Ask the Experts
October 25, 2023 - Updated
November 7, 2011 - Originally Posted

Solder Paste Viscosity

Is it permissible to return a sample of paste to the original solder paste container after the sample has been used to measure viscosity?

L. W.

Expert Panel Responses

Before any expert can truly answer this question, several factors must be divulged. First, why are you measuring viscosity? What instrument and method is being used? What is the complexity of the board? Is it a fine aperture board or a simple board? Lastly, what type of solder paste is being tested--no clean, water-soluble, or rosin?

The viscosity does not solely define the rheology of a paste. Moreover, tests run by users often do not match manufacturer specifications. This does not necessarily mean the paste is unusable. Several factors may influence viscosity readings, including temperature, handling, and sample preparation.

If paste is damaged through handling, the severity will depend on characteristics such as flux chemistry, paste type, and alloy type. The damage first appears as a viscosity change but can show up later through excessive solder balling. This is more common in water-soluble versus no-clean and is rarely an issue in RMAs since most are older chemistries that tend to thicken.

Viscosity testing will likely degrade most solder pastes, some more than others will. The first sign of degradation is a change in viscosity. The viscosity will first decrease in the beginning, however a rapid rise will occur after the paste has been sitting for an extended period. Depending on the type of board in use, the paste may be salvageable. The chances of re-using the paste are considerable if the board does not contain fine pitch components and is 0.5 mm and above.

Proceed with caution. Assemblers should consider the cost of the board, components, and repair if the paste turns out to be corrupt. Generally speaking, the amount of paste you save may cost you more in the long run.

Karl Seelig

Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

It is probably not a good idea. It would be better to perform the viscosity check just prior to printing, and then dispense the sample directly onto the stencil or the paste pump on the printer.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

Yes, It is permissible to return a sample of paste to the original solder paste container after the sample has been used to measure viscosity.

Since you take out the Paste only for the viscosity measurement. The viscosity of the paste can only be change if you use it for screen printing or you kept this paste in the uncontrolled environment. MSDS of the solder paste will help you to get the Solder Paste handling Instruction.

Sandip Thakor
Quality Engineer
Matrix Telecom Solution P Ltd
Sandip Thakor has 9 years of experience in electronics industry specializing in soldering technology. Thakor has experience in lead free installation, process optimization and developing quality standards.

You should be able to return the sample as long as there is no contamination when running viscosity.

Today's pastes typically have very long open times and have solvents that aren't as hydroscopic and are less volatile.

I wouldn't suggest leaving open jars that are water soluble for a long period of time especially if the environment is humid but again I think in this case you should be fine.

Mike Scimeca
FCT Assembly
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.

Reader Comment
Solder paste viscosity measurement is ideal way to know what is viscosity for each sample solder paste whether water soluble, no clean or RMA type of paste after open the fresh paste jar.

It is even important to measure sample solder paste on screen printer to know how much different paste viscosity. All of solder paste viscosity need to measures dynamic viscosity to calculating thixotropic index and temperature.
Tom Fujikawa, Malcomtech International, USA

Reader Comment
Yes, that is fine as long as the sample has not been sitting out for more than 2 hours and as long as it has not been contaminated.
Bob Doetzer

Reader Comment
If you do not mix impurities into the solder paste to measure viscosity it is unproblematic returning the solder paste back to the container.
Florian Schildein

Reader Comment
Solder paste samples may be returned to the original jar, but it's not advisable. Viscosity test samples are usually small enough that their disposal will not have a significant cost impact.

While a solder paste sample is out of the jar, it is exposed to air - drying and humidity absorption, etc. can occur, depending on how long the material has been exposed to the environment, as well as possible other types of contamination.

It is unwise to return the sample to the jar as it will contaminate the rest of the material. These test samples of paste can be reprocessed with waste dross, old solder, etc., so such a small amount should not be an issue.
Mike Martel

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