Electronics Assembly Knowledge, Vision & Wisdom
Questions About Handling Solder Paste
Questions About Handling Solder Paste
We use lead-free solder and store it at a temperature of 4 to 5 degrees Celsius. What is the best practice for prepping this stored paste before use?
Board Talk

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Board Talk is presented by ITM Consulting
Phil Zarrow
Phil Zarrow, ITM Consulting
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, ITM Consulting
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. Today we are coming to you from the elegante ballroom, high atop Mount Rialto where ITM headquarters is located.

We are here to talk to you about electronics assembly, materials, equipment, components, practices and procedures, among other things. So, what have we got today Jim?

Jim
Phil, we have a question that is hard to believe that we haven't dealt with completely before, a fundamental question. It is about paste handling and the printing operation.

We have had several questions about this. I think we have talked about various aspects, but never covered it completely.

So we'll do that today. One question that comes in called solder paste prep before use.

It come from S.S. We use lead-free solder and store it at a temperature of 4 to 5 degrees Celsius. After removal from the refrigerator we let the jar sit at room temperature for one and a half to two hours. We then stir the paste with a chemically resistive plastic spatula for 1.5 minutes by hand.

Is this good practice? How often should we clean the stencil printer? We now clean it every two hours. Another question similar comes from E.R.

We purchase our lead-free solder paste in 500 gram jars and store them in the refrigerator, similar to above. After removal from cold storage we let the jars sit at room temperature for four hours. Then we place the jars in a centrifuge machine horizontally for five minutes prior to use.

Is this a good practice? Would you suggest any alternative mixing practices? Well, I know you have very strong opinions about mixing solder paste.

Phil
With regard to our first question from S.S., with regard to 1.5 hours, I don't know what size container that is at. How did he derive that?

Where did he read that? Where on the internet did he see that? A little fairytale or something.

Basically the same thing for E.R., 4.5 hours, it is very arbitrary. The thing is, you have to do this scientifically. It isn't rocket science. It is solder paste science. Basically you do a very simple design experiment to find out the true time that you need in your environment to get the solder paste up to ambient temperature.

What you do is you take a container or whatever you are using, a jar or cartridge, out of the refrigerator and you insert into it a thermocouple and hook it up to your data recorder or whatever you are using. Basically track how long it actually takes to come up to ambient temperature.

Unless it is a really small container, I really doubt that it is an hour and a half. Four hours, hey maybe it is. It depends on the temperature of your place.

I have seen other people go eight hours. And we see a lot of people doing it overnight.

This way you know. You will have data. Then you will know that for this size container, whatever you are using 500 grams 1,000 grams, 50 grams whatever, this is the amount we allot for that size container to come up to ambient for that material.

So that is a starter, you actually have a real number. Now the other interesting question was the use of the centrifuge.

Those things have been for sale for quite some time. They are rather interesting. I have seen them used in different parts of the world.

I think they are about four or five thousand dollars. I have seen people put it right out of the refrigerator, or as I guess E.R. has been giving it some time there.

Basically, as it spins the particles are rubbing against each other, and that is how you have your warming mechanism. Where I have seen them, they seem to be doing an adequate job.

But the way to know, to answer E.R., temperature, measure the temperature when it comes out. It is in ambient?

The obvious is obvious, so go forth.

Jim
I've never heard of the idea of using a centrifuge. And to me, it is scary. I think of centrifuging as a separation process, not as a mixing process.

And you have the real heavy solder particles and the much lighter flux goo in there. Aren't you going to run the risk of separating, when the particles go to the outside of the jar because they are being affected more by centrifugal force?

I don't know. I have never seen this, you obviously have the experience.

Phil
My experience is limited. I will be honest, it will make a great experiment. If someone wants to sponsor us doing an experiment on this. Yeah, people have done it. You don't see a lot of them in use.

Most people, rather than plucking out $5,000 or whatever these things cost, would rather just stage things on a FIFO basis, which is free. I would say basically FIFO works for the vast majority of the industry, big and small, OEMs and CEMs.

Basically once you know the time you need and you just do things on a FIFO and the ideal people record the time they take it out of the fridge and how much time there is before.

What time you are allowed to use it. Just good discipline. And that works out well.

Now Jim, mixing. What are your thoughts on that?

Jim
I am not a big fan of mixing. Most people I know buy their solder paste in cartridges and dispense directly from the cartridge onto the stencil, laying down the bead across the stencil.

And then maybe knead it with the motion of the squeegee. Good paste, modern paste should be designed not to settle.

That is one of the reasons you keep it in the refrigerator. Although, there are now pastes that you don't even have to refrigerate.

So that they don't settle. So therefore, what are you doing when you are mixing.

And S.S. even raises the issue, we stir with a chemically resistant plastic spatula. What is the implication?

When you open the jar and stick something in it you are inviting contamination, or something that could possible degrade the solder paste. My recommendation is get away from jars.

Buy it in cartridges, don't mix, dispense it directly onto the stencil.

Phil
Right, and then there is the oxidation factor when you are stirring of course. Whether it is sizable or not remains to be seen, but you are inducing oxidation of the solder paste.

And the other reason we prefer cartridges over jars is because with jars you are constantly opening and closing, opening and closing, exposing it. Whereas basically that cartridge stays shut.

You remove the cap, you spritz some out, and you put the cap back on. It is much better and to my knowledge most solder paste companies do not charge a premium for requiring the solder paste in cartridges, as opposed to jars.

Just our recommendation, that is our feeling. So good, I think we answered two questions there, or a whole bunch of questions, and clarified the subject.

Just remember Board Talk, it melts in your mind not in your ears. However you are handling your solder paste, hot or cold, whatever you do with it, please don't solder like my brother.

Jim
And don't solder like my brother.
Reader Comment

I have done some tests with pulling a jar of lead free paste from the fridge and centrifuged it for 5 minutes and placed a thermocouple in the center to measure the temp and after running a few test, found that if centrifuged for 15 min you will reach room temp.

Michael Peters, ACDI
Reader Comment

When improving upon our solder paste handling for 0.5Kg jars of KOKI lead-free and KESTER lead paste types, we spoke with reps to acquire and verify current data sheets and took the most helpful extra step of speaking with the companies solder paste engineers. For the two points of temperature and mixing, ranges are detailed in data sheets and, per engineer recommendations for that jar size, 24 hours is plenty.

If needing to reach room temp in a short time, you should verify ideal temperature range with a thermometer. Be concerned with temperature not time. Temperature can be reached in a short time, but the very first time we looked ahead for our use and warmed paste overnight, we had better than ever results with paste release through apertures and from squeegee blades.

Given that these pastes can be stored at room temperature for one month, refrigerated for six, it is not a big deal to warm the paste in advance and keep ambient. Also, some more stressed key points per conversations with the engineers: storage time estimations are median, warmed paste should not be returned to refrigeration, used/in use paste should be kept separate from not used, sheer-thinned paste on stencil should be recycled at the end of every shift, used paste life varies from 1-5 work days depending on the amount of shifts and time exposed, and use the ideal paste bead size detailed in the data sheets.

For the second point of mixing the paste, engineers stressed that settling does occur and 1-3 minutes of gentle mixing is needed, taking care to not scrape the sides of the jars with your proper spatulas.


Greg Edwards, EMAC Inc., USA
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