Solder Paste Stencil Inactivity

How long can we leave solder paste on a stencil without any activity, wiping, etc.?

S.K.

Expert Panel Responses

This is highly dependent on both the paste in use and the environmental conditions in the factory (specifically the conditions in the printer itself). Additionally, your results may be affected by how difficult your printing challenges are (are you trying to print very small apertures, for instance?). For easier printing jobs, you may find that much longer idle times are workable, however for very challenging jobs with narrow process windows, much shorter idle times may still be problematic.

You should consult with the paste manufacturer to see what their recommendation is given the specifics of your paste, process and environment.

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.

Solder paste can be left undisturbed on the stencil for 1-4 hours and remain printable. The actual length of time depends upon the solder paste and environmental conditions. It is best practice to knead or mix the solder paste before printing, after pauses of that length. It is best to contact your solder paste supplier for specific recommendations.

Tony Lentz
Field Applications
FCT Assembly

Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

The time solder paste can be left on a stencil without printing or wetting it depends on many different factors like solder paste formulation (flux system e.g. water-soluble, resin, epoxy, halogen/halide content…), solder powder size, temperature and humidity conditions. Manufacturers typically indicate abandon time frame within the product datasheet (typically 1 to 4 hours).

Manuel Schöllig
Product Manager Soldering
Inventec Dehon

Product manager for soldering products, including solder pastes, liquid and tacky fluxes.

The safe duration of leaving solder paste on a stencil without wiping or cleaning can vary depending on the type of paste, the storage temperature, and the level of exposure to air and moisture.

Typically, it is safe to leave solder paste on a stencil for a few hours without wiping or cleaning. However, as the duration increases, the risk of clogging also increases. In general, it is recommended to wipe or clean the stencil after each use to minimize the risk of clogging and ensure optimal paste transfer performance.

In some cases, the manufacturer of the solder paste may provide recommendations on the safe duration for leaving the paste on the stencil without wiping or cleaning. In such cases, it is advisable to follow the manufacturer's recommendations to ensure optimal performance and minimize the risk of clogging.

Dr. Eyal Weiss
CTO
Cybord

Electronic and Mechanical Engineer with 25 years experience in technology development. CTO of Cybord, developing electronic component authentication technology using deep-tech visual inspection and big-data. Author of a book and 24 papers.

The amount of time solder paste can remain on a stencil without activity is going to depend on the formulation of the solder paste. It would be best to check with the solder paste manufacturer. If you are using an automated screen printer, some of these machines have a knead feature allowing you to get the paste back into a workable viscosity before printing.

Brien Bush
Manufacturing Applications Specialist
Cirtronics Corp.

Mr. Bush has 20 years experience in electronics contract manufacturing. Major areas of expertise include through hole, SMT, wave and selective soldering.

I never let the solder stay idle for more than 4 hours. Some paste solders can remain on the stencil longer than others. For example, no-clean solder can stay on the stencil longer than water-soluble because of the flux properties. Always follow manufacturer’s recommendation.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman

Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

4 hours is a timeframe heavily brought up in this topic. But it will not be the rule as different parameters can influence the paste quality - the flux composition, temperature and humidity conditions, the quantity of paste on the stencil. If you know that the process will be idle for a while, the right thing to do is to not leave the paste on.

Georgian Simion
Engineering and Operations Management
Independent Consultant

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

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