Most bed of nails PCB test fixtures probe from the bottom only — the board sits face-down so test points contact pogo pins mounted on a probe plate below. This page covers the cases where bottom-only access isn't enough: top probing, side probing, and test plug engagement for board-edge connectors.
When you need multi-sided probing#
Bottom probing works when all test points sit on one side of the board. You need top or side probing when:
- Test points on both sides. Components or test pads on both the top and bottom of the PCB require simultaneous contact.
- Tall bottom-side components. Components on the bottom prevent flipping the board, so top-side test points must be reached from above.
- Board-edge connectors. USB-C, Ethernet, HDMI, and other connectors can only be engaged from the side — pogo pins from above or below won't reach the contacts.
- RF shielding or thermal interfaces. Top-side contact is needed for grounding shields or verifying thermal pad connections.
If your board fits any of these, read on. If all your test points are on one side, standard bottom probing is simpler and cheaper — see Design considerations below.
Top-probe plate assembly#
When a DUT requires test points on both sides, the fixture uses a dual-sided probing configuration with a top-probe plate mounted above the board.
DUT orientation depends on which side has more test points — the majority side faces the bottom probe plate. Due to the mechanical complexity of the top-probe plate assembly, dual-sided probing is only available in Production-level fixtures.
FixturFab designs a custom top-probe plate with its own test probes and a top-located Test Point Carrier Board (TPCB). The top TPCB routes signals down to the primary TPCB in the fixture base, keeping all electrical connections accessible at the interface connector.
As the pressure plate closes, top-located test probes contact test points on the upward-facing side of the DUT. The top-probe plate aligns to the bottom plate through guide pins, maintaining probe-to-pad registration across both sides.
Side probing#
PCBA connectors are prime candidates for test point connectivity in bed of nails test fixtures. When auxiliary test points can't provide adequate access — often due to fine pitch or connector pin density — direct connector engagement from the side is the answer.
FixturFab offers three approaches to side-connector probing, all with automated connector insertion and disengagement. Side actuation features are available in Production-level fixtures only.
Side Approach Mechanism (SAM)#
The stroke-controlled Side Approach Mechanism provides automated connector engagement with consistent force and travel on every cycle:
| Specification | Value |
|---|---|
| Maximum contact force | 150N |
| Maximum contact stroke | 16mm |
| Maximum operating stroke | 7mm |
| Maximum lever arm length | 60mm |
| Torque | 1.5Nm |
| Weight | 0.14kg |
Side-actuated test plugs#
Test plugs mate directly with board-mounted connectors, replicating a real cable connection during test. FixturFab supports test plugs for industry-standard connectors including USB-A, USB-C, RJ45 Ethernet, HDMI, and DC power jacks.
Test plug selection depends on the connector type, required pin-out, and the signals you need to route. If your board uses a connector not listed here, contact engineering — custom test plugs are available for most standard and proprietary connector formats.
Side-actuated test probes#
Custom side plates can contact connector test points using pogo pins instead of male test plugs. This approach improves cycle sustainability — pogo pins handle 100,000+ cycles where repeated mating of a test plug wears the board connector faster.
Design considerations#
Bed of nails test fixture design gets more complex when you add top and side probing. Bottom probing is simpler by a significant margin — fewer mechanical tolerances, fewer alignment surfaces, and lower material costs. Here's what changes when you go multi-sided:
- Tolerance stacking. Each additional probing direction introduces alignment surfaces. A top-probe plate must align to the bottom plate, the DUT locators, and the pressure mechanism simultaneously. Tighter tolerances mean higher-precision machining.
- Actuation force. Multi-directional probing increases the total force the pressure system must deliver. Top probes compress against the DUT from above while bottom probes push from below — the fixture frame absorbs the combined load.
- Cost and lead time. Dual-sided and side-actuated fixtures require additional design time, more precision-machined components, and longer assembly. Expect higher cost and longer lead time compared to bottom-only fixtures. These features are only available on Production-tier fixtures.
- Maintenance. More probe plates and mechanical assemblies mean more components to inspect, clean, and replace over the fixture's lifecycle. See maintenance guide for recommended intervals.
Design for test recommendation: Limit test points to a single side of the PCB when possible. Single-sided probing results in faster turnaround, lower cost, and simpler fixtures. If your board layout forces multi-sided access, bed of nails testing from multiple directions is a solved problem — but it's worth checking whether a DFT review could consolidate test points to one side before committing to dual-sided fixture design.
Specifying in Studio#
When configuring a fixture in Studio, upload your board files (Gerber, ODB++, or pick-and-place) with test points marked on all relevant sides. Studio identifies which sides need probing based on your test point placement.
For side-actuated connectors, note the connector types and locations in your configuration. If your requirements are unusual — multiple side approaches, non-standard connectors, or tight clearance constraints — contact our engineering team to discuss before placing an order.
Related documentation#
- Selecting test probes — Probe tip styles, pad-type matching, and grid spacing for all probing directions
- Pressure application — Actuation mechanisms and force requirements, including multi-sided configurations
- DUT presence detection — Board presence sensing, including top-side switching probes
- Fixture bases — Base specifications and which bases support top/side probing options
- Specifications — Overall fixture tolerances and alignment specifications
- Signal interfaces — TPCB options for routing signals from top and side probe plates
Ready to configure a multi-sided fixture? Configure your fixture in Studio — upload your board files and we'll handle the mechanical design. For boards with unusual access requirements, talk to engineering first.