Test Point Accuracy

We often confuse on how to increase the test accuracy in Print Circuit Board test situation, our fixture house are accepting all the challenge 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 Firgure A.

Probe Accuracy is namely a probe tolerance in manufacturing processand mating with the receptacles.   A probe system is assembly of a tube, a spring , a plunger at least. Each parts are manufactured by machine or mankind, 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, the probe accuracy is nearly 0.02-0.03mm(including the probe plunger shaking tolerance and manufacturing tolerance.) See as Figure B.

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

Drilling Holes Offset value is measured from the hole center to the test point center, we normally foucs on more on the drilling hole tolerance to keep a stable and better circular tolerance.

So we ever asked a good partner in the world classic fixture house to make a experiment, 256 holes of 39mil grids were drilled through, ever we have the a series of holes 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.