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IC Test Probe Needle Configurations

There are a number of technologies used to produce the test needles mounted in the probe card that vary by test level (pre- or post-packaging) and IC function.  Many wafer- and die-level test needles are produced from metallic wires fabricated with alloys which were developed by Deringer-Ney.  Common types of wire needle configurations include cobra probes, cantilever probes, vertical probes, and LED probes.  Other common, non-wire probes include so-called MEMS probes and pogo pins.

Cobra Probes

Cobra Probes are formed pins manufactured with starting wire diameters of approximately 0.001-0.004 in (25-100 μm) and are typically used for small-pitch logic testing.This type of probe is named for its self-contained ribbon-shaped spring that gives it a “snake-like” appearance as shown in Figure 1.  The working tip may be found in a flat configuration or as conical points.  Force exerted by the tip of the cobra probe on the test pad is a combination of beam bending and beam buckling.  The forerunner to today’s cobra probes were so-called “buckling beam probes”, developed by IBM in the 1970s.  In fact, Deringer-Ney’s Paliney® family of alloys was named in the original inventions as materials suitable for this application. The cobra probes are typically the IC-facing surface of the probe card, backed by complex electronics that route and condition the signal to and from the prober.    




Vertical Probes

Vertical probes, as well as other straight wire needles, are often used in a manner consistent with cobra probes.  A key difference is that probe card design utilizes clever guiding with multiple perforated plates to offset the tip and tail of the probe, thus controlling the buckling of the probe.  This simplifies probe manufac-ture relative to cobra probes.  As with cobra probes, typical starting wire diameters are approximately 0.001-0.004 in (25-100 μm).

Cantilever Probes

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

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

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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