Is X-ray Inspection an Essential Tool? Print E-mail
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Written by Alan Cable   
Saturday, 05 March 2011 00:09

Absolutely, and its benefits go beyond quality assurance.

An essential part of selective soldering process development is inspection and verification of the results. Assemblers need to qualify their process, and x-ray inspection is the most reliable method for knowing solder connections are reliable on the inside as well as on the outside.

We’ve had x-ray capability in our selective soldering lab for more than two years. X-ray is critical because when processing samples for a customer, we need to be able to verify the integrity of the solder joints that we’re creating. If we can’t, then the process we are developing may not be robust either. Our position is anyone who does not have access to x-ray inspection or solderability and ionic contamination testing is flying by the seat of their pants.

Compounding the problem: During process development, we generally have the opportunity to process just one or two of a customer’s boards. We cannot dissect, section or destroy those boards, or conduct destructive testing or evaluation afterward. But voids are a fact of life in virtually any kind of soldering, and they can have a negative impact on the integrity and acceptability of the solder connection. These voids could be filled with a chemically-active material that could have a deleterious effect on the joint over time. And there have been times we could not even see a topside fillet because the component was covering it and obscuring it from view. X-ray inspection removes any doubt, and allows an accurate assessment to be made.

Once installed, the x-ray machine proved its worth immediately. We found three of 10 samples that we ran, on average, had imperfections we otherwise would not have recognized from a surface visual inspection. This was an eye-opener.

And, once we observed voids in the barrels, we were able to come to some hypotheses about how they were formed. We discovered that in some cases it was due to insufficient or defective interior plating, a component solderability problem or a cleanliness problem. We could not always remedy the situation ourselves, but we could inform the customer that they might have a problem with certain areas of the assembly. Sometimes the problem was due to the type of flux we were using, where the flux would outgas and cause voiding. This gave us the opportunity to change the flux type and mitigate the problem. In other cases, we would modify the process by the addition of topside preheating, lengthening the dwell time over the soldering nozzle, or tweaking any number of other process variables. The result would be a robust process that would meet the customer’s requirements. In the end, as we say, the machine itself is not the panacea; it’s how you use it, and how you develop a robust process beforehand using key tools.

In some cases, we examined completed samples our customer had provided to show how they wanted the board soldered, and found voids in their samples as well. This surprised them, in many cases making them aware for the first time they had problems.

PCB assemblers concerned about the integrity of solder joints for Class III standards ought to consider having an x-ray inspection system as part of their own process development laboratory, particularly if they are a high-volume or high-mix assembler, no matter if they are hand, selective, or even wave or reflow soldering. It’s also key to compliance with the appropriate IPC acceptability standards.

Any good process engineer will want to know these things ahead of time. In another scenario, our x-ray machine saved us from a very uncomfortable situation with a prospective customer that was evaluating our machine. We had been sent samples to process and were having a difficult time making the results of our process development suit the customer’s requirement. It was a situation that could have become unpleasant; they felt that our system was not up to the task, but we suspected that the samples they were providing were defective. Thanks to an x-ray machine and a solderability tester, we were able to show defective plating in the barrels of the samples. Our prospective customer went from argumentative to appreciative, because now they knew that the problem was with their board supplier, not us, and the evaluation saved them headaches that would have occurred down the road.

I consider an x-ray, wetting balance tester and ionic cleanliness tester essential to developing a good selective soldering process. I’ll talk about these latter two in a later column.

Alan Cable is president of A.C.E. Production Technologies (ace-protech.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Monday, 07 March 2011 15:17
 

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