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Flip Chip is a methodology in which the the connection between the padcells and bond pads can be done after some layers . Advantage of this technique is the routng lenght between padcells and band pads are less means reduced.
With flip technology die is turned upside down and directly attatched to the substrate using slder bumps.
due to this some percentage of top metal layer is not avaliable for signal routing.
flip chip technique avoids the traditional wirebonding machines , in which the connectivitity through gold wires is done in the sewing machine order, i.e., serially.Hence it is a time consuming technique also for the designs with high pin count it may take quite long and much prone to shorts between the wires.
Flip chip technique involves the direct contact of the die( faced down ) to the polymer tape on which predesign pattern of the I/O grid is layedout and the connections can be made parallel by solder bumps.
Another advantage of flip chip is that you can get a lot more pads on the chip since you have pads over the entire chip, not just on the sides. This can be very good for power and ground connection since you can have low inductance, low resistance power grids, as well as have more signal i/o. Since IR drop & I/O for a large, high power chips are a big problem, microprocessors have been one of the first adopters.
One disadvantage is that it's pretty difficult to do thing like micro probe or FIB the chip if there are problems with the design. All you have access to is back side of the chip, and you have to dig through the substrate to get to your probe points. (Hopefully there's no active under it....)
**broken link removed** is an excellent free resource on the vast array of flip chip technologies... there's a lot more to it than just solder bumping (which is very limited in bond pad pitch), there's also gold ball bumping, MicroPosts, and polymer bumping, just to name a few.