TEST POINT ACCURACY

We are often confused about how to increase the test accuracy in Print Circuit Board test situations, our fixture house are accepting all the challenges in how to make the test probes accurately get the test points. As the name suggests, test point accuracy is the accuracy of test probe getting on the test points in the in-circuit test and functional test, test point accuracy includes drilling holes offset and probe accuracy itself.

As we considered, A probe point accuracy is composed of two-part tolerance as the Figure A.

Probe Accuracy is namely a probe tolerance in the manufacturing process and mating with the receptacles.   A probe system is an assembly of a tube, a spring, a plunger at least. Each parts are manufactured by a machine or mankind, and we see the tolerance will be contributed by different parts' tolerance and finally come to a total number, 

we ever do the test for a normal 39mil test probe, and the probe accuracy is nearly 0.02-0.03mm(including the probe plunger shaking tolerance and manufacturing tolerance.) See Figure B.

Drilling Holes offset is caused by the fixture house hole drilling process, as we know fixture houses have a variety of drilling house machines and different skilled technique operators. 

The drilling hole offset value is measured from the hole center to the test point center, we normally focus more on the drilling hole tolerance to keep a stable and better circular tolerance.

Figure A                                                                                                                                                                   Figure B               

So we asked a good partner in the world classic fixture house to make an experiment, 256 holes of 39mil grids were drilled through, ever we have a series of hole numbers( measured by optical detection). see as the Frigure C.

We see the distribution of deviation values are normally presented. and the average is normally 0.0098mm. 

Figure C          

In conclusion, both the test probe processed manufacturing and the great drilling board process will affect the test point accuracy. Spring contact probe accuracy is more important for better getting in touch on the test pad or points.