Cantilever probes, more typically used for testing memory, are fabricated from wire approximately 0.003 in (75 μm) starting diameter, with lengths of 2-5 in (50-125 mm). Generally, the cantilever probes are held at an acute angle relative to the DUT, and their pointed tips make contact with the IC test pads. Force exerted by this probe design is that of a simple bending beam, as the name would imply. Most of the overall length of the probe is used as a connection to route signals to and from the supporting PCB and associated electronics. Flexibility of the wire is a key characteristic to ensure consistent board attachment for each probe.
LED probes, as the name implies, are used primarily for verifying the function of light emitting diodes (LEDs). This style of probe has a larger cross-section than the others discussed above, typically 0.014-0.020 in (350-500 μm). When testing LEDs, particular attention is needed to the surface finish of the probe such that detected illumination values do not vary from probe to probe.
MEMS probes are typically wafer-level test needles and are manufactured using techniques similar to those used in the fabrication of ICs, but reimagined to produce freestanding structures. This is referred to in the industry as “microelectromechanical systems”, or “MEMS” technology. A typical method for MEMS needle production includes photolithographic patterning of a shape over a sputtered seed layer, plating using ECD, perhaps over the core of a different metal, and release of the MEMS needles from the substrate.
Pogo Pin Probes
Pogo pins, unlike the other probe types discussed, are usually designed to contact balls of soft solder alloys at the die- and packaging-level test during wafer bumping and die attachment respectively. These pins are micro-assemblies typically consisting of 4 parts: the probe tip is the working surface that contacts the DUT; the spring, which dictates load-displacement behavior for the pin; the plunger that compresses the spring and transmits the signal back through the probe card; and the barrel that houses all of these components. The probe tip will have a major diameter of about 0.010-0.020 in (250-500 μm), making it quite a bit larger than most wire probes.
Figure 2 below illustrates the details of a pogo pin probe using both SEM and optical mi- croscopy techniques.
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