Test point
A test point or test pad is a contact attached to a circuit board which, during the test of a circuit board in an in-circuit tester, establishes the electrical contact between the network node to be tested and the test needle of the adapter.
Test points are usually just a thickening of a conductor track where the solder mask has been left out. In some cases, vias on a circuit board are also used as test points in order to save space.
The distance between two test pads is called the pitch. This distance is often still defined in inches, so that today many test points still have a distance of 2.54 mm (for larger circuit boards). With the finer printed circuit boards , metric units are increasingly being used. The circuit boards are always equipped with more compact structures and smaller test pads (saving space), which also increases the demands on the testing machines.
Influence of the test point distance on the electrical test with adapters
- Pitch> 2.54 mm: old standard for larger circuit boards
- Pitch> 0.80 mm: conventional adapter with spring pins
- Pitch> 0.30 mm: rigid needle adapter (adapter is aligned using guide holes)
- Pitch> 0.15 mm: Rigid needle adapter (adapter is aligned using camera correction values)
Influence of the test point size on the electrical test with adapters
With the size of the test point, it must be possible to compensate for all inaccuracies in circuit board production, such as shrinkage and elongation. If the adapters are guided via catch pins, the following tolerances must be taken into account when testing with adapters:
- Adapter accuracy: tolerance between the pilot pin and the individual measuring contacts
- Catch pin to guide hole: clearance required so that the circuit board does not jam in the adapter
- Guide hole for layout: Guide holes and print layout are not produced in the same process
- Layout accuracy: shrinkage, stretching and twisting of the layout
- Pad edge: the slightly convex surface at the pad edge should not be contacted
The required pad size can now be determined from the tolerances. It should be noted that certain tolerances can cancel each other out.
Conclusion: Above all, the manufacturing tolerances determine the size of the test points required. If the structures to be contacted are smaller than the tolerance analysis allows, the contacting must be optically aligned, whereby certain tolerances can be eliminated.
Influence of test points and connection lines on the electrical function of the circuit
If the use of test points leads to additional cable runs that connect the test point to the electrical network on the circuit board, there is a risk that additional faults may occur in the assembly. each additional line can act as an antenna via which additional interference radiation can be emitted. This can lead to malfunctions in other circuit components or in neighboring devices. The other case is also possible, however, in which external interference can be coupled in via the additional connecting line. This can impair the switching function of the module.
The risk of interference can be minimized by attaching the test points directly to the connection line without using additional copper tracks on the circuit board. If this is not possible, the connection line between the test point and the actual electrical conductor run on the circuit board should be as small as possible.
Alternatives to test points
Test via customer interface
With fully populated circuit boards, there is often the situation that there is no more space for the test points. In this case, an attempt is made to use customer interfaces that are present on the module for contacting the electrical networks. Often only a part of the electrical network can be contacted via the customer interface. Depending on the application, the number of test points can be reduced.
Boundary scan
Test points on the circuit board can also be saved when using the boundary scan test method. This test method can be used in the field of digital signals. This test method can reduce the number of test points required on the assembly.