The iWatch has had a number of iterations, yet they seem to be minimal visible changes from the outside and as well as the listed specifications. But, looking at the X-rays shows with each generation, the density of the components inside is ever increasing. In Gen 2, the S2 module contains components within the module, but nowhere else. Gen 3 added components to the rear of the module on the S3, but didn’t make use of component stacking that was common in smartphones at the time. Gen 4 makes use of the common stacking techniques (Apps/memory, modem/ram, etc.) and takes it to another level with high-density passive modules in the PMU section.