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What is Solder Paste Working Life on a Stencil?



What is Solder Paste Working Life on a Stencil?
Our solder paste has an exposure life of ten hours. After ten hours of operation do we need to remove all of the paste on the stencil and scrap it? The Assembly Brothers, Jim Hall and Phil Zarrow, share their experiences.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
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 Jim Hall and Phil Zarrow of ITM Consulting, otherwise known as the Assembly Brothers. Today we are coming to you from high atop Mount Rialto, at the ITM printer extravaganza lab.

We are here to talk about all kinds of things related to electronics assembly, materials, equipment, components, practices and procedures. Jim, what are we talking about today?

Jim
Well, it is solder paste life on the stencil and it comes from S.S. Our solder paste has a specified life for exposure on the stencil at ten hours.

We run our stencil operation continuously for forty hours, adding 250 grams of new paste every two hours. The additional paste becomes mixed with the existing paste.

After the first ten hours of operation do we need to remove all of the paste on the stencil and scrap it? Currently the practice is to keep adding new paste until the end of the forty hours of continuous production.

First off I want to say, S.S. thank you for giving such a clear, concise description of what is really going on. That makes it much easier to give a reasonable answer.

Phil
Well, as reasonable as you would expect from us.

Jim

Well for me, putting on my lean six sigma hat, I would say what are your defect rates? If you are currently continuously running your process, adding your 250 grams every two hours, if there was a problem with that you should see it, in an increase in defects.

The most obvious we think about when leaving paste on the stencil is that it is going to dry out or degrade in other ways and it is not going to print well.

So my question is, are you inspecting your printing post print, either 2D on the machine or 3D post print. If you are, are you seeing your defects increase, the number of boards you have to wash and redo over that forty hour period?

If you are, then it may benefit you to completely change the paste and clean the stencil every ten hours, or at some other interval. Then further down at the end of the line the flux can be compromised and you don't get as good wetting.

Well, what are your results from final inspection and test over that forty hour period? Are you seeing an increase in that defect level?

If not, then your current practice of adding the paste every two hours is probably adequate for, as my brother said, the conditions and the specific paste you are using, the level of difficulty of your printing, and your soldering.

Phil
Right, the operation, your procedures and your materials, you've got a good solder paste. So again, that is one of the other variables.

Some other solder paste might not give you as optimum, or some might give better. So that is another variable in the mix as well.

Yeah as Jim said, you've basically got to take hold of what you have there and go with that. The end result is what kind of yields are you getting.

Jim
Basic statistical process control, continuously measure what you are doing and look for trends. And even if you're not getting bad defects, can you see some trends in there that are indicating there is a degradation going on.

We know what you are looking for. You are looking for reduced printability and reducing wetting, more solder balls, maybe even bridges if you are getting slump problems.

But those should be measurable. You should be measuring them in your process and using them to improve your process through feedback.

Phil
Right, as the saying goes "In God we trust, all others bring data." Well good, you just squandered another five or so minutes listening to Board Talk.

We appreciate it and we look forward to your other questions and inquiries and comments. And in the meantime whatever you do, wherever you go

Jim
Don't solder like my brother.

Phil
Please don't solder like my brother.

Comments

Very enlightening for us laymen, and extremely entertaining.
Malcolm Montanjees, Equipment Services LLC
Continuous replenishment (250 grams every 2 hours) means you will always have a very high percentage of paste that has been on the stencil for less than 10 hours. If you are having variation in print volume deposited, there are pastes with longer stencil life.
Mitch Holtzer, Alpha an Alent plc Company, USA
That was quite an interesting question today, what's the solder paste life on the stencil after all that mechanical friction and exposure, as well as your answer, thank you you remind us the basics guys.
Abraham Noriega, CST-Kavlico, Mexica
Just so you know our process is somewhat the same except were NOT adding the 250grams every two hours, maybe half that. The other thing that we do that is very different is we may put the paste in a jar when we go from lead free to lead and it might be hours before we go back to using it again. Our stencil life it 10 hours but when we did the paste qualification I actually let about 250 grams sit for an additional 16 hours, 24 total, and the paste was still workable. If we do notice any issues with the paste rolling on the stencil during printing we will scrap it and get fresh paste. I've never tried to relate stencil live back to defect rates because we are continually changing over.
Gary Bikun, Arc-Tronics, Inc., USA

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