Ask the Experts
June 26, 2018
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.
Expert Panel Responses
It is completelydependent on paste type, we have several types where you change half of itafter four hours and it lasts for 7 days, so suggest you trial first and seefor 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 thesolder paste on a regular schedule, possibly every 4 hours, and clean thestencil either manually or with a machine. If you don't clean yourstencil your apertures will close and the proper quantity of solder paste willnot be put down. Your solder paste supplier will provide you withthis information. Also, you need to watch your room temperature andhumidity ...
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 assumptionthat your 250g additions represent about half the volume on the stencil. Youwill have made a total of five of these additions at the 10-hour point. Afterthe first addition, you'll have half of the original paste left. After thesecond addition, you'll have one quarter of the original paste left, and soforth. So after five additions, you'll have only 1/32, or about 3% of the originalpaste left. If your addition is actually only a third of the total pastevolume, you'll have 13% left, and if it's two thirds of the volume, you'll haveless than half a percent left, so as you can see the replacement ratio makes abig difference. The more paste volume that is on the stencil, the harder itwill be to replace it.
The environment also plays a role. If the temperature riseinside the printer is significant, the paste may age prematurely. If theenvironment is well-controlled, you may actually have longer than 10 hours.As they say, the proof of the pudding is in the eating. Onlyyour print quality can tell you whether the paste has aged too much. Younormally be better off with periodic additions than with wholesale replacement,since you will have less dramatic changes in paste behavior when replacing onlypart of the paste.
My recommendations are:
- Ensure you have less than 5% of the original paste left afterthe end of the rated 10-hour period by controlling the amount on the stencil
- Validate that your prints continue to be of good quality afterthe 10-hour period by careful visual examination and/or by comparingmeasurements
- Validate that your defect rates through reflow don't increase asyou run by monitoring DPMORecognize that the results you obtain are specific to the paste,equi0pment, and environmental conditions under which the tests are run.
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 theway to go in my opinion.
I think that a processof reducing the amount of paste to the cycle should be implemented towards theend of the "40" hour cycle so that when you reach that time frame youhave a minimal amount of materials to disposes of.Disclaimer - I have no vested interest in thesale of solder paste.
- Are you seeing issuesrelated to leaving the paste on the stencil for the time period you indicate
- Adding past to thestencil / process is common practice during the working cycle as materials arebeing consumed with every print stroke
- If you are seeing nosolderability issues I see no concerns about running your material for theentire 40 hour cycle time.
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 timethan recommended.
The issue with solder paste isthat 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. Thisleads to solderability problems and even clogging of stencil apertures resultingin mis-prints. Simply adding additional paste will not refresh theexisting paste where the flux has evaporated, and in fact can result indiluting the active flux across dry paste reducing its effectivenessoverall.
We run timers for paste on stencilsand when it goes off, the stencil is cleaned off and fresh paste is applied.
Director of Marketing
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.
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.
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 dataThis 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