Ask the Experts
March 24, 2023 - Updated
April 9, 2012 - Originally Posted

Syringe Dispensed Solder Paste for Hand Soldering

You recently had a discussion about the application of liquid flux for hand soldering operations. Would you advise using a syringe to dispense solder paste as an alternative to dispensing flux only? If use of solder paste dispensed from a syringe was acceptable, what expiration would you apply to these syringes?


Expert Panel Responses

Solder pastes in syringes have lower viscosities and usually lower metal content 80-85% compared to printable solder pastes which have 90+%, separation is normally the issue. The shelf life is less than liquid fluxes which have a 1 year shelf life or more; solder pastes in syringes tend to be anywhere from 3 months to 6 months and require refrigeration at 10 Centigrade.

The usual issues with syringe dispensing are separation of flux and metal in the syringe and/or needle blockage.

For liquid flux applying flux in excess, is the issue. So both have advantages and disadvantages in use.

Peter Biocca
Senior Market Development Engineer
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.

I would only use a syringe to dispense solder paste in applications where the volume of paste was not critical, such as attachment of a metallic EMI shield. For component leads, never. Solder paste for component mounting needs to be tightly controlled, either by stencil printing or by jetting.

In terms of expiration, that depends a great deal on the paste chemistry. If you get the syringe tubes from the paste manufacturer pre-loaded, then use the given expiration date. If you are loading syringes yourself, work with your vendor to determine how long a working life you have depending on how you fill the syringes.

Doug Pauls
Principal Materials and Process Engineer
Collins Aerospace
Doug Pauls has a bachelors in Chemistry & Physics, Carthage College, BSEE, Univ of Wisc Madison. He has 9 years working experience for US Navy - Materials Lab, Naval Avionics Center Indianapolis. 8 years Technical Director, Contamination Studies Laboratories. 11 years Rockwell Collins Advanced Operations Engineering.

Syringe-dispensed solder paste can be an acceptable substitute for wire + liquid flux; some pastes work better than others for this application. Many solder paste manufacturers sell "dispense grade" pastes that have lower viscosity than pastes intended for stencil printing.

If you are not buying in the syringes but re-packaging from tubes, the shelf-life of properly filled and stored syringes should be the same as for the original packaging. Best practice is to label the re-packaged syringes with the material, lot and date of expiry information from the original container.

The key to success with this process is controlling your dispensing operation and ensuring that the flux is completely reacted. Hot air tools can be a good match for dispensed solder paste; some pastes react badly to heating with an iron. Discuss your intended soldering process with the solder paste manufacturer for guidance on paste selection and process optimization.

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.

Using syringe packed solder paste is a very common application for SMT rework. Some of the key things to control in your facility:
  • Keep the solder paste refrigerated until a couple of hours before you plan to use it.
  • Use ONLY solder paste that has been formulated for dispensing applications.
  • Use ONLY a tapered dispensing needle, this prevents solder paste separation and reduces needle clogging.
  • Set your dispenser at a low pressure, pressures over 35PSI can cause the solder paste to separate.
  • At the end of your day, recycle the needle and re-cap the syringe. This protects the remaining solder paste in the tube.
Recycle the un-used or partially used syringes according to the expiration date printed on the syringe.

Edward Zamborsky
Regional Sales Manager
OK International Inc.
Ed Zamborsky is a Regional Sales & Technical Support Manager for Thermaltronics, located in New York. His position requires frequent customer visits throughout North America and the Caribbean and his position encompasses not only sales but the role of trainer and master applications engineer for all of Thermaltronics products. His expertise includes such specialties as hand soldering, convection and conduction reflow techniques, array rework, fluid dispensing equipment, and fume extraction. Ed has authored many articles and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead-Free Hand Soldering, High Thermal Demand Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

Dispensing solder paste can be a science in itself and in my opinion best to stay away from. When dispensing solder paste the nozzle inside diameter (ID) size becomes a big factor. If the nozzle is too small, the solder will separate in the nozzle, so it is important to understand the relationship between particle sizeof the solder paste and Nozzle dimensions.

Many solder company's sale solder paste specifically designed for dispensing, so if this is something you plan to follow up, you should make sure that your vendor understands that you will be dispensing the material when discussing solder paste selection.

Dispensing grade solder paste will have higher flux content than standard solder paste, this is by design to help lubricate the nozzle or dilute the number of solder particles being forced out the nozzle at any given time,reducing the number of opportunities to clog the nozzle.

Consult your vendor for recommendations on shelf life for dispensing grade solder, typical solder paste shelf life is around 6 months, however left at room temperature, the metal in the solder paste will start to separate, so it is best practice to dispose of unused material after a couple of days usage.

Doug Dixon
Douglass Dixon is the Chief Marketing Officer for 360 BC Group, a marketing agency with offices throughout the US. 360 BC specializes in consulting and implementing successful marketing programs that utilize the latest in marketing, sales and technology strategies. As an electronics veteran, Dixon has worked in the industry for over 30 years for companies like Henkel, Universal Instruments, Camelot Systems, and Raytheon. Dixon's electronics industry experience includes a broad skill set that includes engineering, field service, applications, product management and marketing communications expertise.
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