Design For Testability Guidelines

DESIGN FOR TESTABILITY GUIDELINES
Information Compiled by Scott Cufr

All test targets are to be on one side of the PCB but both top and bottom are accessible.
Test Target Priorities: 1) Component Leads 2) Test Pads and 3) Via's
Center to center spacing between test points or adjacent drilled locations should be as follows:
- 1) Two .100" probes with a minimum center to center spacing of .085".
- 2) One .100" probe and one .075" probe with a minimum center to center spacing of .075".
- 3) Two .075" probes with a minimum center to center spacing of .070".
- 4) One .100" probe and one .050" probe with a minimum center to center spacing of .070".
- 5) One .075" probe and one .050" probe with a minimum center to center spacing of .060".
- 6) Two .050" probes with a minimum center to center spacing of .050".
Test targets should be at least .050" away from components on the side of the PCB being tested.
Test points are to be distributed evenly over the surface of the PCB. Areas of high probe density should be avoided.
The preferred size of test targets is .035" (or .040" on top-side targets). Square pads are preferred, due to the increased target area (27%) over the round shaped targets. Targets that are less than .030" will require the probe tips to be guided to the targets.
Solder mask should not cover any test pads or vias.
Test targets should be at least .100" away from the edge of the PCB or "break-away" areas. This will allow the gasketing material to meet evenly with the board surface and create a good vaccum seal.
PCB's should have a minimum thickness of .062". This helps minimize board flex.
Tooling hole diameters should preferably be .125". The tolerances should be held at +.002/-.001 and the locations should be placed in opposite corners.
Tooling hole to pad tolerance should be +/- .002".
Avoid placing nodes on SMT devices. This can cause damage to the device and provides minimal probe contact area.
Avoid placing test points on excessively long leads (.170" or more). Additional machining may be required for leads of this length and probe contact is compromised due to deflection.