Test Point Placement For PCB Assemblies

A test point is a defined place on the board where a probe can make contact for electrical test. It is the interface between the design and the test fixture, and its placement decides whether the board can be tested at all on automatic equipment, or whether every unit has to be handled manually.

Test points are usually added at the end of a layout, when space is scarce and the design is nearly complete. That is the worst possible time to place them, because a probe needs clearance that a fully routed board may no longer have.

Why Test Points Matter

Automatic test with a bed of nails depends on reliable contact between a spring loaded probe and a defined pad. Without a test point the probe has to land on a component lead or on a via, and both give a less reliable contact and a greater risk of damaging the assembly.

The cost of test is affected as well. A board with well placed test points can be tested in a few seconds, while one that needs manual probing takes minutes and depends on the skill of the operator. On a production line that difference dominates the total cost of test.

Bed Of Nails And Fixture Design

A bed of nails fixture holds a grid of probes in a plate, and each probe contacts one test point. The fixture is built from the test point data, so the positions have to be defined precisely and consistently on every board in the batch.

Probe travel is limited, typically a few millimetres, so all test points must lie within a narrow height band on one side of the board. A board with components of very different heights can require a fixture with stepped probe plates, which is considerably more expensive to build.

Size, Shape And Finish

A test point is normally a square or round pad of 0.9 to 1.5 mm with a 0.6 to 1.0 mm opening in the solder mask. The mask opening must be clear of ink and of residual flux, since both raise the contact resistance and can give a false reading.

The finish matters as well. A hard, non-oxidising finish such as gold or a thick tin layer gives a stable contact over many probe cycles, while a bare copper pad oxidises and needs frequent cleaning or replacement of the probe. The pad rules that apply are described under PCB pad design standards.

Probe Access And Keepout

Each test point needs a keepout free of tall components, so that the probe can descend vertically onto the pad. The usual rule is a clear circle 1 to 2 mm in diameter above the pad, and more where the fixture uses a wide probe head.

The keepout should also account for components on the other side of the board. A board probed on both sides needs clearance on both faces, and a large component on one side can block a probe on the other if the fixture cannot support the board locally.

Placement On Both Sides

Test points can be placed on either side, and spreading them across both faces makes routing easier. The trade off is a more complex fixture, which has to contact both sides and therefore needs either a double sided probe plate or two separate test steps.

Where the volume justifies it, a double sided fixture is worth the cost because it removes routing constraints from the layout. For a small volume, keeping all test points on the solder side is usually the simpler decision and the cheaper one to tool.

Common Mistakes

The most common mistake is a test point that ends up under a component after a late change, so the probe cannot reach it. The second is a test point on a net that also carries a sensitive analog signal, where the capacitance of the probe disturbs the measurement.

A third is a test point with no connection to the net, left in the layout after the netlist changed. How the test data is transferred to the test engineer is described under PCBA development process.

Test Points On Power And Ground

Every net that carries power needs a test point, and those test points should be sized for the current the probe will carry rather than for the signal level. Some test systems inject current through the probe, and a small pad with a thin trace will heat or drop voltage under that load.

Ground test points are usually placed in several locations across the board so that the fixture can reference the same ground at more than one point. A single ground probe on a large board can give a misleading reading because of the impedance between the probe and the component being measured.

Test Points And High Speed Nets

A test point adds capacitance to the net it touches, and on a high speed net that capacitance behaves as a stub which reflects energy. The effect is small for a short via but significant for a pad connected by a long trace, particularly at multi gigabit data rates.

Where a net cannot tolerate the added capacitance, the test point is often removed and the net is verified by a functional test instead. That decision should be recorded, so that a later revision does not silently add the test point back onto the net.

Documentation For The Test Engineer

The test point data has to reach the test engineer in a usable form: a list of nets with coordinates, a defined probe side, and a note of any net that must not be probed. A simple spreadsheet export from the CAD tool is usually enough.

The document should also state which test points are optional spares and which are mandatory. Without that distinction a fixture is often built with every pad included, which raises the cost and can reduce reliability by adding probes that are never used.

Maintaining Coverage Over Revisions

Coverage usually erodes over the life of a product. A revision adds a net without a test point, or moves a component over an existing pad, and the loss is not noticed until a batch escapes test. A coverage report generated at every revision keeps the problem visible.

The report should compare the current netlist with the test point list and flag every net that has no probe. Reviewing that report takes a few minutes and prevents the slow decline that otherwise turns a fully testable design into a partially testable one.

FAQ

How many test points are needed? At minimum one for every net that must be verified, plus power and ground. Adding a few spares costs nothing and rescues a design when a late change forces a move.

Can a via be used as a test point? Yes, if it is large enough and not tented, and if the probe can land on it without damaging the mask. A dedicated pad is more reliable over a long production run.

Do test points need to be on a grid? Not strictly, but a grid reduces fixture cost because the probe plate can be drilled or milled more simply. Pitches of 2.54 mm and 1.27 mm are traditional.