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? Phil Zarrow and Jim Hall, The Assembly Brothers, discuss this scenario and offer their suggestions.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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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.


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?

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.

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.

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.

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?

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.

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.

And don't solder like my brother.


Here are some answers: 1. For best results, try to reach the paste viscosity as per the Technical Data Sheet for your paste, always available online from the paste manufacturer.

2. People who say a centrifuge should not be used, or is dangerous, or will separate the paste from the flux, etc., are INCORRECT. I can tell you that you DO need to use the right centrifuge that is made for solder paste, not whip it around in a high-speed medical or chemical centrifuge or some other type of centrifuge, they won't work! The proper centrifuge accommodates either the 250 or 500 gram jar or the 500 to 600 gram Semco tube. When spinning at the right RPM (rather slowly, mind you) it MIXES the paste and achieves complete homogeneity of the flux throughout the paste, and you can actually SEE the past slowly folding over onto itself while mixing in one of the somewhat transparent Semco tubes. At the right RPM, for approximately 5.3 minutes (for the paste I use most, others make take a few minutes longer), BOTH the viscosity AND the temperature of the paste will be per the TDS and at ambient temperature!

3. Do NOT believe that just because a paste comes in a syringe or a tube it does not need to be mixed; some don't, but MOST DO. I wish I could tell you which ones do but I cannot. However, your printing operator can tell you, because as soon as you take the cap off the jar or squeeze some out of a tube for the first time, a gob of brown liquid flux pours out first. It is also very apparent in the jars when first opened. The weight of the metal forces the liquid flux to the top while refrigerated. Guess who told me this? Two of the largest distributors of solder paste. They store the paste upside down so the separated flux is on the bottom of the jar or tube when first opened! Some pastes separate much, much worse than others and it is a big problem. Your operator may not realize this however, but without that separated flux your paste loses a huge amount of wetting action, especially on oxidized (old) components.

4. One more thing; if you have paste in a new syringe or tube, and you mix it properly the first time it is removed from refrigeration with the right type of centrifuge for the right amount of time, you CAN dispense some into a jar for immediate use (for the job on hand) and you CAN re-seal the freshly mixed tube and put it back into the fridge without any issues with condensation. And for the next ten days or so you CAN continually take out the tube once or twice per shift, dispense some more into a jar, and put the tube right back into the fridge. Then you mix what's in the jar lightly and slowly for two to 5 minutes and it is ready to go!

This practice extends the working life of your paste, it prevents flux separation while refrigerated, and with no loss of flux and no drying out of paste as a result your printing process will improve tremendously! When mixing in the jar, do NOT whip air into the paste, as that is one of the worst things you can do when mixing. That is why some folks say a centrifuge is not a good idea. They just don't know the right way to do it.
Richard Stadem, Analog Technologies Corp.
Comparing heat generated by friction and warming naturally is not a equal comparison. Solder paste is a thixotropic material - meaning it is thick when static and will thin when shear-stressed. It thickens when the stress is removed. Paste may never recover its engineered properties when the mixing energy is so high. The temperature may have reached a desired value, but the effect of mixing on the paste chemistry cannot be assessed. Therefore, mixers are not recommended for solder paste preparation.
Timothy O'Neill, AIM
It seems to me that we are confusing a solder paste mixer and a centrifuge for medical use, the latter among other applications separates fluids.

In the case of the centrifuge for use with solder paste, mixes the metal part with the flux and does not need to be at room temperature to mix.
Roger, Camtronics
I want to make a small contribution for solder handling.
Personally, I first did a temperature study for the refrigerator in order to evaluate solder paste behavior.
With the results you can decide the behavior of your refrigerator.
Then I did a study to decide the acclimatization time at room temperature.
Using TAMURA solder paste, specification 20° C-25° C
Starting with 5° C taking out the 500gr jar at room temperature I was taking a reading every 30 min.

In the readings were the following:
30 min: 17.7° C
60 min: 20.5° C
90 min: 21.8° C
120 min: 22.3° C

Based on these results, you take into consideration the time you will need to set your solder paste

In the case that you want to take the solder directly from the refrigerator and not wait for the time at room temperature, the same procedure will be taken only by putting the jar in your mixer and taking the temperature for minutes will give you the time you need to reach the desired temperature, in my case it was the following:

1 min: 22.4° C
2 min: 23.2° C
3 min: 23.6° C
4 min: 24.6° C
5 min: 24.8° C
In this case my best option will be 3 minutes

Conclusion :
1.- Jar Solder paste is removed from refrigerator and live 2hr at room temperature
Result are satisfactory according specification (20 - 25° C)
2.- Jar Solder paste is removed from refrigerator and mixed for 3 minutes.
Result are satisfactory according specification (20 - 25° C)

Tools used:
Fluke thermometer
Malcom Softener SPS-1
Manuel Rodriguez, Katolec de Baja California
Another option would be to use a room temperature stable lead free solder paste like the Loctite GC10. Since it is temperature stable there is no need for refrigeration, and a one year shelf life. Might be an option for you...
David Brand, Henkel Corporation
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
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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