3.5 Box Build and System Integration

Mechanical Integration & Potting

The PCBA is installed into its chassis or enclosure.

Torque Control: Electric screwdrivers with calibrated torque sensors are used to prevent stripping plastic bosses or cracking the PCB. Torque data is logged for traceability.
Ruggedization (Potting): The enclosure may be filled with resin to seal electronics against moisture and vibration.
- Epoxy: Rigid, extremely strong, chemically resistant, but difficult to rework.
- Urethane: Flexible, abrasion-resistant, suitable for thermal cycling.
- Silicone: Soft, high-temperature resistance, easiest to repair but expensive.

The hardware is inert silicon until the software is injected.

Flashing: The firmware binary (Hex/Bin file) is loaded into the microcontroller.
Padding: Unused memory space is typically filled with 0xFF (the erased state of flash memory) or 0x00, depending on the checksum calculation method required.
Serialization: A unique identity (MAC Address, Serial Number) is assigned to the device. This creates the "Digital Birth Certificate" of the unit in the database.

While ICT checks if components are present, FCT checks if the device behaves.

The Test: The device is powered on and subjected to real-world stimuli (buttons pressed, screens checked, sensors activated).
The Fixture: FCT fixtures are often custom-built to simulate the user environment. Unlike the "Bed of Nails" used in ICT which contacts test points, FCT often connects via the external I/O ports (USB, Ethernet).
The Gate: FCT is the final "Go/No-Go" gate. A unit that passes FCT is deemed ready for the customer.

The "Out-of-Box Audit" (OBA) simulates the customer unboxing experience.

Cosmetic Inspection: Checking for scratches, gaps, or printing errors on the enclosure.
Packaging: Verification of accessories, manuals, and labels.