Extending Shelf Life for Jars of Solder Paste

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.

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.

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

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.

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.

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.

Timothy O'Neill is the Director of Product Management for AIM Solder. AIM Solder is a leading global manufacturer of assembly materials for the electronics industry. Mr. O’Neill has 25 years of industry experience is a Certified IPC Specialist.

Mr. O’Neill’s responsibilities include developing product and technical information; he is a technical writer and presenter for industry trade publications and events and has been recognized as a Speaker of Distinction by the SMTA.

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.

1. 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.
2. 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.
3. 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
4. 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.
5. 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.
6. 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.
7. 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.
8. 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!

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?