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AnandTech: The MacBook Air CPU Mystery: More Details Revealed:

Why did Apple and Intel opt for a hotter than necessary chip for use in the MacBook Air? Here’s where our trail goes cold but we suspect that in order to bring the smaller CPU/chipset packaging to market earlier, some tradeoffs had to be made. Remember that CPU packaging controls far more than how big the chip is, but also governs FSB frequency, power delivery and getting data in and out of the chip itself.

The shiny die connects to hundreds of pins on the bottom of the package. The more pins that need to be connected, the higher the FSB frequency and the smaller the chip the more strain this puts on the packaging technology itself. It’s quite possible that one side effect of the small form factor CPU package is worse power delivery, requiring that the chip be given a higher than normal operating voltage.

The bigger concern however has nothing to do with packaging technology or operating voltages, but overall thermals. The MacBook Pro runs very hot and while the 20W TDP of the MacBook Air is significantly lower than the 35W TDP of the Pro, it’s high for such a small chassis. We won’t know for sure how hot the Air will get until it’s in our hands but the SSD route seems like an even better bet now that we know a little more about what we’re dealing with. Cutting down heat in that thin chassis will be very important, and moving to solid state storage is the only real option you have there