Bead probe technology-Advantages-
Bead probe technology
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.
The soldering process that forms the bead probe leaves a coating of flux. Depending on the manufacturing process used, this flux can have varying levels of hardness. Flux with a waxy hardness can reduce the deformation force from the bead, preventing proper contact with the test probe during the first pass contact. This becomes less of an issue on subsequent contacts as the flux is displaced. Test probes with serrated ends of an appropriate size can also aid in measuring bead probes where flux is an issue.
Bead probes require the trace being tested to be located on the surface. This makes it unsuitable for testing high-density boards with many obscured or internal traces and buried vias.
Boundary scan integrates test components into the integrated circuits (ICs) mounted on the board, giving the ability to read or drive the ICs' pins. This allows for testing of interconnects for which physical access is not an option, such as BGA components or signal routes sandwiched between plane layers. A boundary scan controller uses four or more dedicated pins on the board to control test cells serially and receive the measured values. It has the disadvantage of needing board infrastructure to support boundary scan.
Test Access Component (TAC) uses a device such as a 0201 as a target for a large probe as in the solder bump examples. The advantage of this technique is that it provides two target points at each end of the package. The disadvantage of this technique is it can add process and cost to the PCB.
A technique has been described which opens up windows in the solder mask to create test points located directly on PCB tracks. This technique uses a conductive rubber tipped probe to contact the test point which could have a conductive Hot Air Solder Levelling (HASL) finish.