John Ternus, Hardware Engineering VP and Bob Borchers, Product Marketing VP recently spoke in a podcast about the process that led to the creation of the 4th gen iPad Air.
Borchers mentioned how the addition of the Touch ID sensor and its placement on the top button was an engineering feat. Additionally, Ternus says that the sensor had to have a narrow aspect ratio and sensitive enough to capture a broader fingerprint during the initial enrollment process.

For the iPad Air Ternus mentioned that the top part had to include both antenna and Touch ID sensor and how they wanted to ensure the two wouldn’t interfere with each other. He revealed that they went through great lengths of hardcore engineering and silicon to make the sensor deliver the same security as its predecessors.
The latest iPad Pro models sport the A12Z Bionic processor, while the iPad Air uses the A14 Bionic chip. Borchers explains that the A12Z is more on graphics while the A14 is more of a jack-of-all-trades processor.