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today i cut a fairly complex chip in half. it comes in this BGA (ball grid array) package. inside was something i didn't quite expect... 
inside it is *another* BGA package! it's the actual silicon chip, which is directly soldered onto a small printed circuit board that is the "outer" BGA package.
annotated cross section of a modern BGA-packaged SoC.
annotated cross section (closeup) of a modern BGA-packaged SoC. you can see the individual fiberglass bundles embedded in epoxy that make up the PC board substrate! also check out those stacked laser microvias.
so why is it made this way? the BGA ball spacing (pitch) is 1mm. the solder *bump* spacing is about 0.15mm. your board would need 10 layers and several layers of microvias just to wire it up -- expensive! think of this package as a breakout board.
IBM developed this technology in the early 1960s for their System/360 computers. these are SLD (solid logic dense) modules from that program.
instead of an entire integrated circuit, the modules held several tiny chips containing just one transistor or two diodes. three solder bumps attached them to the conductive traces.
this is an IBM transistor from 1964 next to a modern 0402 capacitor!
on the other side of the module, you can see the printed resistors.
