The information below is taken from MDK v.0.2
Rear module is composed of three sub-assemblies: the module base, printed circuit board (PCB), and shield. The module shell is not considered part of the module assembly. To the extent that it may deviate from a standard shell geometry, it will be created in accordance with a module developer’s geometric specifications, but will include a consumer’s aesthetic customizations and is presently envisaged to be manufactured as part of the fulfillment process (i.e., not by the module developer).
MDK requires the module base must be composed of a piece of machined anodized aluminum and one or two pieces of Hiperco-50 inserts (1×1 modules have a single Hiperco-50 insert). Hiperco-50 is an alloy with enhanced magnetic properties and thus enhanced holding force between modules and the Endo. The inserts must mount to the sections of the module base that correspond to the locations of electro-permanent magnets (EPMs) on the Endo. When activated, EPMs in tandem with the Hiperco-50 inserts should provide sufficient magnetic force to secure modules into their slots on the Endo throughout all nominal usage scenarios. When released, EPMs provide a residual magnetic force sufficient to prevent modules from falling out unless the user deliberately removes a module from the Endo. Users should be able to remove modules with minimal effort when the EPM is in the release state. EPMs require no sustained electrical power in either attach or release states and only a short electrical pulse to transition between states.
Except for cutouts needed for external interfaces (e.g., USB connector), the external dimensions of the module base must conform to the shape defined by the CAD model and drawings provided in the reference materials of MDK (Module Developers Kit). This requirement ensures the module is compatible with third-party provided module shells and fits snugly into the Endo frame. Custom milled cavities are allowed in the inside of the module base as long as the structural integrity of the module base is maintained and can survive module compliance specifications.
The back of every module must have a clearly marked module label. This label must include the Ara emblem, the name of the module developer, the name of the module, and a module icon signifying the module function. If applicable, module labels should include regulatory markings such as the module FCC ID and CE markings.
Figure below defines the specifications for positioning text and icons in the module label in relation to the interface block. If a module has multiple interface blocks, the module label must be positioned in relation to the leftmost interface block as viewed from the direction in figure below. Module labels should be applied with a laser-etched process.
The PCBs (interface block PCB and main PCB) contain the circuitry for the module, including circuits to enable interfaces to the Endo, as well as custom circuitry of the module. Any non-electronic components that are part of the module (e.g., batteries, sensors) must either mount to or be in place of the PCB. Table below provides maximum dimensions available for rear module PCBs. PCBs smaller than these dimensions are allowed. The main PCB must be securely mounted to the module base sufficiently to survive vibration and shock specifications. A smaller PCB for the interface block should be soldered to bottom of the main PCB and must conform to the shape and dimensions in the reference implementation to fit flush with the corresponding cutout in the module base.
A shield is necessary to prevent users from making unintentional contact with sensitive components on the PCB while changing module shells. The shield also acts as a Faraday cage to protect modules from potential interference and ensure a uniform RF environment for modules which are intentional RF emitters. The shield must be made of nickel-plated steel or a functionally and aesthetically similar metal. Cutouts for components that exceed the standard envelope are allowed (e.g., camera lens). The shield must securely mount to the module base.
The envelope for a module comprises the standard dimensions for the module to conform to the Ara Endos. While a module will ideally fit within these very specific dimensions, modules are allowed to exceed the standard envelope in the Y (top-bottom) and Z (thickness) directions. Modules are NOT allowed to exceed the envelope in the X (side-side) direction. Table below provides the absolute maximum exceedance limits; however, module developers should also use good engineering judgement to determine the stress levels from user levied forces and torques and ensure exceedances are structured to accommodate common usage scenarios including scenarios where the module and phone are in the user’s pocket. Modules with dimensional exceedances must have a custom shield and ensure all electrically active components except antennas and batteries are encapsulated within. Whenever modules exceed the standard envelope, the exceedance should conform to the Ara design language specification.