Test Points

The circuit boards we design also have some testing to go through. The testing will determine how well the boards have been manufactured. For this testing to happen, the board must be prepared with specific elements that a test probe can access. These elements are called test points and their inclusion on PCB designs is important.

There are two main types of test points used on a printed circuit board.

• The first is an easily accessible point for a technician to access with hand-held testing equipment. To help with this, these points often have a post soldered into them for clipping on a test lead. These test points will be marked with a reference designator and usually the name of the net in silkscreen for easy reference, such as “GND”.
• The other type of test point is used primarily for automated test equipment. The automated test points on a circuit board are through hole pins, vias, or small landing pads of metal that are designed to accommodate the probes of automated test systems.

There are three main test systems that will use these test points on the board:

• Bareboard Test: A bareboard test consists of an electrical continuity check performed after fabrication of the physical board. Connections between all component pins ensure that no shorts or opens exist. Once the physical board is checked, it is ready for assembly. During a bareboard test, all component pins on each side of the PCB are probed. Doing so requires a fixture containing probe pins that contact the PCB at specific locations.
• In-Circuit Test (ICT): In-circuit testing verifies that the board and components function together as intended and occurs after board assembly. During in-circuit testing, test engineers require access to each signal by probing a pin or routing via from the bottom side of the board. As in bareboard testing, in-circuit testing requires a test fixture that serves as a receptacle for the probe pins.
• Flying Probe: Like ICT, this test is run after assembly on each net of the board by accessing the test points. Where it differs, however, is that the flying probe equipment uses two to six probes that move around to each test point instead of using stationary probes in a fixture like ICT does.

Using Allegro PCB Editor to assign test points to your board:

Allegro PCB Editor offers a Testprep utility for adding test points to a design. This can be invoked from the menu Manufacture > Testprep. You need to set up the parameters first, as shown in the picture below, before the test points are assigned. These parameters will cover various details such as how close the test points can be to each other and additional design for test rules as well. Once the parameters are set, the test points can then be added automatically or manually by the designer in an iterative process.

Test points are typically assigned first to any available through hole pins, and then to vias. If there are not any vias available, the tool will give the designer the ability to add a new via onto a trace. There will also be a few options available when assigning test points, such as adding a specific reference designator or changing the via pad shape from round to square.

Team PCBTech

Cadence Design Systems