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Welcome to our latest X-ray University Episode, where we’ll be discussing the intriguing practice of BGA stacking. This technique involves stacking one Ball Grid Array (BGA) on top of another, offering solutions for design adaptations and supply chain challenges.

We’ll explore why BGA stacking is utilized, from adapting to design changes to overcoming component obsolescence, and we’ll also highlight the importance of X-ray analysis in ensuring quality control throughout the manufacturing process. Join us as we briefly overview BGA stacking and its impact on modern electronics manufacturing.

Today we’re going to be talking about BGA stacking. And we’ll start with what is BGA stacking? BGA stacking is when you literally stack one BGA on top of the other. Now, this second or the bottom edge, usually an interposer or substrate, a reducer, an expander, different companies call different things, but that’s when you have a BGA on the top assembled onto another BGA array, and then this package is assembled onto the PCB.

Now usually what would we do is you have the top one assembled on to the bottom one, and then this whole package assembled on top of the PCB. Why would you do that? Because that increases the complexity of the assembly by quite a bit. What would you do that? There are a few reasons you can have design changes, right?

If, for example, you have been using one type of BGA and you decide that because of a mistake or because of changing requirements of your product, you have to go with a different type of BGA. You can use this new, bigger one with the same footprint as the old one. Thus do not require you to redesign the PCB, which can be very costly.

Right? So you have two options. Either you redesign hope should be the origin’s design and interposer, or that will be what you made to this new BGA to the old footprint you had. Another reason is obsolescence, right? So if the component becomes obsolete now, you have to go with, you know, an equivalent or an analog component. They very likely will have a different footprint.

So you design an interposer to interface that new BGA with the old one that you have. Again, you have the option to redesign the whole board, but that’s usually quite expensive. Lastly, and this is one of the reasons that we saw a huge uptick in this technique in the past few years with the supply chain shortages due to COVID.

Is that all of a sudden that component you’ve been using that you design your board to use, that doesn’t exist anymore, you just can’t find it. So you end up having to choose and pick something else that’s available on the market and something that might not have the same footprint as the old one that you’ve been using. For that reason, you have to design an interposer. It’s going to allow you to interface this new component to the old. With print, you had the same story, different reasons. They all create complexities in the assembly, and to show you how these complexities and how this is going to show up in an X-ray machine, we’re going to use one of our test cards to exemplify how you can identify and how you can fix these issues on your assembly.

Okay, Anthony, this is the BGA stacking issues video. Now that we saw why and how BGAs are stacked, let’s go check here in the X-ray machine. Exactly how that stack looks like and how to identify potential problems of BGA stacking. So starting with the stack itself, you’re going to see we have two different ball sizes right here.

So the balls that are larger in diameter and the balls are a smaller diameter. So that’s representing the BGA itself, the component and the interposer. In this case, there are quite a few head and pillow and opens on this package, and I have some images I want to share with you that so you can identify how they look like on the straight-down view.

An open is going to look like this. You have this other ball right, which is the dark sort of circular object, and if you don’t have a connection, you’re going to end up being able to see the pad itself like this. A clean pad shows that you have an open. There is no wedding between the soldered onto the pad.

We have then the ability to move the sensor at about 40, 45 degrees to have that side view of the component. So that’s what some people call what. And I have to, I don’t like that because there’s no such thing as a half of a dimension. But, you know, it’s not 3D, it’s not computed tomography, it’s not a 2D. It’s future DX, but it’s a tilted view. 2D OC. But if you’re here two and a half D, that’s what it means. You have a tilted view onto the component. So this is a really nice one to show you have three different balls here in this row. This one has some wedding, as you can see here. You know, there is, there’s a fill it of solder connecting the ball to the pad, and you can see that fill it right here. This one, there’s nothing. You see the pads right here, nice and clean, and the ball is also nice and clean. And if you see a round ball in this tilted view, it’s usually a bad sign because it means that the ball didn’t collapse, which means that there was no wedding either because of temperature was insufficient, the temperature of the reflow was too low, or the temperature was right and you just didn’t leave it there enough to properly soak and get that solder to melt and wet to the pad, or something happened on the pad. Some surface was dirty or the surface finish wasn’t proper. And as a result, even if the solder was melting, the ball wasn’t able to get to the pad and make that nice clean metallurgical connection that you want to see between the BGA and your board.

So all these things play a role or because the BGA was sitting too tall, right? And you just saw the ball melted here, but it was too high because a couple of early issues or potato chip or popcorn, right? So the BGA was too tall. And even if you do the solder was melted in liquid. It wasn’t able to physically touch the pad. And as a result, you ended up with the solder, the ball melting, getting liquid, and I’m not connecting to anything and then solidifying again far away from the pad. So we know the reasons are diagnostic features that you have to go back and figure out where in your manufacturing process the mistake was made, but this is how you identify it on the x-ray.

And that’s why if you’re just taking x-ray images, you’re collecting data. Data by itself is useless. Data you don’t use has a very specific name. It’s called noise data. You don’t use noise data that you act on that you use to identify processing issues in your manufacturing line. It’s got information. Information is data you act on. And what we want to do is have you using your x-ray machine as an information source, not as a data source.