Why DFT must be considered early
DFT (Design for Test) is designing a PCB layout for electrical testability. Unlike pure functional design, DFT asks: "Can a test system later reach and measure every relevant node of the circuit?" If that question is only raised after routing, it is usually too late — contact areas are missing, or test points sit so awkwardly that neither an in-circuit test nor a flying probe can test the assembly economically.
The effort is asymmetric: in the schematic and early layout an extra test point costs practically nothing. Added later to a released revision, the same test point becomes a redesign. That is why test points belong in the same thinking phase as fiducials, panelisation and component clearances — not at the very end.
General test point rules
Regardless of the chosen test method, some basic rules apply, taken from the SMTEC Design Guidelines (chapter 6):
- Positioning: For the flying probe, all test points ideally sit on the same side of the board. For ICT and functional test (FCT) double-sided contacting is possible, but it drives up the hardware cost of the test adapter.
- Node coverage: Every node of the circuit should carry a test point. Only what is electrically reachable can be tested later.
- Vias as test points: Avoid where possible. If unavoidable, the via must be at least 0.8 mm in diameter and filled — otherwise contacting by the probe or spring pin is not reliable.
- Supply nets: VCC, GND and other supply rails need at least two to four test points because of their higher current draw. A single contact would be overloaded during testing.
In-circuit test (ICT) requirements
The in-circuit test contacts the assembly through a bed of nails made of spring contact pins. For this adapter to contact the board precisely and reproducibly, the requirements are tighter than for the flying probe:
- Test point diameter: at least 1 mm.
- Minimum spacing: 2.03 mm between two test points — this matches the grid pitch of the spring contact pins.
- Tooling holes: Every board needs two tooling holes arranged asymmetrically across the diagonal. These can optionally be added by SMTEC in the panel border.
- Tooling hole design: not copper-plated, diameter at least 2.0 mm, ideally 3.0 mm.
- Distance to tooling holes: test points next to tooling holes keep a minimum clearance of 5.0 mm.
- Coordinate stability: when the assembly changes, keep the coordinates of test points and tooling holes — the same test adapter stays reusable, which saves considerable cost.
- Paste data: test points must not be included in the paste data, otherwise solder paste would cover the contact areas.
Flying probe requirements
The flying probe moves individual probes freely across the board and needs no board-specific adapter. That makes it ideal for prototypes and small series — its geometric requirements are correspondingly more relaxed:
- Test point diameter: 0.8 mm or larger.
- Minimum spacing: 1.6 mm between two test points.
- Mind component height: test points should not sit too close to tall components, so the probe can reach the contact point unobstructed.
- Onboard programming: ICs cannot be programmed in the system during flying probe testing. If programming during test is required, the route is via ICT or a separate programming step.
ICT or flying probe?
Both methods have their place — the choice mainly depends on volume and test depth. The following table compares the key DFT requirements side by side:
| Criterion | In-circuit test (ICT) | Flying probe |
|---|---|---|
| Test point diameter | ≥ 1 mm | ≥ 0.8 mm |
| Minimum test point spacing | 2.03 mm | 1.6 mm |
| Tooling holes | 2 asymm., ≥ 2.0 mm (ideally 3.0 mm) | not required |
| Contacting | double-sided possible | one side only |
| Onboard programming | possible | not possible |
// Source: SMTEC Design Guidelines, chapter 6
Rule of thumb: for single prototypes and small pre-series the flying probe is usually the faster and cheaper route, because no test adapter has to be built. As volumes grow or onboard programming is needed, the ICT plays to its strengths — provided the layout has the right test points and tooling holes from the start.
ICT and flying probe are request services at Speedplace. For the flying probe you ideally supply IPC data, a convertible bill of materials, the schematic and the test points. For the ICT we additionally need a specification sheet and the defined test points. The earlier this information is available, the more smoothly the test can be designed.
Test concept for your assembly?
ICT and flying probe are request services. Start your project in the online configurator and note your test requirements — we will come back with the right test concept.
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