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Solder Paste Life on the Stencil
Our solder paste has a specified life on the stencil of 10 hours. After 10 hours of manufacturing, do we need to remove all the paste and scrap it?
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Solder Paste Life on the Stencil
Our solder paste has a specified life for exposure on the stencil at 10 hours. We run our stencil operation continuously for 40 hours, adding 250g of new paste every 2 hours. The addition paste thus becomes mixed with the existing paste. After the first 10 hours of manufacturing, do we need to remove all the paste on the stencil and scrap it? Currently the practice is we keep adding new paste until the end of the 40 hours of continuous production.
S.S.
Expert's Panel Responses
It is completely dependent on paste type, we have several types where you change half of it after four hours and it lasts for 7 days, so suggest you trial first and see for yourselves as it may just be fine.
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.
Yes, you need to remove the solder paste on a regular schedule, possibly every 4 hours, and clean the stencil either manually or with a machine. If you don't clean your stencil your apertures will close and the proper quantity of solder paste will not be put down. Your solder paste supplier will provide you with this information. Also, you need to watch your room temperature and humidity ...
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Rick Perkins
President
Chem Logic
Rick Perkins is a chemical engineer with more than 33 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.
You probably don't need to change it. I'll make the assumption that your 250g additions represent about half the volume on the stencil. You will have made a total of five of these additions at the 10-hour point. After the first addition, you'll have half of the original paste left. After the second addition, you'll have one quarter of the original paste left, and so forth. So after five additions, you'll have only 1/32, or about 3% of the original paste left. If your addition is actually only a third of the total paste volume, you'll have 13% left, and if it's two thirds of the volume, you'll have less than half a percent left, so as you can see the replacement ratio makes a big difference. The more paste volume that is on the stencil, the harder it will be to replace it.

The environment also plays a role. If the temperature rise inside the printer is significant, the paste may age prematurely. If the environment is well-controlled, you may actually have longer than 10 hours. As they say, the proof of the pudding is in the eating. Only your print quality can tell you whether the paste has aged too much. You normally be better off with periodic additions than with wholesale replacement, since you will have less dramatic changes in paste behavior when replacing only part of the paste.

My recommendations are:
  • Ensure you have less than 5% of the original paste left after the end of the rated 10-hour period by controlling the amount on the stencil
  • Validate that your prints continue to be of good quality after the 10-hour period by careful visual examination and/or by comparing measurements
  • Validate that your defect rates through reflow don't increase as you run by monitoring DPMO Recognize that the results you obtain are specific to the paste, equi0pment, and environmental conditions under which the tests are run.
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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.
Simplicity is always the way to go in my opinion.
  1. Are you seeing issues related to leaving the paste on the stencil for the time period you indicate
  2. Adding past to the stencil / process is common practice during the working cycle as materials are being consumed with every print stroke
  3. If you are seeing no solderability issues I see no concerns about running your material for the entire 40 hour cycle time.
I think that a process of reducing the amount of paste to the cycle should be implemented towards the end of the "40" hour cycle so that when you reach that time frame you have a minimal amount of materials to disposes of. Disclaimer - I have no vested interest in the sale of solder paste.
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Jerry Karp
President
JSK Associates
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
We opt for even a shorter time than recommended. 

The issue with solder paste is that the flux, which acts as a binder in addition to the soldering benefits, evaporates out of the paste over time due to the granular structure. This leads to solderability problems and even clogging of stencil apertures resulting in mis-prints. Simply adding additional paste will not refresh the existing paste where the flux has evaporated, and in fact can result in diluting the active flux across dry paste reducing its effectiveness overall. 

We run timers for paste on stencils and when it goes off, the stencil is cleaned off and fresh paste is applied.
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Matt Stevenson
Director of Marketing
Sunstone Circuits
Matt Stevenson has over 20 years experience in the PCB industry. Serving in roles as a Chemical Lab Technician, Process Engineer, Quality Engineer, Quality Manager, and Marketing Manager. He has proven himself to be an invaluable resource.
Since the original solder paste has been mostly replaced with the fresh paste, you do not have to change the paste after 10 hrs. unless the printability or the reflow properties of the paste start to show abnormal behaviors.
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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.
Reader Comment
My suggestion is to perform a test: - Verify how many time you have to add new solder paste - Every time you have to add new solder paste, first remove the old one, mix and put it again - Check every hour the evolution of SPI data and AOI data This will provide you information about your process, and will help you to know when you have to scrap the solder paste over the stencil.
Domingo Jose Lebron Berdugo, Magneti Marelli - Automotive Lighting
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