Building an Automated Test Jig: Requirements & References

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Blue Clover Tester Chassis

As the number of projects We’re supporting increases the growing support requirements are growing out of control.There is a dire need of a standardized approach to test the electronics. Currently at Hobbs ElectroOptics and ElectroOptical Innovations we rely on standard flying probe bare board testing and automated optical inspection followed by built in self test. Some projects have a functional test stand. Tuning is done mostly manually. Like so many other things in this field that this introduces a high mental load; you have to be operating at your best to pull it off. The aim then is to have a testing approach that is more fool proof and easier to transfer.

Requirements

  1. Adjustable: Allow test decisions to be delayed until late in the process. Ideally the design for test rules will allow all but the most critical tests to be worked out after the design is complete.
  2. Reusable & Standardized: Parts should be interchangeable as much as possible with the customization in the easily customizable parts. It’s okay to have 100 lengths of M3 screws, its less okay to rely on a custom die cast from a single source.
  3. Self explanatory: The design should scream its function. The operator should be able to figure out the operation without a manual.
  4. Minimum support: As this is end of project deliverable the device should be serviceable by other engineers. Custom electronics should be avoided is a COTS version exists. A data acquisition device should be used instead of integrating our own.
  5. Quick Turn: The device should be manufacturable with minimal handwork. Ikea style flatpack is fine, dead bug wired electronics is not.
  6. Interchangeable parts: This is related to standardization. Design in replaceable parts to avoid an entire system redesign. The form factor is relatively unconstrained so this should be straight forward.
  7. Mechanically Robust: Parts that wear out should be easily replaced. The design should not require the operator to be ultra careful to avoid breakage.
  8. Electrically Robust: Excellent ESD protection and EMI protection (or insensitivity). Ground loops should either not be possible or not effect the test.
  9. Compact: Should be easy to store and ship. There are secondary benefits to this such as being able to carry additional stock and keeping material cost low.

For the standard industrial method of doing this see National Instruments

Design For Test

The way I see it there are two classes of designs that use these jigs:

  1. Flexible: The design can support changes to increase testability.
  2. Inflexible: Either the design is already completed or its complexity such that it is better for the test system to carry some of the complexity.

Flexible designs can use more standard interfaces and even share jigs across similar projects. If the design is simple and the testing requirements are low, then a reusable or standardized jig may be ideal. Clamp fixtures, tag connectors, or the like may be a good option. The entire complexity of a formal test jig may not be required.

Clamp Fixture

Clamp Fixture

Higher complexity flexible designs may use a standard layout of pads to speed up development plus sections of the jig to be reused. This could be done by designing a footprint or algorithmically placing pads with something like https://github.com/snhobbs/kicad-parts-placer. Placing pads by hand each time should be avoided.

Inflexible designs will need the fully custom jig. Complexity can be reduced by minimizing the number of pin types if possible. There are many ways to connect to features and a few design rules can ease complexity later. A single type of test pad, through hole, via, or pad should be used unless required. If a different pin or pitch is required this should be treated as a complexity and cost tradeoff. Major design changes like double sided testing should be treated similarly. See References & Resources: DFT Guides for DFT guides.

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