OMAP3530 Package on Package Layout

If you've read my prior post, you know that we've built up some package on package (POP) boards. We built up some test boards with dummy components and some working Beagleboards. For the Beagles, we didn't do any of the design or layout work here. We just used the open source Gerbers and BOM files provided at www.beagleboard.org.

BB pop cut out populated I've been reading through the Beagleboard.org message board and pondering the issues related to such a design. At first thought, it seems that placing the part in the layout program shouldn't be too much of a problem. I think most CAD software will just let you set one part on top of another. You may have to ignore the DRC checks for that part, but other then that, it shouldn't be a problem. The Centroid (X,Y,theta) file shouldn't be a problem either. The two reference designators will just have the same XYt value.

What I'm really wondering about though is the library component - the footprint. How do you make a component with pads on the top, in addition to pads on the bottom? What copper layer would you put the top pads on? You'd need to put the pads for the top POP part on that missing copper layer too. Will CAD packages let you create extra copper layers with a Z-offset? Layers that float in the air? Weird.

I don't have much experience with big-system CAD packages so I don't have the answer to these questions, but I expect it's probably not to difficult. Here's my first guess: just make an extra layer and call it pop layer. In the library tool, put the top pads of the OMAP chip in that layer and the regular pads of the POP memory chip also in that layer. Then when you export the Gerbers for board fab, don't include that the pop layer or at least make sure the fab house knows that it isn't a copper layer (or any other layer to go on the board).

On the other hand, I guess maybe you could just not worry about it and in the schematic and layout, simply treat the POP set as one chip. Just make sure that the assembly files (Centriod, BOM and any build instructions) tell the assembly house where to place the POP part. Hmmm. I don't know what would be best.

Has anyone reading this made a POP layout? What did you do?

Duane Benson
Goober snorts

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Comments

The pads on the top of the processor package have no impact on the layout at all. There is no need to have them reflected in the footprint because they in no way touch the PCB and do not require anything in the PCB itself to facilitate their usage. You just mount the POP memory on top during the assembly process.

I don't think you're missing anything. I'd say you're right. It's probably just a documentation issue and as long as the manufacturing folks know to put it there, it's all good. And thanks for the compliment!

I don't quite understand why it should be of any concern for either schematics or PCB design, that the part is a PoP instead of a normal BGA? You draw the library component just as you would for any BGA. The pads on top of the lower package are ignored in the library component. The only tricky thing is the Centroid file, which can be hand edited so that the top package is placed on the lower package, in the same coordinate. Or am I missing something?
Btw: I follow many blogs, but yours is one of my favorites!

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