Bead probe technology

Bead probe technology (BPT) gives the electronics industry a new alternative in providing electrical access (called “node access”) to printed circuit board (PCB) circuitry needed to perform In-Circuit Testing (ICT). Node access is critical for components on the PCB to be electrically connected to the electronic test equipment that is performing the In-Circuit Test. The technology combines a new test point placement method with a well-known probing technique, as described below.

What is bead probe?

Bead probe is a probing method used to connect electronic test equipment to the device under test (DUT). It complements traditional probing solutions that use a sharp test probe to connect the test equipment to the DUT. The BPT primarily targets high-speed and high-density printed circuit boards which generally have insufficient conductive locations (test points) on the DUT to be connected to a test probe.

Bead Probes are made from a very small “beads” of solder that fit atop of a printed circuit trace as shown in Figure 1. They are constructed so as to have no effect (ideally) on the width of a trace, indeed perturbing the local geometry of the trace in only the Z dimension. Beads are only a few mils in height, enough to clear the surrounding soldermask. The bead will be roughly elliptical in shape and may be 15-25 mils long.

Figure 1 shows a graphic representation of the bead on the trace from both end and side views.

How bead probe created?

Bead Probes are constructed with solder paste and solder reflow technology. The solder paste bead is placed on the PCB after opening up the solder mask and exposing the copper trace. Solder paste is applied along the trace and reflowed. The resulting solder-beads are formed as a test point sitting on the PCB traces. It's than can be probed using traditional bed of nails fixtures that have been fitted with flat-headed probes.

Figures 2 to 4 show typical bead probes formed after reflow and probed successfully.

Benefit

The benefit of using bead probes is in the small size of the beads. With widths of just the thickness of the PCB traces and lengths of about three times the trace width, the beads are sometimes only visible using magnifying glasses. While traditional test points require additional board real estate, the small size of the bead probes allow them to be placed anywhere on the PCB trace, providing direct access to board circuitry without the need to route dedicated test circuits. This increases design flexibility, speeds up the layout process and makes it possible to add test points without changing the layout. In addition, because of their small size, bead probes do not affect the signal quality of the signals transferring within the PCB trace, even with high speed signals that are common in today’s products. Thus the bead probe methodology is ideal for the testing of high-speed, high-density PCB assemblies.

References

# [http://www.home.agilent.com/agilent/editorial.jspx?action=download&cc=US&lc=eng&ckey=873640&id=873640 www.agilent.com/see/beadprobe]
#A New Probing Technique for High-Speed/High-Density Printed Circuit Boards by Kenneth P. Parker of Agilent Technologies
#Applying a New In-Circuit Probing Technique for High-Speed/High Density Printed Circuit Boards to a Real-Life Product by Chris Jacobsen and Kevin Wible of Agilent Technologies
#Implementation of Solder-bead Probing in High Volume Manufacturing by Madhavan (Mady) Doraiswamy and James J (JJ) Grealish of Intel Corporation