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Can High Particle Concentrations Impact PCB Assembly?



Can High Particle Concentrations Impact PCB Assembly?
At our SMT facility we currently control ambient temperature, relative humidity and particle count. Is particle count a good parameter to control?
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
And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers, Pick and Place. Today we are broadcasting to you from the ITM Consulting broom closet. Wow, what a great segue into today's question, Jim.

It is from J.P. He says at our SMT facility we currently control ambient temperature, between 20 and 30 degrees C, relative humidity between 40 and 60%, and particle count of .5 microns, not to exceed 100,000 particles per ISO class 8. Is particle count a good parameter to control? How can having high particle concentration affect our PCB assembly process, printing process, reflow, etc.?

This is kind of interesting, Jim.

Jim
Typically, we don't need clean room type environments. This example given here I think is classic. A half micron particle is very, very small. It is probably not going to affect anything.

To give us some reference to that, most of us use type 3 and type 4 solder paste powders. Few people are using type 5, which has smaller particles if you are printing really fine details. In a type 5 powder, the smallest powder typically used today, the smallest particle is 15 microns.

So they are measuring particles of half a micron. That is 1/30th of the size of the smallest particle in the finest solder paste. To me that says that no, those particles are not going to be significant as physical particles.

Now if they are chemicals and then they interact with the flux, that is something else. From a mechanical, particle standpoint, no. I think what we are concerned about is not average normal particles, but those rare large particles that we call FOD.

What does that acronym stand for Phil?

Phil
Foreign object debris. And what Jim is saying is the type of stuff we see is the proverbial hair on the circuit board or other dust.

We see situations where there is dust falling from the upper structure from the room we are in. That is not good.

Jim
A classic problem for me, at least up here in New England, is that we will switch from a heating season to an air conditioning season. Sometimes those are entirely different duct work systems.

A classic example is that if you turn on the heat for the first time in October and all the dust has been settling it does come out. You can get some larger particles.

The point I want to make about this is that what is the smallest particle that can hurt you is decreasing continually because of the miniaturization of our circuits, smaller components, smaller apertures in our stencils.

If you think about stencil printing, the smallest particle that will plug an aperture is getting smaller and smaller because the apertures are getting smaller. They are still significantly bigger than half a micron. They are on the order of mils, which would be 50, 100, 150 microns.

Phil
Let's not forget the orifices in our pick-up tooling and our pick and place machines, which has gotten incredibly smaller and it doesn't take much to clog them up.

That is definitely a situation that a number of our listeners have had. So yeah, it is a matter of perspective. I can't think of how FOD might affect reflow. I am trying to imagine a scenario.

Jim
The defect might result in reflow such as some solder balls or bridges because you have a large FOD stuck in your solder paste deposit. That might cause solder balls to occur or a bridge to occur during reflow. The FOD was probably deposited in a previous process, between printing and placement.

Phil
J.P., I hope that we answered you questions there. Cleanliness is next to Godliness but thank God we don't have to go to any great extremes with surface mount assembly.

Beyond that, I'll just say that regardless of what kind of particles are floating around when you finally get to soldering, don't solder like my brother.

Jim
And don't solder like my brother.

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