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Fireside Chat: The X-Factor – How X-ray Technology is Improving the Electronics Assembly Process (Part I)

Creative Electron is excited to launch our Fireside Chat with the Xperts series with “The X-Factor – How X-ray Technology is Improving the Electronics Assembly Process (Part I).”   There’s none better than Dr. Glen Thomas to guide us as to how and why X-ray inspection has become such an important tool in electronics manufacturing.

This presentation offers a terrific overview of SMT manufacturing and how X-ray inspection informs every aspect of an SMT lines effectiveness, and helps to improve efficiency as well as quality.  Check out the full line-up of upcoming Fireside Chats here.



Dr. Bill Cardoso:           All right. Good morning. This is the first of our new series Fireside Chat with the Xperts. This is a new set of online presentations that we’re going to give. Since, with the 2020 coronavirus pandemic, we haven’t been able to travel and give these presentations live. So, we’re going to be doing that from our homes now as we enjoy the shelter-in-home lockdown. So, the first conversation we’re going to have is a generic presentation we’ve given a couple of times that it will cover a lot of the background on X-ray inspection. Some of the applications on how X-ray is used on a daily basis in our industry to improve manufacturing.

Dr. Bill Cardoso:             So, today’s presentation, as you can see here, we have quite an aggressive schedule. We’re going to cover quite a few topics. That’s the reason why we split this conversation into two sessions. So, we have today at 10 o’clock Pacific time and next week, or next Wednesday, we’re going to have the second half of this presentation. We’re going to keep on … We’ll basically cover everything we missed. We didn’t have a chance to cover today. And every Wednesday at 10 o’clock, after that, we’re going to have sessions of half an hour. We’re going to keep it short and to the point, so that we can fit in our busy, busy days. Even though we’re home, we are still quite busy.

Dr. Bill Cardoso:             So, I’m really happy to introduce you, today, to Dr. Glen Thomas, who is our VP of Technology. He is a guy who doesn’t really need a lot of introduction. He’s been doing X-ray inspection for 20 plus years, even though he’s super young, he’s been in this since high school, right Glen?

Dr. Glen Thomas:            Pretty much.

Dr. Bill Cardoso:             And, so today, we’re going to … Glen’s going to give a presentation. Next Wednesday, I’m going to cover some of the topics that we didn’t cover today. And with that, you have the floor, Glen.

Dr. Glen Thomas:            Thanks.

Dr. Bill Cardoso:             Let me give you control so you can run the presentation yourself.

Dr. Glen Thomas:            All right.

Dr. Glen Thomas:            How do we change, how do we change slides?

Dr. Bill Cardoso:             Let me give it to you, right now.

Dr. Bill Cardoso:             You should be able to do it now.

Dr. Glen Thomas:            What changes it? I guess we forgot to go over this earlier, didn’t we?

Dr. Bill Cardoso:             I know. Well, you just let me know and I’ll change the slide. Yeah.

Dr. Glen Thomas:            So, essentially, what we see here is we see a typical SMT line, right? We have the screen printer, SPI, pick and place, reflow and AOI, as well as X-ray at the end. This would be the ideal setup. Some companies will make a choice, whether they want to go with AOI or X-ray and ideally, you would want both. Most of the time, they will choose between AOI and X-ray depending on their needs and how many problems they have. Typically, with the X-ray and AOI, they both compliment each other. They’re not really inclusive one or the other, but most customers tend to pick one or the other. So, that’s just the way that beast works. Okay, next slide.

Dr. Glen Thomas:            So, essentially, you get a lot of defects with this manufacturing process for bottom components, bottom terminated components, and in solder joint integrity, in general. Essentially, with X-ray for the SMT line, what we’re looking for is solder joint integrity. We want to eliminate the excess solder. We want to, which would be solder bridges and in insufficient solder. And, in that insufficient solder, you’re looking at voiding and you’re looking at lack of solder joint integrity due to not having enough solder or solder paste, solder masks. So, in these three areas, you have quite a few different defects that would fall under those three areas. In X-ray, we’ll actually find that it excels in most of the areas, especially the insufficient solder. Okay, next slide.

Dr. Glen Thomas:            Again, we can look at misalignment, polarity and missing components, and this is where AOI does a really great job. And X-ray, it’s just a matter of being a plus. We can find misalignment, we can find polarity, in most cases, and we can find missing components, as well. But that’s not the main focus for X-ray, that would be more of a function of AOI. Okay, next slide.

Dr. Glen Thomas:            This is where AOI fails and X-ray excels, is in the voiding, the bridging, the solder balls, excess solder and insufficient solder. We can go over gull wing components with insufficient solder, later in the presentation, but X-ray excels at these five key areas of SMT inspection. Next slide.

Dr. Glen Thomas:            Solder joint measurement data, collected during X-ray test, can be used statistically to analyze, identify manufacturing drifts, trends and other processes, right? So, essentially, what we’re saying is we can use X-ray. We can look at a solder joint. We can determine based on what characteristics the solder joint has, we can go back and we can pinpoint areas in the process that are most likely to cause that issue. So, the X-ray will be able to give you an idea and monitor multiple processes and be able to show you drifts. So, if you notice a drift in the process, you will use X-ray to look at that and you can see a drift and you can actually go back and fix your process on the fly while you’re still producing shippable product.

Dr. Glen Thomas:            One of the key problems that we see a lot of is people will use X-ray only as a last resort and not necessarily manage their processes. So, what they do is they use X-ray as an, “Oh no, we have a problem when we can’t ship product,” right? Smart contract manufacturers and smart OEMs, use the X-ray to look at the process consistently, so that they can see drifts, fix the drifts before they actually have a problem. Okay.

Dr. Glen Thomas:            Okay. Basic principles of X-ray, X-ray, whether it’s medical or industrial all uses the same principle, right? We use an X-ray source, typically for small fine pitch components. We would use a five micron source or better. In some cases, if you’re looking at magnification on very small components, you will need a much smaller focal spot than even the five micron. Five micron focal spots, three micron focal spots are pretty much the standard in surface mount inspection in solder joint integrity these days.

Dr. Glen Thomas:            We always need a sensor. The old days, sensor was film. We had some digital detectors and back in the day, they were pretty crude. We considered them digital, but they were actually just a phosphorous and screen with a digital or a CCD camera. And then we went to image intensifiers. They became the de facto standard for many years in industrial X-ray and in surface mount technology.

Dr. Glen Thomas:            Now flat panel detectors are the standard. It’s rare that you’ll find an X-ray system with an image intensifier. And, if it is using an image intensifier, it’s going to be based on the application and the need, rather than just a general imaging device. X-ray sensors with digital detectors excel. And there’s a lot of reasons for that. The main reason is the digital gives us a better gray scale contrast, more spatial resolution and we’re not doing as much conversion. The image intensified systems have analog 256 gray scale. And then we pick that up with a camera and there’s a lot of inefficiencies. Then we’ve got to convert it analog to digital. So, you have some more loss in the inefficiencies. So, digital detectors, flat panel detectors are going to be around and they’re going to stay around there. They’re actually beneficial for companies when they do image processing. Okay.

Dr. Glen Thomas:            As far as … Let’s go back to one other slide, the back slide. As far as image processing, back in the day, image processing was pretty straightforward. We took an analog image, fed it out to a monitor, and we would look at it and you would use a dry erase type marker or a crayon and draw on the actual monitor. If you saw an issue, right, you would, we had three or four times frame averaging, and then we used last image hold. So, you would do a last image hold rundown. You would circle the problem, run down and get the engineer and show him the problem. Then we went to image processing, which. was kind of straightforward.

Dr. Glen Thomas:            It was about the late, mid to late eighties, we had some image processing available. There were only, really, three companies that were building image, processing computers at that time. And they were kind of, kind of archaic compared to what we can do with image processing today. But that also allowed us to use digital printers or actually thermal printers at the time.

Dr. Glen Thomas:            So, now we could save that image, right? And it became much more useful as time went on. Image processing today is actually the key to image X-ray imaging for all types of components, because we have the ability to do some algorithms. We can automate the process. We can save data and use that data to monitor the process. So, essentially anybody can build an X-ray system with an X-ray source and the detector. The secret is really having a beneficial software to analyze and understand the information that you’re getting from the X-ray machine. Next step.

Dr. Glen Thomas:            Why do we need X-ray? Back in the early, or, mid to late eighties, some bright engineer decided that bottom terminated components made a lot of sense, made the boards smaller, gave them a lot more inner connections. It just made sense to make a bottom terminated component. What they forgot about was, what we’ll talk about later, is the ability to inspect that and to actually verify that your process is working.

Dr. Glen Thomas:            So, essentially BGAs revolutionized the X-ray industry for X-ray imaging and surface mount technology. Up to that point, we would, we built a few systems, but most X-ray systems at that point were to look at wire sweep and we would look at components and we would look at bare boards for inner layer registration. It didn’t do a whole lot of X-ray for solder joint integrity, but the BGA changed that whole concept, and actually built the industry that we see today. Next step.

Dr. Glen Thomas:            So, if you take a look at a BGA, you’ve got a lot of balls underneath the solder joints. Those solder joints, there’s a lot that can happen in there. You have solder paste applications, you have solder masks, you have the component itself, placement of the component. You have co-planarity issues with the board or with the component. So, you have a lot of different issues that can happen on a BGA. In a perfect process, you put the BGA down, you apply some heat to it, the BGA self aligns and everything goes well.

Dr. Glen Thomas:            But it’s, the process of placing a BGA sounds fairly straightforward, but it’s actually quite a complex process that requires more or less perfect execution prior to soldering the BGA to the board. So, if he can take a look at the X-ray image, you see that we see some solder balls. So, essentially, those solder balls will give us a great indication as to whether we’re have a fairly decent connection to the substrate, right? In this ball in this image, you can see that we have some balls with no voiding. All the sizes of the balls are pretty standard and circular. So, it’s a fairly decent X-ray image of a BGA. And that’s what most people strive for in the industry. Next one.

Dr. Glen Thomas:            QFNs, QFNs are interesting, as well. Instead of the standard ball, we have a pad and we would need to mount or solder the components to that pad on the substrate. It’s a different process. Some of the same processes, as far as solder paste application, placing the component and your general placement of the products, pretty close to a BGA. One of the problems you have with QFNs is you have a lot less mass as far as the solder paste. So, your profiles can get a little bit weird as far as heat. And, they’re close to impossible to inspect with AOI. Same as a BGA.

Dr. Glen Thomas:            As far as the solder joint integrity on it, this image, those all look pretty decent. It’s not too bad. And when I say decent, what you’re looking for in this component versus a BGA is you’re looking for the solder fillet, the grounds, darker areas around the circumference of each connection. And in this case, if you had a bad solder joint, you would literally see instead of a nice rounded shape, then the nice flow from light to dark, you would see just a straight line. It should be easy to pick up. But those do look like pretty good solder joints. So, the difference between BGA inspection and QFN inspection is essentially we’re looking at solder fillets more so than the solder balls themselves. Okay, next.

Dr. Glen Thomas:            Right here, perfect example of a QFN. Essentially, what we see is, we see some voiding underneath the ball, the pads, in a few places, but essentially, we have a straight out open. There’s no solder fillet at all. And by solder fillet, you can look at the connections and you can see the black blobs all along the bottom. And by inspecting a QFN, you’ve got a couple of voids in there, but for the most part, you have two opens, essentially. They’re pretty easy to pick up and pretty straightforward. AOI would most likely could pick that up. Because it would be probably outside the component a bit on this component, but at what AOI wouldn’t be able to tell you is if you had solder flow up under the leads and you had a nice solder flow, but X-ray will give you that indication that you have solder flow. Because you can have a solder ball on the outside of that pad and outside of that pin and still not have solder flow or a real connectivity.

Dr. Glen Thomas:            And we can also see that all of those balls are somewhat round. When solder melts, it likes to puddle, and it will puddle in a circular fashion. When it’s cold and it doesn’t have a good solder connection or the solder paste application is not actually working well. What you’ll get is you’ll get a lot of angular solder joints. And, in solder joints, angular is always bad. It means that you have a cold solder joint and you have a lack of adhesion. So, even with AOI, you might be able to pick that up.

Dr. Glen Thomas:            There’s been all kinds of different concepts with visual imaging, a lot of silly scopes where they would try to look down under a BGA. And it just makes no sense for a production environment. Looking under each BGA is silly. It’s just pure folly because you can only see two or three rows in efficiently. And with that, you really can’t get an overall view of the solder joint, the circularity, the solder fillet. You really can’t look at BGAs with visual, using a scope with some type of fiber optic input. Next step.

Dr. Glen Thomas:            Void measurements, that’s the key, one of the major keys to the ability to monitor your process. You can look at the percentage of voiding. You look back at that voiding. Most voiding in BGA happens at the screen printer, your solder paste integrity. If you open a bucket of solder paste, you plop some in your printer and you go at it in the morning. You’ll get some really nice results. As time goes by, somebody might leave the lid off of that solder paste. The, the factory heats up from nice, cool morning to a later in the day, it gets a little warmer or in some cases it gets colder in the building.

Dr. Glen Thomas:            You will have a change in your solder paste consistency, and you might get some contamination in your solder paste. Voiding is also an indication of maybe your substrates weren’t clean. You have metalization issues in the substrate. They were shipped and they became, corroded to a degree while they were shipped. So,, the voiding will tell you a lot about your process, but it mostly goes back to the introduction of voids in your solder paste. Essentially, you don’t have a void until to you apply some heat to the board and run it through the reflow oven. And then, any impurities, in any air or any junk that you may have in the solder joint, will … Those gases will expand. And that’s what creates your voiding.

Dr. Glen Thomas:            For BGA voiding, you typically can do about 25%. That’s about usually the number that most people will consider a good solder joint. If you have less than 25% voiding, in all cases in BGA, you don’t want that voiding to penetrate the circumference of the ball. What that does is, it gives you some nice edges and will create a vibration crack over time and create issues. So, voiding will let you monitor your process pretty efficiently.

Dr. Glen Thomas:            And as far as voiding on that, one of the processes that you would have is to generate the report. Generating reports are really key to being able to monitor your process from shift, day to day, from shift to shift or from product run to product run. You put away a dirty stencil and you came back and ran the product again, and somebody forgot to clean the stencil. So, now you have a bunch of opens or you have a bunch of bridging, and that’s all based on the fact that your stencil was probably clogged or dirty, things like that. So, you can really apply X-ray to your process and get some good information. You can also generate a report for later reference and use it as a learning tool. Next slide.

Dr. Glen Thomas:            Solder voids and LED assemblies. Solder voids and LED assemblies are the death to an LED. Solder voids are a major problem for LED substrates. The substrate to the LED attachment is critical to remove heat. So, if you get a lot of voiding in your LED, you’re going to get a lot of warranty returns. And in most cases, if you have a lot of voiding, you just put a $5 bill in each box and call it a day. This is going to cost you money in the long run. So, the X-rays give you the ability to look at those solder defects and fix the problems.

Dr. Glen Thomas:            Most of the problems with LED is just the basic concept of how they’re attached and the power that’s applied to them, essentially. You can always tell when someone really doesn’t understand LEDs, when you buy a flashlight and it has a two pound heat sink on a AA flashlight. It’s, they’re trying to pull the heat away because they had no clue how to do it mechanically with the substrate to LED attachment. Okay, next slide.

Dr. Glen Thomas:            Let’s see where we go. Essentially, it’s what you’re looking at right now. Led inspection, major flaws in the process and major voiding. Essentially, the LED process is the same, in most cases, to a BGA inspection. We’re looking for excessive solder, lack of solder and primarily, in this case, voiding. The major difference is we’re looking at a square or rectangular pad instead of a BGA pad.

Dr. Glen Thomas:            So, the LED voiding calculations are important and they use a different criteria than a BGA void would. We’re interested in, not only the amount of voiding, percentage wise, as compared to the pad, but we’re looking at the largest void as well. As well as in this case, you can see a lot of extraneous BGA ball or not BGA, but solder balls underneath the component, as well. So, there’s quite a few different problems with this component as it’s mounted. Okay.

Dr. Glen Thomas:            Another one, essentially here, we’re just looking at voiding in the attachment, it’s pretty straightforward. It’s pretty simple. This one we would use a rectangular instead of the BGA ball. Voiding is pretty easy to pick up, but also we can see a lack of solder fillet, as well. That’s why you’ve got a nice rectangular, outer edge around this component. It doesn’t have a real nice solder fill. This could be a solder mask issue, more so than a solder paste issue.

Dr. Glen Thomas:            Same thing, same concept. I found a new love for the color contrast. Over the years, that was a worthless environment. You would take a nice, back in the day, 256 gray scale image, and you would apply some color to it and you would bring it down to 32 colors, pretty much worthless. The new algorithms that we’re doing, and the ability with computer power, as well as monitors, the whole concept and digital detectors with thousands and thousands of gray scale, allow us to do a much better job with color.

Dr. Glen Thomas:            At one time, pseudo color was just used to impress the VPs up in the office and to impress the customer to a degree. The advantage of colors and the pseudo color is that we can tie a color to a specific gray scale value. And it makes for a much faster inspection routine. In this case, we’re looking at the solder joint itself, in red, and then we’re looking at the traces in blue. Following traces in an X-ray is a really tedious task because of the multilayers.

Dr. Glen Thomas:            So, having the ability to take a layer or a density and assign a color to it, and then follow that color, it makes a lot of sense. If you were training a person that has no concept of X-ray and you tell them that you want to see red in all of the solder joints, that’s one way to train in an operator pretty quickly, and it works. But in this case, we’re applying some other filters, which gives you a contoured effect as well. So, it really makes those voids stand out. All right, next one.

Dr. Glen Thomas:            Automated X-ray, essentially, in this case, what we’re doing is where we were looking at an automated X-ray system, which essentially, comes in, component comes in, we take an image, we instantly measure the voiding and then move on to the next part, pass or fail. And that gives us the ability to provide quite a bit of data on the products. The future of X-ray imaging is going to be automated X-ray. Where we eliminate the operators concepts of what is actually a good solder joint and what’s not. One of the problems with operators and with manual X-ray is, it’s all dependent on the person, what they consider a good X-ray, for one, which means the settings of the machine can change drastically.

Dr. Glen Thomas:            Monitors, you look at a monitor, one person sees a perfect image. Other person says, “No, it needs a little bit more contrast or it needs less contrast.” So, there’s all these inefficiencies when operators are involved in X-ray. They decide that it … There were some medical studies years ago that they took 50 or 100 radiologists, and they showed them exact images. And they came up with 50 or 100 different concepts of what that image was. Not only was acceptable or not, but what the outcome for the patient was, and they brought those same radiologists back two weeks later and gave them the same images. And even the radiologists couldn’t agree on what was a good image and what was not, and what was passable. So, taking the operator out of the equation is key to getting decent and consistent results, to be able to monitor your process. All right, next one.

Dr. Bill Cardoso:             Glenn. So, we got you to the top of the half an hour, 10:30. I know time goes by. So, we can start to on a BGA inspection next week, but I wanted to open for questions for about five minutes. If people have any questions they want to address now, and of course, they’re always welcome to email us or text or a tweet, Facebook, LinkedIn, Instagram us. We are on every platform and happy to address. So, anyway, open to the floor, you can raise your hand if you have any questions. Otherwise, we’ll wrap up the video today and start again with our next Fireside Chat on Wednesday with again, with Glen and I in this same Zoom meeting, same time. So, it sounds like we’re going to take questions offline today, Glen. Thanks again for the great presentation. Thank you all for attending. And I’ll see you guys again next week. Thanks so much. Bye bye.


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