Design for Test: Board Design

When designing a new circuit board, it helps to imagine a test station that will do the following: Scan, Test and Mark the device under test.

Broadly, this means you need: a unique identifier already present on the board, a way to quickly test it, and a method to mark it as passed (or mark it as NG if it fails)

Here are some more detailed tips:

Dimensions

  • Size the board so that multi-board panels can be less than A4 size (210mm x 297mm).

  • Using A5 (148mm x 210mm) or less enables more fixturing options.

Mounting Holes

  • Use unplated mounting holes (plating affects the tolerance) with a diameter of 3.1mm to allow for commonly sourced tooling pins and standoffs. Mounting hole diameters of 2.1mm or 2.6mm can be accommodated if 3.1mm is too big. The mounting hole needs to be slightly larger than the tooling pin diameter so the board can be easily inserted into the fixture.

  • Arrange mounting holes in an asymmetrical pattern to prevent improper loading into the test fixture or housing (“poka-yoke”)

Top Side

  • Place all SMT components on top if possible to reduce stencil and other setup costs.

  • Limit component height to 10mm to avoid interference with the pressure plate.

  • Include Keep-out space for Board ID label (ours are 8x12mm).

  • Include Keep-out space for “PLT Tested” label (ours are 12x24mm).

  • Include circular (6mm diameter) Keep-out areas for push rods spread across the board, not directly above any test points as they can cause collisions when closing empty test fixture. The best place for these is directly above the PCB support pins.

  • Include 3 Fiducial marks.

  • Include PCB part number and rev number in the silkscreen (PCBA part number can be on a label).

Bottom Side

  • Create distinct test points as components and label them “TP1”, “TP2”, etc. Use test points of 1.0mm diameter with centers 2.54mm apart (100 mil pitch) if space allows. Bigger is okay. This makes it easy for DuPont cables to plug onto the test probe receptacle tips.

  • The smallest possible pitch is 50 mil, which implies test points of 0.635mm diameter with centers at least 1.27mm apart. Quick-turn fixtures require hand-soldering of test probes.

  • Use Keep-outs of 5mm diameter circles for PCB support pins, if feasible.

  • Include 3 Fiducial marks, especially if there are components to assemble.

DON’Ts

  • Avoid DIP switches

    • confusing because you have to describe to an operator how they should be set.

  • Avoid using potentiometers requiring manual tuning, there are digital alternatives.

  • Avoid test points on both sides because that increases the complexity of the test fixture.

See also