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Fireside Chat: Identifying Voids on Bottom Terminated Components (BTC) Assemblies with X-ray Analysis

In this installment of Creative Electron’s Fireside Chat with the Xperts, Carlos Valenzuela is back to stoke the embers and help you watch your bottom…terminated components (BTC).  This discussion reviews X-ray inspection of BTCs, including BGA, QFN, and LED components utilizing 2D, 2.5D (oblique viewing), and 3D X-ray techniques.

X-ray inspection is often the only way effectively inspect BTCs for defects, and this presentation reviews many of the common defects, as well as some of their root causes.  The bonus track includes Carlos’ thoughts on keeping “design for inspection” front of mind while designing for manufacturing.  Watch this and other Fireside Chats with the Xperts here.

Transcript:

Dr. Bill Cardoso:

Good morning. It’s Wednesday, 10:00 AM, so it’s time for another Fireside Chat with the Xperts. And today I’m happy to introduce my good friend and VP of Engineering, Carlos Valenzuela, and today Carlos is going to address a topic that is a recurring question we have from our customers which is, void detection and how can X-ray help identifying, and of course, as time goes by, mitigating and reducing the amount of void on your solder joints. So with that, Carlos, you got the floor.

Carlos Valenzuela:

Oh, thank you Bill. So the presentation we got today is on … Just skip one. Identifying defects on BTC assemblies with X-ray analysis. So BTC is Bottom Terminated Components. So X-ray is one of the main analysis for this type of component. It’s probably the most used analysis for defect detection on bottom terminated components. So here’s a quick summary of what we’re going to go over today. If you probably joined another one of these webinars, you’re probably familiar with the SMT manufacturing process, where things are introduced or defects are introduced. A little quick intro on X-ray inspection, BTC and the need for X-ray. QFN, BGA, LED inspection. All of those are different types of BTCs. BTCs and obstructed views. The difference between 2D, 2.5D, and 3D X-ray inspection. And DFM and DFI, which is designed for manufacturing and designed for inspection and kind of the differences between those two.

Carlos Valenzuela:

So the design for inspection. So this is something that nobody really does, right? As engineers, you design something and you expect it to work and when it doesn’t, you’re kind of surprised, right? So you kind of have to prepare for that. You have to design with inspection in mind, and those are the couple of things that we’re going to go over today. And it becomes very troublesome and more expensive to troubleshoot or inspect components that are not designed to be inspected or with inspection in mind. So here is a traditional SMT kind of production line, so you have every single step of the process and every single step can introduce some sort of defects.

Carlos Valenzuela:

So defects associated with the solder paste. So you can have excess solder, solder bridges and insufficient solder. So basically shorts and opens. Then you go into the component placement, you can have polarity, you can have missing components, you can have misalignment, all sorts of issues. Some of them can be caught on an AI system, but talking about BTCs is going to be easier to catch them on an X-ray system. And on the reflow oven, this is where voiding is introduced. This is where the solders basically melt and it kind of behaves sporadically sometimes. So you can have insufficient solder.

Carlos Valenzuela:

You can have excess solder, missing solder balls, or more complex defects on BDA, so like head-in-pillow, and then coplanarity and things like that. And bridging and voiding, which are maybe the easier one. One of the main things that we want to also introduce is when you have an AXI usually it’s at the last step of your process. An AXI can analyze data, can collect data from other machines and give you a kind of like an overall process of how your line is doing depending on what sort of certain defects are happening at certain different processes. So you can go back and fix the process to have a more efficient overall result.

Carlos Valenzuela:

Here’s how basically an X-ray image is produced. You have the main components, which is a source and a detector, and then you have your sample moving in and out or closer or further away from the source, creating magnification. There’s a perfect example for like a BGA or some sort of QFN. You are penetrating the object and you are seeing inside and inside you can see the balls of the BGA or the voiding of a QFN. Next one. So here we can see a side view of the BGA. So BTC, that essentially is the BGA. We can see the balls, and then we can see the PCB. So with X-rays where we’re going to be penetrating these three things from a top view and seeing how the balls are actually attaching to the solder and to the PCB itself.

Carlos Valenzuela:

So here you see a top view. So here we got the PCB, we got the BGA, you see the little balls on the left side, and here is an actual X-ray image. So you can see all of these are just black circles. We don’t really see any sort of easy defects on this one, we can see very even colors, not a lot of voiding, and no missing balls, which is an easy one. Here we have a QFN and you can see a lot of bond wires, which is good, but right in the middle, we can see a lot of voiding. So that’s important because that’s where a lot of the heat dissipation is happening. So there’s not enough solder, the QFN could actually run a little hotter or not be able to supply enough current that it’s supposed to.

Carlos Valenzuela:

And that leads to failed product. Here, another quick view, we have an open pin. So over here we can see a circle that doesn’t have enough solder, so it’s not making a connection to that pin. Like I said before, some of these might be caught, but by some other test equipment, like an AOI. Once you see void, our TruView™ platform has void analysis tools like this one you can see over here, we can see a lot of voiding on this component and our tools are actually catching the voids on a void detecting tool. And on the bottom right, you can see a 3D rendering of it. Sometimes it’s easy to kind of picture that on a 3D aspect, you can kind of see the actual holes, even though they’re not, there could be holes actually. So you can actually generate report from this data. You can see actual percentages of voiding, so you can start analyzing what’s a good threshold for your product.

Carlos Valenzuela:

LED is another huge market for X-ray inspection, we’ve actually written a few white papers on how voiding percentage can correlate to failure rates and loss of product. So essentially here we can see the LED at the top, the die attach in the middle, and we can see a void, which is essentially a hole or an air bubble, there’s no connection in that area to the substrate at the bottom. And if we don’t have enough solder then the LEDs will fail. And they’re probably going to pass all sorts of tests. They’re going to turn on, they’re going to do all sorts of electrical tests, they’re going to be fine, but they could fail before they’re supposed to, or before warranty expires. So every single one fails warranty expires, you don’t have a product.

Carlos Valenzuela:

Here’s a couple of images samples of what an LED looks like under X-rays. So you can see it on the top, on A, the picture of the LED. We can see on B, it’s just an X-ray image. C is actually detecting the voids. And D is just the 3D rendering of that pad. Then here’s on our TruView™ software. You can see an LED on the left side, in our paddle boarding tool, detecting 28.45, and that’s a little bit on the high side for an LED, so this would probably be a failed LED. And you can see on the left side, there’s a lot of solder missing around the pad itself, so this one is probably missing some solder plus voiding and here again, another 3D rendering of it, and you can see on the right side, there’s a lot of color mapping, for different operators, different colors are better.

Carlos Valenzuela:

They’re able to kind of process that image better instead of just the black and white, we can go into any sort of scale. You can use like a rainbow, it uses a whole spectrum of color instead of just black to white. This is something new that we’ve been working on. It’s basically a live void analysis for LEDs. So as new product goes into the field of view, it automatically gets analyzed with our vision tools. Moving on to BGA inspector, BGA Inspector is our proprietary BGA analysis tool. So if you see the image on the left, we can see the BGA and we can see our software automatically select all the voids in the field of view and process them. But what’s pretty important in this case is that you can do a lot of reporting.

Carlos Valenzuela:

So once you get the data, you can analyze every ball, you can see if it’s good or not. But how you document it is also an important process. So if you can see on the right side, there’s an inspection report being created. So this can be saved under server, it can be saved on PDF, share with your customer, whatever you want to do with it. And this is kind of just reporting the data that you’ve acquired. If you don’t do this, then you just kind of relying on a person to look at it, see if they pass or not, and then moving on. So documenting is a big, big part of the process. To create better BGA tools we use what is called dual energy, and because the balls on the BGA tend to be very dark, because essentially it’s just solder, getting a good image on that will mean that you’re over blowing or over saturating the rest of the image, sort of what you see on the high energy image. And what dual energy allows you to do is to get two images and combine them together.

Carlos Valenzuela:

So if you see on the left side, we have a high energy image, and on the right side, we have a low energy image and our algorithms will combine those two images, giving you what we see the bottom, which is a dual energy image. And this will allow us to see the void inside, but to also see the traces that are going to the balls. So we can see there’s some blown traces. If there’s some missing traces or something like that, we can troubleshoot not just the BGA, but what’s around it and what it’s connecting to. This has been our go-to image to kind of describe every single defect that can happen on a BGA.

Carlos Valenzuela:

I mean, if somebody actually produced this image, they’re having really, really bad problems, right? But you can see some, like I discussed earlier, a couple of narratives. I mean, it’s misaligned, there’s some missing balls, there’s some shapeness, there’s some that because of excess solder, there’s a lot of voids, splatter, there’s everything, this is a perfect example of what can go wrong in a BGA. And one of the most important ones is head-in-pillow number three, which is sometimes you can detect it on a 2D inspection system, you need oblique or angle views. And what it means is, is that the ball essentially sits on top of the pad, but it doesn’t really melt into it, it doesn’t really adhere to it, kind of like your head on a pillow. So we can see here it’s … And the big problem with this is that sometimes they do work because they’re actually touching, but then you move the device a little bit and then it stops working. So those are the hardest one to troubleshoot.

Carlos Valenzuela:

So going back to what I discussed earlier about design for inspection, what we’re trying to avoid is making a system that requires complex 3D inspection. It’s harder to do, it’s more expensive, it takes more time, and it’s very complicated. So if you design a board or some sort of a device with inspection in mind, it can be a lot easier to troubleshoot and to actually analyze it. So here, we’re going to go over some examples of complex devices, like in this first example, see, we get a lot of capacitors on top of other devices. So capacitors tend to be very dense, so we can’t see behind it, we can’t analyze what’s happening behind, if there’s like a whole device missing behind a capacitor, you might not be able to tell.

Carlos Valenzuela:

So this is what we call 2D. So same device just by getting an oblique image or an angle view, you can start to see some of the defects that were very hard to detect in the first one. We see on the bottom, there’s a short right there, but because there’s a capacitor on top of it, we can’t see it. But by tilting or getting this angle view, you can kind of separate the two layers and getting some better views. You see there’s a short on the top, there’s some voiding over here, but again, this is adding a step. So what’s CT, CT is what we call 3D X-ray, it can be kind of complex, but I’m going to try to describe it as simply as I can.

Carlos Valenzuela:

So let’s see right here in the middle, we have our, let’s say this is our PCB. So the component at the bottom are the triangles, the actual PCB is the circle, and the components at the top is a square. So as we get different images at different angles as we’re rotating, the way that they are projected on the screen changes. So we can see number one at the bottom, the circle is in the middle, but we get the square on the top and the triangle at the bottom. If we go to number five, the triangle and the square flipped. So the software starts to understand what layer is on top and what layer is on the bottom, and that will allow us to separate layers and analyze layers separately. So we see here on this example, we’ve got a QFN on the top, which is red, PCB is blue, and resistor is gray, so they’re on top of each other in the image.

Carlos Valenzuela:

But as we do a CT and we rotate the board around, or the imaging around, we can actually split up and we can see how on the top side, the QFN is soldering into the board. But we can also see the bottom side, which is a resistor. So by splitting up or layering the board, we can separate the analysis and make them easier or something that it would have been very hard with 2D or 2.5D. And here’s a quick view of the three different X-ray styles we have. So we have 2D, which is just top view, 2.5D, which is oblique or angle views, and then 3D, which is the actual tomography. So being able to move, rotate your sample around, slice, and manipulate it as you please.

Carlos Valenzuela:

So it’s kind of been the topic of this presentation, the design for inspection. And like I said earlier, as you go to design or as an engineer, you never design your thing to fail, but that’s something to keep in mind. So keeping things away from each other, if you can move a capacitor two millimeters to the left or two millimeters up, so it’s away from another component. And those might be hard now because things are getting smaller, things are getting more complicated. But the goal is to have as simple as an inspection tool as you can, which is a 2D inspection, it’s better, and it’s a lot cheaper and faster.

Carlos Valenzuela:

Here are more examples. One of the things, especially in power components, you’re dealing with bigger heat sinks, you’re dealing with transformers, things like that, that will be very hard to penetrate. You’ve seen the bottom, there’s no way that you can analyze those BGAs and what companies do now, they do like sampling. So there’s a hundred balls in that BGA, they can test 50 and if those 50 pass, they just assume that the rest are okay. And it might be okay, but it’s not the ideal route. So if you can be able to X-ray every single ball, that would be the perfect scenario. Same concept here. We can see some capacitors on the left side. Well, capacitors everywhere actually. You can see right in the middle, there’s a short, but is it a short or is it a component on the other side? Those are the things that are kind of confusing. And this is actually two BGAs on top of each other. So you can see here, there’s a BGA on the left side, there’s also another one.

Carlos Valenzuela:

Any sort of inspection tool is going to struggle with this, especially because the balls on one side are bigger than the other one, so we’re going to rely on some visual operator inspection on these ones. And this is as simple as it gets, this board is not overly populated, this capacitor doesn’t have to be here. I mean, it can be two millimeters to the right. It can be just somewhere else. I mean, this is not a very busy or populated board, so it could have been made a little bit easier or inspectable.

Carlos Valenzuela:

Then another BGA, you can see this one has a lot of balls in it and they’re just some resistors right in the middle. And sometimes it can be avoided, sometimes you have to live with it. Here again, another sample. And going back again, this is just a more complex construction. This just has, I can see some transformers, I can see capacitors, I can see QFNs, this is going to be a struggle. I mean, this is going to be a complex application and you have to simplify it, you just have to concentrate on the most important components at a time. So like in this presentation, we’re looking at more voiding on QFNs, which there’s a lot of voiding in the QFN right in the middle. There’s another one here on the right, there’s a couple of big voids in there, but to automate this process, it’s going to be very difficult. Again, another good image of a larger QFN.

Carlos Valenzuela:

This one, like I said earlier it’s just overlapping two components right on top of each other, they can get complicated to analyze. Some heat sinks, I described earlier with power components, like this one looks like it’s some sort of a computer card or something. They tend to run pretty hot. So there’s a few key things on it actually. There’s one in the middle and there’s a little purple one, these are aluminum. I mean, some aluminum, they’re going to be denser. So they’re going to block your view and if you see here because we’re trying to penetrate the heat sink, we’re just overpowering the rest of the board. So the rest of the board just looks white, we can’t look at the image, we can’t look at anything essentially, but if we remove the heat sink, we can actually see a lot more. We can actually see every single ball and analyze it. We can also see here there’s some sort of, it looks like a transistor or some sort of device that has some voiding, not a lot, but we weren’t able to analyze that without removing the heat sink.

Carlos Valenzuela:

So just to summarize, we are trying to avoid 3D, it’s essentially the concept of it. 3D is awesome, but it is a more complex design, more complex platform. So when designing components, we’ve got to keep a DFM, design for manufacturing and design for inspection in mind. You do want to make your devices easier to manufacture, but you also want to keep them easy to inspect. There’s going to be some applications that require 2.5D and 3D X-ray, that’s just the reality of it. 2.5D is not complicated, it’s just moving angles and it moving either the camera or the sample just to get the side view that you want. But the way things are going, it’s going to be harder and harder to keep a 2D inspection model. Things are getting very small. I mean, the Apple watch has so much stuff inside, there’ll be no way that you can inspect something like that on a 2D inspection system. Things are getting so tiny and so small, very efficient and space and real state. So like I said, design for inspection, and we went through that one. Oh, and we’re done, any questions?

Dr. Bill Cardoso:

Yeah. We have a couple of questions that came on the chat.

Carlos Valenzuela:

Okay.

Dr. Bill Cardoso:

And I’m glad we have a couple of minutes so you can address them. So the first question is, does the location of the void on the ball matter, if the void is in the center of the ball, on the top of the ball, and the bottom of the ball, does that matter?

Carlos Valenzuela:

Yeah. I mean, we want to detect every single void and I guess it’s going to affect it differently, but our tools will just detect voids and how the location of the void makes your device react. It might be different, but the location will matter, ideally you don’t want any voiding.

Dr. Bill Cardoso:

Yeah. And then of course you get the location in Z, right? You have to do a 2.5D or 3D inspection.

Carlos Valenzuela:

Yes.

Dr. Bill Cardoso:

To figure out if the void is close to the board, to the component or right in the middle of the ball, which would be a different story. We have another question. I think we have time for another question. Is there any issues with the location of passive components? I think this question refers to one of the slides where you showed a QFN and the ball is pretty much empty and there was one passive component right on top of the pins basically blocking any chances of finding a short circuit between those two pins. Were there any issues moving those, I guess-

Carlos Valenzuela:

No, I mean from my little, I do have some experience in PCB design, passive components are usually filters and things like that. So they can be moved by all means, taking design in mind. So maybe moving it to the opposite side of the board doesn’t make sense because you’re adding traces and you’re making it more complex, but things like moving a capacitor two millimeters to the right, it wouldn’t affect it. There are some more complicated things for communication protocols and things like that, that the trace has to be a certain length that will be more complicated, but for passive components, it wouldn’t be an issue.

Dr. Bill Cardoso:

Thanks so much, Carlos. We really appreciate your presentation, great job again. And for all of you guys tuning in, we have another Fireside Chat with the Xperts next week. So tune in Wednesday, 10:00 AM Pacific, and this video is going to be available online in the next couple of days. So if you know anyone who wanted to join and missed, make sure to send the link so they can watch this online. Thanks so much, and see you next time.

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Creative Electron.