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.
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?