PCB Online Test Fixture:
The test fixture used by bare board manufacturers is also called a bed of needles. However, in the bare board test at the end of the wiring process, the probes are hard and thin, and they are in contact with the top and bottom surfaces of the circuit board. The test needles on the needle bed fixture for online testing are flexible, and the required test pad area is relatively large. Keep in mind that only one-sided testing of the assembled board will reduce the cost of the test fixture during in-line testing.
The following is a set of verified design rules, which can reduce the cost and complexity of test fixtures.
1. The pad diameter of metallized holes and test through holes is a function of hole size. The diameter of the test pad dedicated for probe testing is not less than 0.9 mm. It is also feasible to use a 0.6 mm diameter test pad, but the corresponding circuit board area is not more than 0.700 square millimeters.
2. The clearance around the test probe depends on the assembly process. The clearance around the probe must be maintained at 80% of the height of adjacent components, with a minimum of 0.6 mm and a maximum of 0.5 mm.
3. The height of the components on the test surface of the probe on the circuit board must not exceed 5.7 mm, otherwise the high components on the test surface of the circuit board can only be tested after the test fixture is cut. The test pad must be at least 5 mm away from the high component.
4. No components or test pads are arranged 3 mm from the edge of the circuit board.
5. All probe areas must have flux or conductive non-oxidizing coating. No solder mask can be printed on the test pad.
6. The probe contact point must be on the pad, not on the end point, on the leadless SMT component array and on the pin of the pin component. Contact pressure will cause the circuit to open, but the virtual solder joints still look good at this time.
7. Use probes to test both sides of the circuit board. A through-hole can be used to switch the test point to the other side, preferably the bottom side (no components or through-hole soldering surface). In this way, a highly reliable and relatively inexpensive fixture can be manufactured.
8. If it is possible to use standard probes and highly reliable fixtures, the center hole of the test pad should be 2.5 mm.
9. Reduce the use of gold fingers as test pads, because the test probes can easily damage the gold fingers.
⒑ The test pads should be evenly distributed on the circuit board. If the test pads are unevenly distributed or concentrated in a certain area, it will cause circuit board bending, probe failure and vacuum sealing problems. The other half is placed in the pad on the other side.
There are two purposes for this.
1. Do not exceed the test equipment limit of “up to 5-6 test points per 100 square millimeters”.
2. Broaden the distribution of test points to reduce the pressure of the high-density distribution of test points. During the vacuum or mechanical action of the clamp, this pressure can cause the clamp to deform.
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