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PCBA Inspection Concerns



PCBA Inspection Concerns
We inspect PCBAs on a relatively hard ESD mat. Will moving the PCBs on the mat will scratch and stress the soldered components on the bottom side? Jim Hall and Phil Zarrow, The Assembly Brothers, share their own expertise on this matter.
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 of ITM Consulting, the Assembly Brothers. Coming to you from high atop Mount Rialto at ITM headquarters.

We're here to talk about electronic assembly, materials, equipment, components, practices, procedures and who knows what else. Jim, what is today's question?

Jim
Well it comes from T.S. We inspect PCBAs on a relatively hard ESD mat.

QA claims that during inspection moving the PCBs on the mat will scratch and stress the soldered components, bottom-side. Are there studies on this matter? Are there any dedicated tools on the market designed for this purpose.

Phil
Well, that is different, I have to say. A relatively hard ESD surface. Well, I don't know of any particular studies. Perhaps the ESD people might.

As far as tools, there is other methodologies to be used. Jim, I remember when we were talking about different types of ESD mat surfaces.

There were some that are foam and much gentler with regard to any kind of propensity toward scratching or otherwise damaging the components or the board surfaces. That would be one aspect of it.

But depending on what you are using for inspection, are things remaining stationary and you're magnifying methodology over it or just visual, or exactly what you are doing there. There are a number of other systems where the board is actually held by the hands of the operator and moved under a scope or a mantis, or something along those lines.

But there are softer surfaces if that is a genuine concern. Of course, my question is has anything been observed or is this the QA people crabbing about things.

Has this actually been observed? Is there a real problem in this situation or not? Jim, what are your thoughts?

Jim
Well, I guess I'm leaning toward the QC. I don't like the idea of physically touching soldered surface mount parts on either side of the board, much less lying it down.

Yes, if it is stationary, certainly using a foam would be better. But if you need to move it, it shouldn't be sliding on any surface.

It should be either held by the operator or in a frame. You asked about tools. So there are lots of XY sliding fixtures that grip boards on the edge that are used for rework and inspection, and other manual operations.

That typically have V grooves that grip the board on the edge and allow you to move it under a microscope, or other inspection tool, without touching it. The other issue of cleanliness.

What about that mat? What about dust?

Phil
What about that operator?

Jim
Whatever is on that mat is getting picked up on the next board that is being touched. Think about a populated board going down on a hard surface. It is going to touch on some points, some corners of things, and the taller components.

What is taller? Well a switch, and LED or something like that. Yes, these things could be damaged by sliding them, or even touching them.

If you are just going to be stationary, going down on foam, as you suggested Phil that is okay. But in general I do not like to touch the surfaces of the board or the components any more than absolutely necessary.

Phil
Right, I agree. And again, I think the cleanliness issue is major. If everybody in QA is going to be so finicky, don't neglect the cleanliness.

In fact we believe that best practices is anybody handling the circuit board anywhere in the procedures, certainly prior to soldering, and this might have been after soldering, you really don't want to add any soils or contaminations anywhere ideally. If the board is being handled, it should be handled by a person wearing gloves or finger cots.

That is the absolute best practice. Think broadly on this whole thing,

Good, okay. Well, you have squandered five minutes listening to Board Talk. And as a disclaimer we have to tell you that Board Talk has not been FDA approved.

Possible side effects range from enlightenment to total bewilderment. And on that note, we say that whatever you do and however you are doing it don't solder like my brother.

Jim
And don't solder like my brother.

Comments

There are different performance characteristics of ESD matting. The of material that a facility uses is usually determined by the products being made and the processes involved in the production area. The types of ESD surfaces can range from hard surfaces such as an ESD laminates to softer vinyl or rubber surfaces that will provide more physical protection for the products being handled. Softer materials can also be beneficial in securing boards/products so that they do not slide around while being worked on and softer surfaces can be easier to pick up small components off of (similar to surface of a card table).

Other considerations for ESD mats include the material's resilience to solder and chemicals used at a workstation. ESD mat materials must be cleaned with an ESD cleaner regularly to extend the performance and aesthetics of the materials. Mat materials will wear-out over time and the physical characteristics of the material will change, but the ESD properties should be remain consistent if maintained correctly. Mat materials are meant to be somewhat sacrificial because the cost to replace the material is much less than replacing a laminate or table top that has been damaged.

Jeffry Brake, Desco
I have seen a lot of mechanical damage to SMT parts caused by sliding a board around on a mat. This is especially true for parts with a relatively large body size and small leads and pads. Think Tantalum or aluminum electrolytics, even right angle headers. I stress to our assemblers and inspectors, never to slide any board across a mat. It should be lifted straight up and repositioned. Try to move the scope around not the board. If you must do a lot of "sliding" around, conductive foam (not non-generating) can be used. In some cases the foam can be custom cut, especially if it is a repeated build. You might even use a ESD bag, and slide it around if there is nothing that will puncture it and get hung on the mat.
Alan Woodford, NeoTech
If there are screw holes in the PCBA's, you can sacrifice a bare board and put distance bolts (or whatever you call them over there) in these holes with the threaded part facing upwards. Then you can place your populated board on these screws and slide it over the table without damaging any components.

This also solves the problem with leaning boards causing it to always get out of focus due to different component heights on the bottom side. We use this method sometimes.

Unfortunately, I can't add a picture here, but this is the distance bolt type I mean. Hope you understand :-) https://asset.conrad.com/media10/isa/160267/c1/-/sv/521626_LB_01_FB/image.jpg
Thomas Lundman, CrossControl AB
That's a new angle. I did not encounter this issue/concern in over 10 years of doing ESD audits as a Mfrs. Rep. I'm not aware of any studies covering this but the ESD Association would be a good source if there were.

I have seen a large variety of tabletop ESD matting materials: name brands, no-name brands, vinyl versus rubber. I can think of two scenarios where the mat hardness could come into play.

#1 - an "old" dry/brittle ESD vinyl mat, especially one with years of flux residue, flux removers and other chemical damage. I agree with Phil that contamination may be a larger concern.

#2 - if the matting was originally intended as a floor mat, and was the hard/rigid type, designed for chair casters to ride on. In this case, it's possible for it to be abrasive in nature, whether new or well worn.
Mark Edwards, Hisco Inc.

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