Bead probe technology

[1][2] It makes use of small beads of solder placed onto the board's traces to allow measuring and controlling of the signals using a test probe.

Construction requires a hole to be opened in the solder mask, exposing the copper trace.

A properly constructed bead is the same width as the trace and just enough to clear the surrounding solder mask.

This allows a high degree of flexibility in their positioning, and can in some cases be applied retrospectively to existing layouts.

[5][6] This is especially useful in high speed input/output (HSIO) interconnects, where a standard test pad would interfere with the signal.

Side and end view showing a solder bead providing access to a trace located under the solder mask
Side view of a PCB showing a solder bead and test probe.
Bead probes on a PCIe lane.