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Solder Paste Beyond The Shelf Life?



Solder Paste  Beyond The Shelf Life?
We have some solder paste that has been in a sealed box at 4 degree Celsius for 8 months. The shelf life is 6 months. Can it be used? Phil Zarrow and Jim Hall, The Assembly Brothers, offer their insight and share their own experiences to answer this reader's question.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


Phil
Welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting, the Assembly Brothers.  I think today we have a material question, is that correct Jim?

Jim
It comes from G.H. We have some solder paste that has been in a sealed box at 4 degree Celsius for more than 8 months. The shelf life is 6 months. Can it be used? 

What adverse effect might we expect, if any, using solder paste slightly over the shelf life? Well my brother Phil has very intimate knowledge of the answer to this question.  Don't you brother Phil?

Phil
No, don't use it! There's a limit, they have expiration dates for crying out loud!  And that means it doesn't keep up anymore!

If you want to do that with your medications, I know I do with mine go beyond the expiration date, go ahead, take your chances and suffer the consequences.

There's a reason there's an expiration date. Having been involved in a lot of solder paste evaluations, I've seen paste that even in a short time, go beyond it's expiration date and just turn to crap - for lack of a better word.

So it might look like it's working but you're really rolling the dice as far as additional defects.

We recommend actually going the other way. We try to look at putting almost a half life onto solder paste. The manufacturer recommends an expiration date of 6 months, we try to make sure we're going through our inventory within 3 months.

People wave their finger at you and say, "Well you should buy it in smaller quantities." What a lot of solder paste companies will allow you to do is to make a blanket purchase order, spread out the actual deliveries of the paste so that you're getting the price of the quantity, but yet you're basically getting fairly fresh solder paste in whatever quarterly, monthly basis.

I think most solder paste companies accommodate you. I think they want it because it's a very, very, very competitive industry.

Jim
Just to re-emphasize, it's the most critical material that you've got in your assembly. It's doing a lot for you.

It's going to stencil print well, it's going to be tacky to hold your parts in place, it's going to solder well when you go for the reflow, it's either going to have a safe no-clean residue or you're going to be able to clean it - it's a lot that you're asking from that paste and there's a lot that goes into that formulation.

As Phil said, there's a reason they put a shelf life on it because there's a very delicate chemical formulation and you don't want to take any risks. Because it could affect virtually every joint on the board.

Phil
We like to equate solder paste as being what blood is to your body what your solder paste is to your circuit board and as Jim has said, it's pretty important stuff.

On that note I'll also add, don't solder like my brother.

Jim
Don't solder like my brother either. 

Comments

To say that there is a finite day that solder paste becomes immediately unusable means that you must ignore chemistry and physics fundamentals. Solder paste manufacturers use different chemistries, yet almost all have a 6 month shelf life. I consider that more of a "coincidence" created by a consumer created expectation, not a chemistry governed action.

We currently have a process to actively monitor our paste consumption and inventory on a weekly basis to hold only 2-3 months stock at any time. Before this, we had challenges due to numerous changes with customer requirements that impacted the variety of solderpastes we must utilize. We currently use 7 different solderpastes to accommodate all of our customer requirements, so as build volumes change among them, we can be left with paste longer than anticipated.

In regards to using expired paste, we do not use paste that is expired unless we approve it through an evaluation and re-certification process that we developed internally. While it is very rare that we have paste reach expiration, we do have occasions where a minimally required paste, will expire. (for example, tin/lead water soluble in a type 3 is only used a few times a year) We only consider approving expired paste if they are unopened jars or cartridges and we will not approve expired paste for use on any complex assemblies.

Our process includes evaluating paste viscosity, tackiness, printability on a test substrate, reviewing slump and coalescence, Cyberoptics 3D print verification using the full requirements for normal paste, and a reflow test. Our process uses several steps related to the SMTA Solder Paste Test Vehicle for Miniaturized SMT. If the paste passes these tests, a single production board is ran with the paste and it is evaluated through the same process. If the production board also passes, the paste is eligible use.

Essentially, knowing your paste selections, process parameters, and customer requirements well, will enable you to determine usability of expired paste with minimal risk. Also, a well managed paste inventory will enable you to not need to do so. Since we improved our paste inventory process 2 years ago, we have not had to do an expired paste evaluation.
Andrew Williams, PRIDE Industries
Other than water soluble pastes; at times I have run into the same problem i.e. +3 months past use-by date. Mix paste to ensure proper dispersion of flux and undertake two tests: coalescence and print release. I've had to refrain from using expired paste once when, for some reason, it had absorbed moisture and failed coalescence.
Rob Hills, Tait Radio Communications
It depends how much you have. One jar, toss it. Many jars, send a jar to the manufacturer for re-qualification; it might still be good. In order to provide good product, manufacturers often halve the determined shelf life. For no-clean materials, this is 6 months. However the paste may still be good for up to a year, but the paste should be tested to determine whether it is still in spec. I would not use no-clean paste that is more than 1 year old.
Karen Tellefsen, Alpha Assembly Solutions
Originally being on the manufacturing side I have seen solder paste both fail before expiration or get good results after expiration. You can do the "try it and see if its good" . If it doesn't work you have rework to do, if it does visually looks good you might still have latent defects you cant see. The only real way to know is to test it. That used to mean sending it to a lab or the paste manufacturer and wait to see what they say. I know now there is a new technology that came out this year where the user can test the paste for "Fit for use" and it only takes a few minutes. Either or is the only way to know if your paste is good and that you wont have issues in your process.
Clinton Buldrini, Matthew Associates
In the early days of solder paste the expiration date was gospel. The material was no where close to Homogeneous and the sphere size was all over the map so separation was guaranteed. Today solder paste is almost homogeneous, flux and carriers are much more synergistic (stick together)so as the Alpha gentleman says, paste does not self destruct on the expiration date. If the application is Class III then get the paste re-certified. Many manufacturers prefer expired solder paste as it has had time to "soak"and the pores of the metal spheres have absorbed the flux and carriers.
Ike Sedberry, ISEDS
Don't try to use expired water-soluble solder paste, results will be very ugly. But with no-clean leaded and leaf-free solder paste even after 4 years from date of expiry I had acceptable results. Color of residues was little bit more yellow, but it was soldered and devices works more than 2 years.
Vitalii Gladkyi, Biakom LTD
Solder paste doesn't turn to dust the day the label shows for expiration, but the user bears any risks inherent with using paste beyond its published shelf life. Some pastes are very robust and can perform adequately well beyond their stated shelf life, but other pastes may not be so forgiving and can result in unanticipated costs that far exceed the replacement cost of the expired paste.

With our pastes, we can recertify users' material for shelf life extension by performing some testing on a sample from that storage lot. Assuming the paste is recertified, users can continue to use the paste for the extension period and comply with all material control audit requirements.
Jason Fullerton, Alpha Assembly Solutions
Some years ago when I was a PCBA manufacturing industry employee we used solder paste beyond shelf life with great results. The assembly was a quite simple class 2 application with big SMD components and no fine pitch ones. There was also a hot air application which sample of flat cables soldered by hot air were submitted to pull test force.

Not a single sample produced with this out of date solder paste was reproved. Also we had a failures lab internally in site and the FA lab guys, by doing cross section, did not find a single reason to reprove the assemblies. At the end, our final customer approved to receive all the batch assembled under this circumstances. For me the answer depends type of assembly to be soldered with solder paste.
Glayson Figueiredo, Philips Medical Systems, Brazil

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