PCB Online Test Fixture:
The test fixture used by bare board manufacturers, also known as a bed of needles, is crucial in the bare board test process at the end of the wiring process. Unlike traditional hard and thin probes that make contact with the top and bottom surfaces of the circuit board, the test needles on the needle bed fixture for online testing are flexible, requiring a relatively large test pad area. It’s important to note that conducting one-sided testing of the assembled board can help reduce the cost of the test fixture during in-line testing.
Listed below are design rules that have been verified and proven to reduce the cost and complexity of test fixtures:
1. The pad diameter of metallized and test through holes should be proportional to the hole size. The diameter of the test pad dedicated for probe testing should not be less than 0.9 mm. While a 0.6 mm diameter test pad is feasible, the corresponding circuit board area should not exceed 0.700 square millimeters.
2. The clearance around the test probe should be based on the assembly process. It’s important to maintain a clearance around the probe 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 components on the test surface of the probe on the circuit board should not exceed 5.7 mm. If higher components are present, they can only be tested after the test fixture is modified. The test pad should be at least 5 mm away from high components.
4. Components or test pads should not be placed within 3 mm from the edge of the circuit board.
5. All probe areas must have a flux or conductive non-oxidizing coating. No solder mask should be printed on the test pad.
6. Ensure that the probe contact point is on the pad, not on the end point, especially on leadless SMT component arrays and pin components. Contact pressure may cause circuit openings, but the virtual solder joints should remain intact.
7. Utilize 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 with no components or through-hole soldering surface. This approach can lead to the production of a highly reliable and cost-effective fixture.
8. If feasible, use standard probes and highly reliable fixtures, with the center hole of the test pad set at 2.5 mm.
9. Minimize the use of gold fingers as test pads to avoid potential damage from test probes.
10. Distribute the test pads evenly across the circuit board to prevent bending, probe failure, and vacuum sealing issues. Additionally, consider placing half of the pads on one side of the board and the other half on the opposite side to stay within the test equipment limit of “up to 5-6 test points per 100 square millimeters” and reduce pressure on high-density test point distribution during vacuum or mechanical clamping actions that could result in deformation.
The test fixture used by bare board manufacturers, also known as a bed of needles, is crucial in the bare board test process at the end of the wiring process. Unlike traditional hard and thin probes that make contact with the top and bottom surfaces of the circuit board, the test needles on the needle bed fixture for online testing are flexible, requiring a relatively large test pad area. It’s important to note that conducting one-sided testing of the assembled board can help reduce the cost of the test fixture during in-line testing.
Listed below are design rules that have been verified and proven to reduce the cost and complexity of test fixtures:
1. The pad diameter of metallized and test through holes should be proportional to the hole size. The diameter of the test pad dedicated for probe testing should not be less than 0.9 mm. While a 0.6 mm diameter test pad is feasible, the corresponding circuit board area should not exceed 0.700 square millimeters.
2. The clearance around the test probe should be based on the assembly process. It’s important to maintain a clearance around the probe 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 components on the test surface of the probe on the circuit board should not exceed 5.7 mm. If higher components are present, they can only be tested after the test fixture is modified. The test pad should be at least 5 mm away from high components.
4. Components or test pads should not be placed within 3 mm from the edge of the circuit board.
5. All probe areas must have a flux or conductive non-oxidizing coating. No solder mask should be printed on the test pad.
6. Ensure that the probe contact point is on the pad, not on the end point, especially on leadless SMT component arrays and pin components. Contact pressure may cause circuit openings, but the virtual solder joints should remain intact.
7. Utilize 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 with no components or through-hole soldering surface. This approach can lead to the production of a highly reliable and cost-effective fixture.
8. If feasible, use standard probes and highly reliable fixtures, with the center hole of the test pad set at 2.5 mm.
9. Minimize the use of gold fingers as test pads to avoid potential damage from test probes.
10. Distribute the test pads evenly across the circuit board to prevent bending, probe failure, and vacuum sealing issues. Additionally, consider placing half of the pads on one side of the board and the other half on the opposite side to stay within the test equipment limit of “up to 5-6 test points per 100 square millimeters” and reduce pressure on high-density test point distribution during vacuum or mechanical clamping actions that could result in deformation.
