Test fixture probes — also called pogo pins or spring-loaded contact pins — are the interface between your DUT and your test instrumentation. The right probe depends on three things: what your test points look like, how tightly they're spaced, and how the probe connects to your signal path. This guide walks through each of those variables so you can match probes to your board with confidence.
Probe tip styles#
Each probe pin type uses a different tip geometry, and that geometry determines which pad surfaces it contacts well. Crown tips handle the majority of bed-of-nails test probe applications. The others fill specific gaps.
Crown tip#
Four or more contact points arranged in a ring. The crown shape self-cleans on each press cycle, scraping through light oxidation and solder residue. Multiple contact points mean reliable connections even with slight misalignment — which matters when you're probing hundreds of pads per fixture.
Use when: Solder pads (HASL, ENIG, OSP), component leads, through-hole test points. This is the default choice for most boards.
Spear tip#
Single pointed contact that penetrates surface contamination. More aggressive than crown tips — useful when you're probing through flux residue or light oxidation that a crown tip can't clear.
Use when: Solder pads with contamination, plated test points, boards that aren't cleaned post-reflow. Also the go-to at 50-mil pitch where crown tips are too large.
Cup tip#
A concave tip that centers itself on rounded surfaces. The cup shape catches and holds cylindrical pins, giving you consistent contact without precise lateral alignment.
Use when: Through-hole connector pins, round test points. Not suited for flat pads.
Cone tip#
A sharp point designed to enter holes and contact interior plating. The tip geometry concentrates spring force on a small area, which helps penetrate oxide layers inside vias.
Use when: Unplated vias, unpopulated through-holes. Avoid on flat pads — the point can damage the pad surface without making reliable contact.
Serrated tip#
A flat face with machined grooves that grip pad surfaces. Combines broad contact area with some scrubbing action. Less common than crown or spear, but useful on specific pad finishes.
Use when: Bare copper pads, pads with heavy oxidation where you need aggressive scrubbing without the penetration depth of a spear tip.
Matching probes to pad types#
Start with your pad geometry and finish, then pick the tip that contacts it best. This table covers the common combinations:
| Pad Type | Surface | Recommended Tip | Notes |
|---|---|---|---|
| SMD pad (HASL) | Tin-lead or lead-free solder | Crown | Self-cleaning handles solder residue |
| SMD pad (ENIG) | Gold over nickel | Crown or Spear | ENIG is flat and clean — crown works well |
| SMD pad (OSP) | Organic coating over copper | Crown | OSP degrades over time; crown penetrates light oxide |
| Bare copper pad | Uncoated copper | Serrated or Spear | Copper oxidizes fast; you need aggressive contact |
| Solder-filled via | Solder plug | Cone or Spear | Cone enters the via; spear works if the fill is flush |
| Through-hole pin | Round metal pin | Cup | Self-aligning on cylindrical surfaces |
| Component lead | Flat or formed metal | Crown | Multi-point contact handles varied lead shapes |
| Edge connector finger | Gold-plated trace | Crown | Low contact resistance on gold surfaces |
Contact resistance matters here. Crown and serrated tips typically achieve < 50 mΩ on clean pads. Spear tips can go lower (< 20 mΩ) because they concentrate force, but they're more likely to damage soft pad finishes. If your test plan includes precision resistance measurements, factor this into your tip selection.
Grid spacing and pitch constraints#
Your receptacle pitch determines which probes fit. Tighter pitch means smaller barrel diameters, which limits your tip style options.
| Receptacle Pitch | Max Probe OD | Available Tip Styles | Typical Use |
|---|---|---|---|
| 100 mil (2.54 mm) | 1.37 mm | All styles | Standard-density boards |
| 75 mil (1.91 mm) | 1.02 mm | Crown, Spear, Cone, Cup | Medium-density boards |
| 50 mil (1.27 mm) | 0.69 mm | Spear, Cone | High-density boards |
At 50-mil pitch, you're limited to the smallest probe barrels. Crown tips don't fit — their multi-point geometry requires a wider tip diameter than the barrel spacing allows. Spear tips become the default for flat pads at this pitch.
When your board has mixed spacing — some areas at 100 mil, others tighter — you may need different probe sizes in the same fixture. This is normal. FixturFab fixtures support mixed receptacle pitches; specify the pitch per test point in Studio and we'll match the right probe and receptacle for each location.
Test probe and receptacle compatibility#
Probes and receptacles are paired by series. An E-075 probe fits a KS-075 receptacle. Mixing series (an E-050 probe in a KS-075 receptacle) doesn't work — the bore diameter, spring engagement, and travel are all matched. For detailed receptacle specifications including collar heights, drill sizes, and connection types, see the receptacles guide.
Connection method considerations#
How your probes connect to test instrumentation affects which probes you can use. This is primarily a signal interface decision, but there are probe-side implications.
Wire-wrap and solder cup receptacles work with any probe series. The probe drops into the receptacle; the wiring happens below. Solder cup adds brief thermal exposure during assembly — not enough to damage probes, but something to be aware of if you're hand-soldering near installed probes.
Wireless receptacles (TPCB) constrain probe travel. The receptacle connects directly to a test point contact board, so the probe's compressed length matters more. Check that your probe's working travel matches the TPCB stackup. See our TPCB guide for stackup details.
Special applications#
High-current probing#
Standard probes handle signal-level currents (< 2A). For power rail testing or high-current functional test, use heavy-duty probes with larger barrels and stiffer springs. Kelvin probe pairs (separate force and sense contacts on the same test point) give you precision resistance measurements by eliminating lead resistance from the measurement.
Switching probes#
Spring-loaded probes with an internal switch that closes when the probe compresses. Used for DUT presence detection — the fixture knows a board is seated because the switching probes are compressed.
Multi-sided probing#
Boards that need contact from the top, bottom, or sides require angled or horizontal probe mounting. See top and side probing for fixture design approaches.
Probe specifications quick reference#
| Series | Pitch | Travel | Spring Force | Current Rating | Recommended Receptacle |
|---|---|---|---|---|---|
| E-050 | 50 mil | 2.5 mm | 75 g | 1 A | KS-050 |
| E-075 | 75 mil | 3.0 mm | 100 g | 2 A | KS-075 |
| E-100 | 100 mil | 3.5 mm | 120 g | 3 A | KS-100 |
| P75-LM2 | 75 mil | 2.5 mm | 85 g | 1.5 A | R75-1W |
The Ingun E-series (E-050, E-075, E-100) is what we use in production fixtures — these are precision-ground probes rated for 100,000+ cycles. For development fixtures where cost matters more than cycle life, the P75-LM2 works well at roughly a third of the price. Budget probes are fine for dev work where you're iterating on your test plan and the fixture sees hundreds of cycles, not tens of thousands. When you move to production volumes, switch to the Ingun series.
Probe life depends on current, contamination, and cleaning frequency. The 100,000-cycle rating assumes clean pads and currents within the rated spec. Running probes at higher currents or through contaminated boards shortens life significantly.
Related documentation#
- Receptacles guide — Receptacle specifications, drill sizes, and connection types
- Signal interfaces — Choosing between hand-wired, Auto TPCB, and Consigned TPCB
- Fixture validation — Post-assembly probe contact testing
- Dev fixture guidelines — Dev-tier probe and receptacle defaults
- Dev Pro guidelines — Dev Pro probe and receptacle defaults
- Ingun guidelines — Working with Ingun probe specifications
Ready to apply this? Configure your fixture in Studio with your board files and we'll match probes to your test points automatically. Or browse probes and receptacles in Shop if you're sourcing individual components.