Issues With Solder Paste Transfer Efficiency

Issues With Solder Paste Transfer Efficiency
With transfer efficiency and its relationship to solder paste volume, can you produce a transfer efficiency over 100% of the theoretical volume?
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
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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.


And welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting. The Assembly Brothers. Today we are coming to your from the ITM elegante ballroom, high atop Mount Rialto.

And we're here to talk about electronic assembly, materials, equipment, components, practices and procedures, among other things. And I believe today we have a procedural question.

No, I would call it also an equipment question, Phil. It comes from A.L. While I understand the concept of transfer efficiency and its relationship to solder paste volume, is it possible to produce a transfer efficiency greater than 100% of the theoretical volume?

Can the volume we are measuring with our solder paste inspection system be believed, or is this measurement an anomaly of the inspection measurement process? What is between the lines here, A.L. is getting values greater than 100 for transfer efficiencies as measured by his SPI system.

That is a great question. SPI, Solder Paste Inspection, has finally really come into its own after 20 to 25 years. It is finally being implemented into all different levels.

Of course, more of us are becoming familiar with the equipment and the methodologies and techniques being used by them, learning what it is all about. I'll start by saying that with regard to the equipment regards to SPI is accuracy and repeatability.

So to start off with, make sure your machine is properly calibrated per the manufacturer's specifications. Secondly you should do a Dage R and R. In fact that should be part of your evaluation procedure when you are looking at various SPI machines. Then we get into some of the variables that we see at board level.

Right. Just to finish up on the equipment, because of its tremendous growth, SPI equipment is improving dramatically, almost day by day.  

It seems like every time you open a trade periodical somebody has a newer machine, supposedly better accuracy and everything. The implication is that if you have an older machine, it may not have the greatest accuracy or repeatability.

You need to understand where you are with the piece of equipment that you have and what its capabilities are, as my brother said. If we think about putting on my six sigma hat, variability.

Variability in a measurement system is one of the six m's of your fishbone diagram. You need to consider by Dage R and R, what is the variability in your measurement system, in this case SPI. What about the variability in the process?

Well, let's start real basic. You start with your stencil. How accurate is your stencil?

In terms of the volume, how does your SPI make that calculation? It starts with a theoretical volume. How does it calculate the theoretical volume?

Typically it takes the Gerber data from the stencil drawing and takes the length, width times the thickness of the stencil and calculates the volume. Now you have a stencil that you had manufactured for you. How accurately was that made?

Suppose you had a 5 mil thick stencil and the aperture was supposed to be 10 mils by 20 mils. That is the volume that Gerber data says and that is the value that the SPI is taking as theoretical. What was actually manufactured when it was cut with a laser, electroformed or whatever?

There have been some published data where people have actually measured the dimensions of apertures in incoming stencils. And guess what, they are not perfect.

One source of possibility is that yes, the aperture was a little bit bigger and you did get the theoretical volume calculated from the Gerber was small. So yes, you get more paste than you wanted. Another phenomenon is the dynamics of the printing process.

As the squeegee moves across an aperture, if you see some of these slow-motion cut-away videos, you will see that sometimes depending upon the speed and the angles and all of the other print parameters there is some dynamic elasticity in the paste, Such that as the squeegee goes across the aperture, the paste depresses and then actually bulges up.

If you look at the top of the aperture after the squeegee has gone over it the paste is actually bulging up. So once again you would have a larger volume of paste than the theoretical and give you a greater than 100% transfer ratio.

But probably the most common one that equipment has been dealing with is how your machine measures and calculates the zero plane. No matter what technology you are using in your SPI, it is measuring the top surface.

To calculate a volume, it has to define where zero is, where the bottom of that deposit is. And if you think about a PCB, ideally you want to calculate that volume from the top of the pad, the copper pad, or whatever the surface finish is on the board.

But now as your SPI system is going over and doing its measurement technique with cameras, lasers, or whatever it is using, is it actually identifying where the top of that copper is? You have two other potential surfaces, the laminate on the PCB which is below the level of the copper and you have the solder mask, which could be above or below.

So a classic example is, for whatever reason, the paste is sitting on top of the pad, but the SPI machine is measuring zero down at the board level. So it is adding the thickness of the copper pad to the volume of solder.

That could also give your greater than 100% transfer efficiency. I don't know if we made A.L.'s life easier.

Yes, there is variability and if you look at most specs they allow for transfer efficiencies greater than 100% for some combination of all of these reasons. Ideally we would like to make that go away, but in reality we are probably only going to be able to reduce it to an acceptable level. We can never make variation go away, we can only minimize it.

I think the only thing that I will add to that is hooray for A.L. for using SPI and for asking these questions. We can't emphasize enough how important it is that you and your techs who are working the SPI equipment understand what the process is, how the equipment works, the entire procedure.

We can't tell you how many times we've gone on process audits and here is this really nice, beautiful, state-of-the-art SPI in-line and it is being used as a pass through conveyor because no one knows how to use it. It just becomes an extremely expensive pass through conveyor.

It breaks my heart every time we see it. It is so important that you understand. If the machine has been passed down, if the original operator has gone on to another job, or he got a different assignment, or he got hit by a bus, it is very important that whoever is operating be properly trained. You have made a major investment, and a very wise one, use it to the optimum.

On that note, I think we, not only answered A.L.'s question, but hopefully a lot of other people's. If you don't understand any of the areas we discussed, please learn more about it.

Talk to the people who manufacture the SPI if you are looking at or if you own one, talk to the people about these issues. I am sure they have dealt with them all before.

Great. Well, you have been listening to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting. We would just like to say however you put the solder down, and however you are going to reflow it, please don't solder like my brother.

And don't solder like my brother.


SMD or NSMD pads will also result in 100%+ deposits. Paste that bleeds under the stencil into the relief of an NSMD pad is an uncontrolled variable that will increase paste volume. Another factor is gasketing between the bottom of the stencil and the substrate. Any gap will introduce an artificial snap-off increasing the volume of the paste beyond the calculated volume of the aperture.
Timothy O'Neill, AIM

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