Ask the Experts
November 22, 2019
Extending Shelf Life for Jars of Solder Paste
We have a few unopened jars of solder paste that have just recently gone past the posted expiration date.
The jars have been kept in refrigeration from the time of receipt at 5 +/- 3C. This is a no-clean paste with combination Sn62Pb36Ag2.
We would like have extension of shelf life. How should we proceed?
Expert Panel Responses
You may want to consider storing the Paste at a lower temp. -40°c is optimum.
Round Rock Consulting
Following a successful 20 year career within world class OEMs and EMS providers including Honeywell, Bull Electronics, IBM, Celestica and Plexus, James founded Round Rock Consulting a business consultancy focused on supporting OEMs and EMS providers with product realization strategies.
The first course of action should be to talk with a technical resource at the manufacturer. It's highly likely that the paste is just fine, but a lot depends on the specific flux chemistry. The conservative approach would be to have the paste re-certified by the manufacturer and given a new expiration date.
Another approach would be to do an in-house test to verify printing and soldering performance, and extend based on discussion with the manufacturer. My personal preference would be the latter approach, since it avoids the added risk of shipping it two ways.
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.
You will have to get in contact with the supplier/manufacturer of the solder paste and approach them with a request for expiration date extension. It is not unusual for them to address these types of requests.
Depending on your specific case data (storage temperature, initia lexpiration date), the supplier can guarantee an extension.
Engineering and Operations Management
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at firstname.lastname@example.org.
I would check with the manufacturer of the solder paste and find out just how conservative they might be with the stated expiration date.
However, be ye warned! I have had experiences with solder paste (no-clean as a matter of fact) that was just 3 weeks past the expiration date and it turned to crap. Solder Ball City!
You might do better to send it out for reclamation than taking a chance on it. And consider either buying it in smaller lots and/or scheduled release of the lots based on anticipated usage. Solder paste, like the mind, is a terrible thing to waste.
Mr. Zarrow has been involved with PCB assembly for more than thirty years. He is recognized for his expertise in troubleshooting SMT manufacturing and lead-free implementation. He has extensive hands-on experience with set-up and troubleshooting through-hole and SMT processes throughout the world.
Extending shelf life on solder paste may be a tricky thing to do. I would recommend the following test be conducted, solderability, solder balling, wetting characteristics, flux activity levels, slump, flux weight percentage,viscosity, percent metal, sphere size, plus others.
I would also highly recommend getting in touch with the manufacturer for their inputs as to how to proceed, as perhaps they have a process to refresh the solder paste itself.
Please keep in mind surface mount processes make solder joints quickly and in large volumes, so using bad, defective or expired solder paste to save buying new material could be very expensive as the expired solder paste is not a qualified material after its expiration date.
Vice President, Technical Director
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
Firstly ask the supplier their advice, alternatively carry out a simple solder balling test Cut 3 circles out of a business card using a hole punch (same as putting paper in lever arch folder) carefully hand print 3 circles of paste on a pwb resist area, make sure they are not touching pads or tracks (break off strips are great,if you have ceramic slides these are better) make sure the circles look smooth and not smudged.
Put them through your standard reflow process, the circles should form 1 sphere, look through a low powered microscope to observe any satellite balls,if there are less than 10 satellites the activity is ok.
If the material is also printable then it should be ok for a few weeks more.
How ever check with your end customer as some will not allow you to use product out of shelf life and you may need to seek a concession from them to do this.
Global Product Champion
Richard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.
Some paste manufacturers will re-certify material by running a sample through the same QA tests used during initial production i.e. solder ball, viscosity measurements, to determine if the material is still within specification.
However there are several considerations:
Any potential cost savings could quickly evaporate should any of these circumstances arise. Solder paste is the foundation of the surface mount process and using fresh, quality paste if of the utmost importance.
- Using paste that is past it's expiration date would invite a reprimand from the board customer should there be any issues with the quality of the final assembly.
- The paste performance could be compromised despite proper storage. As paste ages and the flux and metal powder interact, performance invariably degrades. The potential production issues include a reduction in print performance, increased voiding and poorer wetting.
Technical Marketing Manager
Tim O'Neill is the Technical Marketing Manager for AIM Products. AIM is a global supplier of materials for the PCB assembly industry including solders, fluxes and thermal management materials. Tim has a B.A. from Assumption College and post-graduate studies in education. He has 20 years of experience in the electronics soldering industry, beginning his career in 1994 with EFD and was key in business development of their fine pitch solder paste dispensing technology. Tim joined AIM in 1997 and has since assisted many clients with assembly challenges, specializing in Pb-Free process development and material selection.
Expiration date mentioned by manufacturer is with recommended storage condition. It is not recommended to use expire paste, though if you like to use check with manufacturer to send back to do evaluation and and possible remixing.
Supplier Quality Leader
Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.
Did I read James DiBurro's comment correctly? -40°C sounds awfully cold, and I've always read that storage beneath 32°F could cause problems with flux separation.
Leland Woodall, Keihin Carolina System Technology, LLC, USA
Richard Stadem, General Dynamics AIS, USA
- DO NOT store solder paste at -40 deg. C. Solder paste should never be frozen nor allowed to get above refrigeration temperature during shipping or storage. DO NOT let it reach temperatures above 85 degrees Fahrenheit either. DO allow it to reach room temperature only when it is being printed and then use it up within 10-15 days or discard.
- DO refer to the solder paste vendor's Technical Data Sheet (TDS) for the recommended storage temperature (refrigerated, but not frozen), as well as the optimum viscosity for best printing results.
- Mr. Byle is correct, DO consult with the vendor. However, they will NOT tell you the paste is OK. They would then be liable for the results. If you want them to test it for you, they will NOT do that for free and yes, you will have to ship it two more times. That in itself poses a significant risk
- DO NOT allow any paste that sits for 2 months or longer to be used without mixing as it will likely have some flux separation. Mr. Zarrow's experience with solder balls 3 weeks after expiration is NOT due to "expiration" alone, but rather it is caused by attempting to print separated paste without mixing it first. DO mix the paste in its sealed container to ensure homogeneity of the flux and the solids. DO use a centrifuge made for solder paste set at the optimum time to achieve the correct viscosity and temperature (typically takes less than 2 minutes from refrigeration). This is a QUALIFIED PROCESS for handling of solder paste in a repeatable manner.
- While Mr. Lambert is correct that using "bad" solder paste can be expensive, we all know that most solder paste distributors refuse to sell small quantities of paste. So it is likewise very expensive to have to buy that much just to throw half of it away after 6 months. Most distributors have a minimum lot buy of at least ten 500 gram cartridges, some require you to buy 20. DO find a certified major distributor of your paste that sells enough such that their inventory is turned frequently enough that they can sell smaller minimum lots (a certified distributor is one recognized by the paste vendor, who will notify you of recall notices of bad lots, etc.). DO NOT buy your solder paste from the back of a "brokers" 1957 Rambler station wagon at midnight down by the dock of the bay.
- DO NOT punch holes in business cards to make a stencil to re-certify paste. The cards I have on my desk (from solder paste salesmen) range from .006" thick to .036" thick, so the amount of paste deposited is going to vary wildly. Depending on the amount of paste and the surface energy and cleanliness of the substrate being printed, they may or may not agglomerate into a single ball.
- DO NOT use expired paste unless you test it yourself, and have in place a documented agreement with your customers as to how expired paste shall be handled. The customers are savvy enough to know that your costs are their costs also, and will usually allow the use of expired paste provided you can show that you have a qualified procedure for testing and re-certifying it such that there is no loss of quality or reliability.
- DO purchase a copy of J-STD-005 and read it, especially section 6.0 where it outlines rules for re-certification per the test methods outlined in IPC-TM-650 (a free download). Pay particular attention to the stencil pattern shown in Figure 1 of J-STD-005.
And here is your reward for reading all of this: Why not have the small stencil pattern shown in Figure 1 included in every production stencil you purchase, along with two of the 6mm holes recommended in Test Methods 2.4.43 and 2.4.45?
If you purchase a small paste centrifuge, you eliminate the 1 or 2 hour wait for paste to come to room temp from refrigeration, you can print paste tested to TM 2.4.34 (viscosity), and if you print the slump pattern on pads on a rail and the agglomeration pattern onto the equivalent of a fiducial (the small center pad holds the agglomerated solder ball securely) and the wetting deposit is printed on a small square pad, you have essentially tested the paste for the full requirements of J-STD-005 ON EVERY SINGLE BOARD YOU EVER MANUFACTURE, automatically and free of any additional cost!
Richard Stadem's comments are on the money. I like that he refers to published codes rather than stating his own experience or opinion. One thing that should be considered is criticality of end assembly. You cannot use expired paste for mission critical assemblies.
Richard Stadem's summary is the most comprehensive feedback I've ever seen on one page!
Mark Edwards, Hisco, Inc.
The pragmatist in me would also want to extend the life of most everything used in a PCB assembly operation. However, when one factors in the average cost of a 500 gram jar of paste ($50-125/jar, depending on alloy) and the amount of paste by volume used on the PCB, the solder cost is going to be very low versus the complete Bill of Materials (BOM).
Factor in the extra time to extend the paste life and possible process/quality issues outlined above, and I would vote for working on more pressing assembly issues like first pass yield, SMT line uptime, change-over time, etc. Wouldn't you?