Design For Testability
The testability of the printed circuit board (PCB) is not always taken under consideration during the design process. The consequence could be higher costs to the test or a degraded test. Below please find recommendations which will make a printed circuit board (PCB) suitable for test in Torolex test fixtures.
- Preferred test points. Test points are prioritized as: 1. Test pads, 2. Leaded component pins. Test of SMD pads and VIA's should be avoided. Test pads can be covered by solder or gold. Gold is preferred because a solder surface has a layer of oxide (and solder flux) which must be penetrated by the test probes.
- Single sided test. All test points must be placed on the test probe side of the PCB.
- Close test pads. For use of the most common the pads the center to center distance must be minimum 2,54mm (0,100").
- Pad size. The preferred size of test pads is 1,2 mm or bigger.
- Short pins. The length of pins from leaded components should be less than 3mm after soldering.
- Test guide distance. The distance from the center of a test pad to the surface of guide pin hole must be minimum 3mm.
- High components. Test points should have a minimum distance of 4 mm to high components (>4 mm) on the test probe side.
- Connectors. Connector pins should be available on the test probe side of the PCB. If not add test pads on the test probe side.
- Hidden Pads. Required signals may be hidden behind piggyback PCB’s, displays or keyboards. Add test pads.
- Test pad distribution. Test pads should be distributed over the PCB to avoid the load from test probes concentrated in small areas. A single probe loads about 200g.
The most common and recommended test probe center to center distance is 2,54 mm (100 mil). But we also support fine pitch test probes with 1,91 mm (075mil) and 1,27 mm (050 mil) distances. Avoid the vulnerable fine pitch probes if not necessary.
PCB Fixing Holes:
- Opposite corners. Holes used to fix the PCB during test should be placed in opposite diagonal corners of the PCB to maximize the accuracy of the PCB position.
- Min. 2 holes. A minimum of 2 PCB fixing holes should be available.
- Symmetric holes. Avoid symmetric position of the holes to limit the possibility of mounting the PCB 180 deg. turned.
- No holes. If the PCB has no feasible holes the PCB edges may be used. This method is less precise and more complicated.
The size of the holes is non standard. But a size of ø3 to ø6 mm is recommended. The guide pins are manufactured to fit the holes.
If the PCB is already manufactured and you have recognized a problem then please don't hesitate to contact us. A discussion may help find a suitable solution.