Last updated on: Nov. 17, 2023, 1:29 p.m.
Test System Development Process
Test Systems are designed to execute automated Functional Test Plans, and are most often implemented in production-level fixtures (development-level fixtures also occasionally fall into this category). Simple Test Fixture Designs (mechanical-only fixtures) do not require this level of design effort. FixturFab has developed a standard, comprehensive process for designing Test Systems that will be explored in this series of articles.
During the Design Phase, FixturFab engineers complete the mechanical and electrical components of the Test System that was architecturally defined in the Blueprint Phase. Design outputs from both disciplines are then made available to FixturFab customers for review and approval.
Some of the most important initial engineering work must be conducted by FixturFab on the many mechanical aspects of the Test System. Fitting exercises are completed in the Blueprint Phase to ensure the correct fixture size has been selected for the project, but detailed work must now be executed to inform the Electrical Engineering work that must also be done (PCBA size and mounting hole locations for the TPCBs, for example).
An example CAD Model of a multi-DUT Test System fixture is shown in the figure. In this fixture, 6 DUTs are processed in a single test run, so the spacing is a big design element. All components of the Test System, including the CPU subsystem and all instrumentation, are contained inside this particular example, requiring precise and thorough mechanical consideration.
The interior of a Test System fixture can be a busy place, with a multitude of power supplies, communication hubs, computers, instrumentation, various other components, and all the wiring that connects the system together. Careful planning is conducted in the mechanical realm to optimize the placement of all these internal parts and wiring pathways.
Each DUT location that can be simultaneously loaded in the Test System (there may be one to multiple) has an associated mechanical stack-up made of of the fixture’s probe plates, DUT mounting pins, test point carrier board (TPCB), instrumentation, etc. Each stack-up is designed to be mechanically (and electronically) independent of each other, because FixturFab’s Test Runner software framework makes it simple to run each DUT’s Test Plan in parallel, independently and asynchronously.
The back panel of the Test System fixture provides connectivity for power, the monitor, and other peripherals (mouse, keyboard, scanner, etc.). A small fan is common to evacuate heat, and normalize interior moisture levels with the surrounding environment in the event of fast temperature changes. Each back panel is customized to the particular Test System requirements.
Utilizing the KiCad EDA platform, PCBA schematics are prepared by FixturFab for the TPCB and other circuit boards that have been designed into the Test System. Many times, support circuits must be custom designed to implement the Customer’s Test Plan, and these generally fall into the following categories of expertise:
- Power injection circuits (to safely power DUTs)
- Continuity test circuits (supplies, connectors, etc.)
- Driver circuits (LEDs, motors, relay control, solenoid control, etc.)
- Multiplexer & demultiplexer circuits (analog & digital)
- Voltage level shifters
- Peripheral interfaces (memory card, etc.)
- Networking interfaces (Ethernet, WiFi)
- Simulated loads (battery simulation, charging)
- GPIO control instrumentation & related circuits
- Communication translators (USB, UART, SPI, I2C, CAN)
After schematic review and approval by the Customer, the boards undergo layout activities, a bill-of-material is created, and all design files are submitted to a CM for manufacture and assembly. Modern PCBA manufacturing companies can typically turn around the finished boards quickly, and they are back in the FixturFab lab in approximately 2 weeks or less.
A 3D model of the finished PCBA is made and checked by FixturFab engineers to ensure it fits properly into the fixture, as designed. All of the attached components are modeled to make sure we can determine if there is any mechanical interference between the PCBA and other Test System internal components.
As soon as the various mechanical and electrical design work begins to solidify, procurement activities kick in to order any non-stock components of the Test System, to ensure materials are on hand for the hardware build (machining, cabling, etc.). Common items that get purchased at this time are:
- CPU subsystem and related components (DRAM, Drives, etc.)
- Fixture base & blank cartridge plates
- Rack components
- Electrical components for PCB assembly
- Wiring and cabling