Bead probe technology

Bead probe technology (BPT) is technique used to provide electrical access (called “nodal access”) to printed circuit board (PCB) circuitry for performing in-circuit testing (ICT). 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. This permits test access to boards on which standard ICT test pads are not feasible due to space constraints. Bead probe technology (BPT) is technique used to provide electrical access (called “nodal access”) to printed circuit board (PCB) circuitry for performing in-circuit testing (ICT). 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. This permits test access to boards on which standard ICT test pads are not feasible due to space constraints. Bead probe technology is a probing method used to connect electronic test equipment to the device under test (DUT) within a bed of nails fixture. The technique was first used in the 1990s and originally given the name “Waygood Bump” after one of the main proponents, Rex Waygood. They are also commonly referred to as solder bumps. Bead probes were designed for when less than 30 mil is available for test probe points on the PCB. They are used with standard ICT spring-loaded test probes to connect the test equipment to the DUT. Bead probes are made from a very small 'beads' of solder that fit atop of the PCB traces. They are manufactured using the same techniques as other solder features. Construction requires a hole to be opened in the solder mask, exposing the copper trace. This hole is sized to precisely control the amount of metal that forms the bead. Solder paste is applied to the location and reflowed. During reflow, solder flows and is drawn to the copper trace. Surface tension causes the bead to have a curved surface and rise above the solder mask, where it solidifies into a Bead Probe. The bead will be roughly obround in shape and may be 15-25 mils long. A properly constructed bead is the same width as the trace and just enough to clear the surrounding solder mask. The bead is then accessible for testing using a probe with a flat end, which can help compensate for the tolerance build up in the test fixture and PCB. Bead probe can be used in circuits where the pin-pitch is too fine to allow standard test pads. This is becoming more common as pin pitches continue to reduce, particularly in embedded devices. Typically bead probe widths are the width of the PCB traces with a length of about three times this. This allows a high degree of flexibility in their positioning, and can in some cases be applied retrospectively to existing layouts.Because of their small size, bead probes do not affect the signal quality of the signals transferring within the PCB trace. This is especially useful in high speed input/output (HSIO) interconnects, where a standard test pad would interfere with the signal.