What is the difference between atmega32u4-mu chip and atmega32u4-au chip

Using Arduino Microcontrollers
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What is the difference between atmega32u4-mu chip and atmega32u4-au chip

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The AU is the TQFP package
The MU is the QFN

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Simply put, it’s the same CPU chip in a different package! A different package means different pin locations.