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

K.G.

Experts Comments

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.
Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has twenty two years of service in Electronics industry. York has installed over 350 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.
There are some issues that make checking viscosity less useful than it might be:
  • 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 
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.

I strongly 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.
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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.
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.
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 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.
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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 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 humidity.

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
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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.
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