Solder Paste Past Shelf Life

We have some solder paste that has been in a sealed box at 4degree Celsius for more than 8 months. The shelf life is 6 months. Canit be used?

What adverse effect might we expect, if any, using solder paste slightly over the shelf life?

G.H.

Expert Panel Responses

Best to follow solder paste manufacturer's recommendations,however, it would be worthwhile to print some and test it before recycling the expired paste. Test for printability, solderability, voids, fillet formation and fillet wetting angles.

If everything is acceptable, then why not use? It is best to repeat using another jar. If any of the attributes fail or if at all suspicious, return it to the supplier for recycling and replacement.

Gary Freedman
President
Colab Engineering

A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.

If you do not have internal capability to re-certify the material, contact the manufacturer. They should be able to re-certify it. The concerns are as follows:

Viscosity changes that will affect printing and/or slump
Reactions between the metal powder and flux that will consume some of the flux activity (and also affect viscosity)

Internally, you would need to run a print test to confirm that the paste still prints acceptably. You would also need to run reflow tests to ensure that the flux activity as acceptable. A controlled solder spread test would be best, but for that you really need a baseline to compare to.

Finally, you would need to look at hot slump, to ensure that any viscosity behavior changes have not resulted in a dramatic change in slump behavior.

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.

Solder paste consists of three major elements, solder powder of a give size, flux, and a viscosity agent. All three work together to create therheology of the solder paste. The size of the powder allows the paste to be used for specific size terminations, the flux is selected to reduce the oxides on the materials being soldered and the viscosity agent being able to maintain suspension of the powder within the mass of solder powder within the solder paste mixture. Without the proper viscosity material the solder powder would separate and sink to the bottom of the jar.

The other issue with solder paste is the function of the flux.In plated through hole soldering the flux is used to prepare the component leads and the circuit board prior to soldering. The same applies to flux within the core of the wire solders being used in the industry. However the flux in the solder paste has an extra function, preparing the solder powder in addition to preparing the component lead and pad area.

Since the flux is in constant contact with the solder powder, it is continually acting on the oxides on the surfaces of the solder powder or spheres, and this action reduces the activity levels of the fluxes. This is also why it is always recommended to refrigerate the solder paste to reduce this chemical reaction with the solder powder and give a longer shelf life to the material.

Whether or not the solder paste can be used, would depend upon the type of flux being used, be it either a low activity, medium activity, or high activity flux. The adverse affect would be un-reflowed solder paste,insufficient reflow, solder balls, dewetting, etc.

The recommendations would be to perform a solderability test,slump test and flux activity test on the solder paste itself and see if it still performs to the original standards of the material.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation

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.

Shelf life typically defines the warranty limit for a solder paste. Many remain fully functional beyond the published shelf life. It the solder paste prints or dispenses and reflows appropriately it should be okay.

If either application or reflow looks abnormal you probably should not use it. Typical symptoms of age are poor print or dispense and visual soldering defects such as incomplete coalescence and abnormal solder balls.

John Vivari
Application Engineering Supervisor
Nordson EFD

Mr. Vivari has more than 15 years of electronic engineering design and assembly experience. His expertise in fluid dispensing and solder paste technology assists others in identifying the most cost effective method for assembling products.

If the solder paste appears normal after allowing it to come to room temperature, with no visible separation... it's probably OK to use.There may reduced stencil life, rheology changes and a minor impact on wetting, but if it prints and reflows OK... It's OK.

The real issues are: "I'll save a couple hundred bucks by using expired paste and potentially, increase defects and rework due to clogged apertures, so is it worth it?" Even worse... suppose there's an issue after the build ships to the customer and they start asking questions and you're forced to reveal expired paste was used... that could get really uncomfortable and potentially very expensive.

1st Rule of SMT assembly - Solder paste performance is the foundation of the process - it's the last place one should scrimp.

Tim O'Neill
Director of Product Management
AIM

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.

Reader Comment

It sounds like the paste has been stored in the correct conditions so I would first test its performance on the stencil and its wetting characteristics. It is a good idea to test against a fresh batch of material.

Solder paste is a incredibly crucial part to the process and fresher the paste the better. Look at the time it could take to rework all of your boards or the cost of failure and this may be the decider for you.

Chris Ward, SolderKing Assembly Materials Ltd

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