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September 13, 2024 - Updated January 8, 2013 - Originally Posted Solder Paste Life on the StencilOur 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. |
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Expert 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. Technical Sales Manager BLT Circuit Services Ltd Greg York has over thirty two years of service in Electronics industry. York has installed over 600 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. 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:
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.
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. Quality Manager Sunstone Circuits Kevin is part of the Sunstone Management team. In his role as Quality Assurance Manager, Beattieās 25+ year background in Printed Circuit Board manufacturing is a tremendous asset to the Sunstone team. In addition, he brings valuable experience from his previous roles in the following areas: process engineering, new process introductions, support of nearly every manufacturing process, and extensive knowledge of Continuous Improvement, ISO, IPC, and various other industry requirements.
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. 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:
Domingo Jose Lebron Berdugo, Magneti Marelli - Automotive Lighting
- 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. |
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