(12) United States Patent
`Yu et al.
`
`I 1111111111111111 11111 lllll lllll 111111111111111 1111111111 1111111111 11111111
`US006529021Bl
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,529,021 Bl
`Mar.4,2003
`
`(54) SELF-SCRUB BUCKLING BEAM PROBE
`
`(75)
`
`Inventors: Yuet-Ying Yu, Hopewell Junction, NY
`(US); Daniel G Berger, Wappingers
`Falls, NY (US); Camille Proietti
`Bowne, Poughkeepsie, NY (US); Scott
`Langenthal, Pleasant Valley, NY (US);
`Charles H Perry, Poughkeepsie, NY
`(US); Terence Spoor, Marlboro, NY
`(US); Thomas Weiss, Poughkeepsie,
`NY (US)
`
`(73) Assignee: International Business Machines
`Corporation, Armonk, NY (US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by O days.
`
`(21) Appl. No.: 09/558,428
`
`(22) Filed:
`
`Apr. 25, 2000
`
`Int. Cl.7 ................................................ G0lR 1/067
`(51)
`(52) U.S. Cl. ..................... 324/754; 324/158.1; 439/482
`(58) Field of Search .............................. 324/761, 158.1,
`324/754; 439/482
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,027,935 A
`4,423,376 A
`5,225,777 A *
`5,952,843 A *
`
`6/1977
`12/1983
`7/1993
`9/1999
`
`Byrnes et al.
`Byrnes et al.
`Bross et al.
`.. .. ... ... ... 324/158.1
`Vinh .......................... 324/761
`
`OTHER PUBLICATIONS
`
`Probing Considerations in C-4 Testing of IC Wafers The
`International Journal of Microcircuits and Electronic Pack-
`
`aging, vol. 15, No. 4, Fourth Quarter 1992 (ISSN
`1063-1674) pp. 229-238.
`
`* cited by examiner
`
`Primary Examiner-Safet Metjahic
`Assistant Examiner-Jimmy Nguyen
`(74) Attorney, Agent, or Firm-H. Daniel Schnurmann
`
`(57)
`
`ABSTRACT
`
`A self scrubbing buckling beam contactor for contacting an
`array of pads positioned on a device under test is described.
`The contactor consists of three insulating dies: a top, an
`offset and a lower die separated from each other by an
`insulated spacer of variable thickness. Each die is provided
`with holes. The buckling beam has an array of flexible wires
`positioned substantially perpendicular to the dies, each of
`the flexible wires crossing a corresponding hole in each of
`the top, offset and lower dies to allow each wire respectively
`contact a pad of the device under test. By shifting the center
`of the hole of the lower die relative to the center of the offset
`die, the tip of the wire exits from the lower die at an angle
`with respect to the plane formed by the pads of the device
`under test. The exit angle of the wire tip is controlled by the
`relative displacement of the offset die relative to the bottom
`die, such that the exit angle of the tip of the wire at the
`bottom die changes when the probe wire is under compres(cid:173)
`sion. By applying a reciprocating motion to the tip of the
`wire contacting the surface of the device under test, a
`scrubbing motion is achieved that lowers the resistance
`between the pad of the device under test. In this manner, the
`tip of the wire cleans the surface of the pads and prevents
`contaminants from adhering to the tip of the wire.
`
`16 Claims, 4 Drawing Sheets
`
`10 f----~,1r--T~OP_G-U~IOE____,
`15
`THIN SPACER
`20 f------1>-o=r=rs=n~G=u1=DE-1
`
`25
`
`90
`
`NO LOAD
`
`\
`\\
`\\
`II
`II
`II
`II
`\I
`\\
`I
`\
`I
`I
`I
`I
`II
`II
`II
`II
`
`,Ji,
`
`I
`
`THICK
`SPACER
`
`UNDER
`LOAD
`{BUCKLED)
`
`/
`
`/
`
`/2
`
`//''"'·
`
`//
`
`/ I
`\
`
`30 BEARING
`WALL
`BUCKLED
`STRAIGHT SELF-SCRUS-
`
`BUCKLED
`
`.....__
`
`.....__
`
`.....__
`
`---.:
`
`(.0012" TYPICAL)
`
`\
`NO LOAD J
`/
`
`FF1011
`Formfactor v. Feinmetall
`Page 1 of 9
`
`

`

`U.S. Patent
`
`Mar. 4, 2003
`
`Sheet 1 of 4
`
`US 6,529,021 Bl
`
`WIRES OR
`CONTACTS
`
`UPPER DIE
`
`OFFSET DIE
`
`LOWER DIE
`
`-
`
`CHIP WITH
`SOLDER BALL
`OR WAFER
`
`FIG. 1
`(PRIOR ART)
`
`FF1011
`Formfactor v. Feinmetall
`Page 2 of 9
`
`

`

`U.S. Patent
`
`Mar.4,2003
`
`Sheet 2 of 4
`
`US 6,529,021 Bl
`
`---------.-,
`
`TOP GUIDE
`10
`THIN SPACER
`15
`20----·--oF=,FSc-=ET,.......,G,..,..,..,Ul-=-=DE-1
`
`1= - - - - - . . . - 11 - -~ - - - - 1
`
`25
`
`NO LOAD
`
`THICK
`SPACER
`
`'
`\
`\\
`II
`II
`II
`\\
`II
`I
`I
`I
`I
`I
`I
`If
`II
`/
`II
`I I /
`~
`I
`
`UNDER
`LOAD
`(BUCKLED)
`
`/
`
`I
`\
`
`BUCKLED
`
`30 BEARING
`WALL
`BUCKLED
`
`NO LOAD
`
`---
`STRAIGHT SELF-SCRUB" --- --- ---
`
`FIG. 2
`
`//
`I I
`//
`//
`//
`It
`//
`//
`I
`I.
`
`\
`J
`
`NO LOAD
`
`/
`
`FF1011
`Formfactor v. Feinmetall
`Page 3 of 9
`
`

`

`U.S. Patent
`
`Mar.4,2003
`
`Sheet 3 of 4
`
`US 6,529,021 Bl
`
`TOP GUIDE
`THIN SPACER
`OFF.SET GUIDE
`~
`~
`\\
`\\
`\\
`I
`
`THICK
`SPACER
`
`10
`15
`20
`
`25
`
`UNDER
`LOAD
`(BUCKLED)
`
`/4
`
`/
`
`/
`
`~
`
`90
`
`NO LOAD
`
`BOTTOM
`GUIDE
`
`\
`)
`
`----,,....._------=::i+-F---,------
`
`-__
`
`-..
`
`I To
`005"
`~C~ /
`(.005" TYPICAL)
`
`/
`
`BUCKLED --:::: -:..::__
`SHIFTY SELF-SCRUB
`
`NO LOAD
`---- ----
`35
`PROBE TIP
`
`FIG. 3
`
`FF1011
`Formfactor v. Feinmetall
`Page 4 of 9
`
`

`

`BUCKLING BEAM
`SELF SCRUB PROBE
`CONTACT ON ALUMINUM WAFER
`
`CONTACT RESISTANCE
`ohms (NORMALIZED)
`
`~:;1
`
`0.2,
`
`-i
`
`o "'
`
`•
`
`\I -,
`
`"'
`
`1 ',
`
`-
`
`I\ l'{
`
`FIG. 4
`
`NUMBER OF TOUCHDOWNS
`
`. t I
`
`-o~Af ......... q'..
`
`: \ !'ta
`
`d •
`r:JJ.
`•
`~
`~ ......
`~ = ......
`
`~
`~ :;
`~,J;..
`N
`0
`8
`
`'JJ. =(cid:173)~
`~ ....
`0 ....,
`
`,J;..
`
`,J;..
`
`e
`rJ'J.
`O'I
`1J.
`N
`'° b
`
`N
`i,-
`~
`i,-
`
`~ • 1
`
`FF1011
`Formfactor v. Feinmetall
`Page 5 of 9
`
`

`

`US 6,529,021 Bl
`
`1
`SELF-SCRUB BUCKLING BEAM PROBE
`
`FIELD OF THE INVENTION
`
`This invention is related to a probe contactor having a
`plurality of buckling beams, and more particularly to a probe
`provided with self-scrubbing features such that when a force
`is applied to the probe, the tip of the probe changes its
`landing angle, providing a controlled scrub by wiping
`action.
`
`BACKGROUND OF THE INVENTION
`
`2
`buckling to occur. Contact lengths are 16.6 mm for 0.10 mm
`diameter contacts. Contacts are made of paladium allow and
`are coated with insulating parylene. They further have
`swages head which interfaces with gold pads on the foot-
`s print transformer. The upper and lower dies provide both
`location and bearing surfaces. The offset die provides both
`bias prebuckle and vertical retention. The contacts traverse
`all three dies using holes that are aligned with respect to each
`other and which are positioned to coincide with the footprint
`10 of the device under test.
`In present day probes contactors, deflection of a probe is
`proportional to the amount of force exerted on the probe.
`This is particularly true since conventional probes are
`designed for only vertical motion, to a point that these
`15 probes are oftentimes referred to as vertical probing tech(cid:173)
`nology.
`Another type of buckling beam extensively used in the
`industry and known as a Cobra probe, is characterized as a
`hybrid type of buckling beam. The housing, insulating dies
`20 with an area array of holes, and the array of wires, are the
`same as those previously described with reference to FIG. 1.
`The Cobra probe differs in the design of the contact or wire.
`The wire is best described as a hinged-offset hybrid column,
`typically 6.4 mm in length and using a 0.13 mm Paliney 7
`25 wire. The main difference resides in the wire being flattened
`and precurved to form what is known as the active portion
`of the contact. The head is swagged at 90 degrees with
`respect to the active portion. Conventional Cobra probes do
`not have an offset die, with the lower die being displaced
`30 with respect to the fixed upper die by 1.3 to 1.9 mm. The
`contact shank rides essentially vertically in the lower die. A
`limited amount of wiping (scrubbing) of the pads of the
`device under test is achievable, although the construction of
`the Cobra probe precludes an effective wiping action which
`35 is essential to establish good contact with the pads because
`of its characteristic low inertia and vertical retention.
`The problem associated with vertical only movement
`probe is that when probing is performed on Al pads, as
`40 typically found in semiconductor wafers, the probe does not
`easily penetrate and push off the oxide layer on the pad,
`resulting in a high and unstable contact resistance.
`Related patents are:
`U.S. Pat. No. 4,027,935, Contact for an electrical contac(cid:173)
`tor assembly, issued to H. P. Byrnes et al., and of
`common assignee; and
`U.S. Pat. No. 4,423,376, to H. P. Byrnes et al., Contact
`probe assembly having rotatable contacting probe
`elements, and of common assignee.
`
`Present trends in the microelectronic industry portend
`ever increasing chip densities, which translate in the need for
`new probing devices to accommodate the increase in the
`number of 1/0 pads.
`When testing the electrical characteristics of an integrated
`circuit, whether a chip, module, and the like, the probes of
`a contactor engage the pads of the package under test to
`provide an electric path from the device under test to the
`tester. Accordingly, it is imperative to ensure that the probes
`contacting the pads be endowed with a controlled force to
`prevent or at the least, reduce, any damage to the pads.
`Oftentimes, excessive force exerted by the probe on the pad,
`particularly when dealing with a chip, can destroy the pad
`altogether, requiring the additional step of having to recon(cid:173)
`struct the pad in question by way of techniques known as
`solder reflow.
`Present art probes include etched lead frame cantilever
`contacts, which are highly versatile and which are mainly
`used for peripheral footprints. With the advent of more
`complex footprints, commercial cantilever contactors
`became more prevalent. These contactors are characterized
`in that a popout card is plugged into a motherboard. The
`contacts of such devices typically take the form of discrete
`wires extending outwardly and secured around the periph(cid:173)
`ery. The distinct advantage of such probes is its ability of
`serving the dual purpose of acting as a space transformer
`with the motherboard acting as a mounting platform for the
`desired discrete components.
`Cantilever probes are oftentimes provided with several
`layers of contacts in order to adapt such contactors to pads
`in an array formation. Such probes with more than one layer 45
`are difficult to design, specifically having a plurality of probe
`tips to be coplanar for allowing contact to all the pads of an
`array with equal force. Even when multiple layers of probes
`are used, it is still difficult to physically position them
`without they interfering with each other.
`The aforementioned commercial cantilever contactors
`were followed by custom probes, particularly useful for
`array footprint, become prevalent in view of their ability of
`incorporating a vertical acting motion. Both types, the
`cantilever contactors and custom probes provided electrical ss
`contact coupled to a spring action that proved highly ben(cid:173)
`eficial to account for product height variations. More details
`can be found in an article published by the International
`Journal of Microelectronics and Electronic Packaging, Vol.
`15, No. 4, Fourth Quarter 1992 (ISSN 1063-1674), pp. 60
`229-238, entitled "Probing considerations in C-4 Testing of
`IC wafers".
`A prior art buckling beam contactor is shown in FIG. 1.
`It consists mainly of a housing, insulating dies with an area
`array of holes, and an array of contacts or wires. These 65
`contacts employ the principal of a buckling column whereby
`the application of a force beyond the critical force causes
`
`50
`
`OBJECTS OF THE INVENTION
`
`Accordingly, it is an object of the present invention to
`provide a buckling beam probe, wherein after a critical
`buckling point, the probe force remains constant.
`It is another object of the invention to provide a buckling
`beam probe having the benefits of a cantilever probe, i.e., by
`providing scrub action while probing.
`It is yet another object of the invention to provide a
`buckling beam wherein the scrubbing direction and scrub
`length can be accurately controlled by design, independent
`of the amount of compression, thereby allowing the level of
`the device under test relative to the probe head, and the
`amount of probe compression to be controlled precisely by
`the test system.
`It is still another object of the invention to provide a shifty
`self-scrub design, wherein the scrub direction changes dur-
`
`FF1011
`Formfactor v. Feinmetall
`Page 6 of 9
`
`

`

`US 6,529,021 Bl
`
`4
`FIG. 3 schematic diagram of a shifty self-scrub buckling
`beam and an exploded view of its tip, according with the
`present invention.
`FIG. 4 is a plot showing the contact resistance versus the
`number of touchdowns for a self-scrub buckling beam
`making contact with Al pads on a wafer.
`
`DETAILED DESCRIPTION OF A PREFERRED
`EMBODIMENT OF THE INVENTION
`
`3
`ing testing, such that under no load, the tip points to the right
`which is set by the shift of the bottom plate, and when the
`probe is under load, the tip scrubs to the left, which is set by
`the offset guide plate.
`It is a further object of the invention to provide a probe s
`beam using straight wire materials which are not highly
`ductile, e.g., tungsten, as opposed to a Cobra probe which
`body is normally flat, pre-bowed shape and wherein the
`probe head is normally coined or formed to a ball shape.
`It is still a further object of the invention to provide a 10
`probe beam which can be easily repaired and wherein
`individual probe beams can be replaced without removing
`guide plates, thereby dispenses the need for a probe tip
`alignment after replacing the beam.
`
`20
`
`25
`
`30
`
`Referring now to the drawings, and more particularly to
`FIG. 2, there is shown a side view of a straight self-scrub
`buckling beam and an exploded view of its tip, in accor(cid:173)
`dance with the invention. The beam is shown both, in a no
`load state as well as under compressive stress, to be referred
`15 hereinafter as a buckled beam.
`An array of electrically conductive contacts made of thin
`straight wires 90 are held by three insulated guide plates.
`These are: a top guide plate ( or die) 10, an offset guide plate
`20 and a bottom guide plate 30. The guide plates are
`typically provided with an array of round holes having the
`same pitch as the footprint of the Device under Test (DUT).
`The diameter of the holes is slightly larger than those of the
`probe to allow a vertical displacement under load. The offset
`guide 20 hole pattern is offset by dowel pins in a predeter-
`mined direction, normally a few thousandths of an inch, in
`order to force the beam to buckle in a set direction. The
`guide plates are separated by spacers, more specifically, by
`a thin spacer 15 separating the top guide from the offset
`guide, allowing the probe wire to be offset at a low stress
`level, and by a thick spacer 25 separating the offset guide
`from the bottom guide. The thickness of the thick spacer is
`carefully calculated to achieve a certain buckling force and
`a certain probe tip 35 movement. The probe array assembly
`is normally used in combination with a footprint transformer
`35 (not shown) which is positioned above the top guide plate of
`the probe array assembly.
`In order to determine the optimum buckling force for a
`beam probe having a single 'buckle' (i.e., the wire has only
`40 one curvature when the wire is under stress), and assuming
`a critical force of 12 grams on a probe having a diameter D
`of 2.5 mils (i.e., 0.0025"), for a buckling beam made of
`tungsten, the modulus of elasticity E of the probe material,
`also referred to as Young's modulus, is 58xl06 psi. Thus, the
`moment of inertia I of the probe is given by the equation:
`
`SUMMARY OF THE INVENTION
`In a first aspect of the invention, there is provided a
`buckling beam contactor for contacting an array of pads
`positioned on a device under test that includes: a top, an
`offset and a lower insulating die, each of the insulating dies
`being provided with holes, each of the dies being separated
`from the next die by an insulated spacer of variable thick(cid:173)
`ness; and an array of flexible wires positioned substantially
`perpendicular to the dies, each of the flexible wires crossing
`a corresponding hole in each of the top, offset and lower dies
`to allow each wire respectively contact a pad of the device
`under test, wherein by shifting the center of the hole of the
`lower die relative to the center of the offset die, the tip of the
`wire exits from the lower die at an angle with respect to the
`plane formed by the pads of the device under test.
`In a second aspect of the invention, there is provided a
`method of providing of contacting an array of pads posi(cid:173)
`tioned on a device under test by way of a buckling beam
`contactor, the method including the steps of: supplying a top,
`an offset and a lower insulating die, each of the insulating
`dies being supplied with holes, each of the dies being
`separated from the next die by an insulated spacer of
`variable thickness; and providing an array of flexible wires
`positioned substantially perpendicular to the dies, each of
`the flexible wires crossing a corresponding hole in each of
`the top, offset and lower dies to allow each wire respectively
`contact a pad of the device under test, wherein by shifting
`the center of the hole of the lower die relative to the center
`of the offset die, the tip of the wire exits from the lower die
`at an angle with respect to the plane formed by the pads of
`the device under test.
`The exit angle of the wire tip is controlled by the relative
`displacement of the offset die relative to the bottom die, such
`that the exit angle of the tip of the wire at the bottom die
`changes when the probe wire is under compression.
`When a reciprocating motion to the tip of the wire is
`applied when the wires contact the surface of the device
`under test, a scrubbing motion is achieved that lowers the
`resistance between the pad of the device under test. In this
`manner, the tip of the wire cleans the surface of the pads and ss
`prevents contaminants from adhering to the tip of the wire.
`
`45
`
`/-pD 4/64-3.1416(0.0025) 4/64-1.92xl0-12
`
`Since the critical force P that causes buckling of the wire
`( also referred to as slender rod) is obtained from the fol(cid:173)
`so lowing equation:
`
`the wire length is
`
`J2-Np 2EI/64/Irn.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The foregoing and other aspects, objects and advantages
`of the invention will be better understood from the following
`description of a preferred embodiment of the invention when
`taken in conjunction with the accompanying drawings.
`FIG. 1 shows a prior buckling beam contactor, depicting
`the alignment of the holes in the upper, offset and lower dies.
`FIG. 2 is a schematic diagram showing a side view of a
`straight self-scrub buckling beam and an exploded view of
`its tip, in accordance with the present invention.
`
`Let N be defined as a constant dependent of how the top
`and the bottom of the wire are fixed. It ranges between a
`fixed N=4 and a hinged N=2 condition. Assuming N=3, the
`value of the wire length I is 1=0.353". This length determines
`60 how long the thick spacer should be for such a wire.
`Assuming now that the (single buckle) buckling beam
`probe is made of beryllium-copper.
`The critical force=lO gms for a probe having a diameter
`of 2.5 mils. The modulus of elasticity E=19xl0 6 psi. and
`65 P CR=lO gms=0.022 lbs. Accordingly, 1=1.92xl0-12
`. Assum(cid:173)
`ing once again N=3, then the length of the thick spacer
`1=0.220".
`
`FF1011
`Formfactor v. Feinmetall
`Page 7 of 9
`
`

`

`US 6,529,021 Bl
`
`5
`
`5
`Practitioners of the art will readily appreciate the length of
`the thin spacers is only dependent on the offset, and thus do
`not need to be precisely calculated.
`Still referring to FIG. 2, the construction of the guide
`plates is as follows: the offset guide hole is slightly offset to
`the right of the top guide hole to achieve a predetermined
`buckling direction. The bottom guide hole is in-line with the
`offset guide hole. The probe tip exit hole located on the
`bottom guide is slightly oversized and is characterized by a
`relatively short bearing wall (the slightly oversized exit hole
`herein creates space for the probe beam to move around
`slightly under buckling conditions). Experiments have
`shown that the bearing wall on the bottom plate should
`preferably be 250 microns high, which is rather short for a
`typical buckling beam probe construction. The dimensions
`of the oversized bottom plate hole and the height of the
`bearing wall vary in accordance with the desired probe scrub
`length. The greater the exit hole and the shorter of the
`bearing wall allow the probe tip to exit at a bigger angle
`under buckling conditions and thus induce a longer probe 20
`scrub. When the tip is compressed by the DUT, the oversized
`exit hole and the thin bearing wall will induce the probe tip
`to skew to the left (i.e. pointing to the left is caused by the
`offset plate is positioned to the right of the top and bottom
`plate. This forces the probe beam body to buckle, bowing to
`the right. Hence the tip exits at an angle from the bottom
`plate, providing the desired wiping action.
`Referring now to FIG. 3, showing a schematic diagram of
`a shifty self-scrub buckling beam, and an exploded view of
`its tip, the offset guide hole is slightly offset to the right of
`the top guide hole to create a predetermined buckling
`direction. This offset is of the order of 0.005" and is set to
`the right hand side of the top plate. The bottom guide hole
`is further offset to the right of the offset guide hole to force
`the probe tip to exit at an angle, (i.e., the probe tip cannot 35
`make a straight exit under this construction because the
`bottom plate hole is offset relative the offset plate hole),
`pointing to the right when the probe is in free stage mode
`(i.e. the buckling beam is not under any vertical stress). The
`furthest offset of the bottom plate hole relative the offset 40
`plate hole is of the order of 0.035". This inhibits the probe
`beam from exiting as a straight beam in a no-load condition,
`because the beam now is slightly curved right above the top
`surface of the bottom plate. During test, the probe tip will
`make contact under a compressive stress exerted by the DUT 45
`(i.e., the wafer). Then the probe tip exit angle changes
`direction due to the manner the offset guide is positioned
`(i.e., the offset plate hole is offset to the right of the top plate,
`setting the buckling direction).
`The spacer wall is constructed in such a manner that the 50
`probe beam can only buckle to the right because immedi(cid:173)
`ately below the offset plate, a solid wall on the left side
`prevents the beam from buckling to the left. Accordingly, the
`probe tip exit angle has to be changed. Under these
`conditions, the probe tip points to the left when the beam is 55
`buckling. This provides the probe with a wiping action.
`(Note: in practice, the actual scrub length achieved by these
`conditions is about 10 microns. Preferably, the probe tip
`used is needle shaped, with a radius of the tip of 12 microns).
`Referring now to FIG. 4, there is shown a plot of the 60
`variations of the contact resistance versus the number of
`touchdowns (also referred to as cycles). A touchdown rep(cid:173)
`resents the condition when the tester drives the probe down,
`allowing the probe contact the pads on the wafer. Resistance
`measurements are taken by the electronic within the tester. 65
`The plot shown in FIG. 3 applies to a self-scrub buckling
`beam making contact with Al pads on the wafer. The
`
`10
`
`6
`variations of the contact resistance span over 60K cycles,
`and were measured on a Sara tester, manufactured by
`Teradyne. The goal is to maintain the contact resistance to a
`very low level, preferably, below 0.5 ohms for 200k
`touchdowns, allowing cleaning only at a limited number of
`intervals, typically in the range of some 10 kcycles or one
`cleaning per 50 kcycles.
`The results shown apply to a shifted self-scrub type
`having a 40 mil offset, and operating in a non-clean room in
`order to highlight the problem associated with dust particles
`and the way of handling the cleaning of the probe tip in such
`an environment. The control group and straight scrub exhibit
`inconsistent results similar to those observed in prior art
`readings. However, some of the probes in both the control
`group and straight scrub show "good" results even at 60
`15 kcycles. At the end of 60 kcycles, only one probe reads
`above 0.5 ohm contact resistance, denoting that the probes
`have not been cleaned up to this point. Cleaning usually
`consists of lightly brushing off any contamination on the tip,
`preferably, with a soft camel hair brush.
`The contact resistance (in ohms) shown in FIG. 4 is
`normalized with respect to the same probe operating on a
`gold pad used as a standard. More specifically, some five
`readings were first taken one a golden wafer for each
`channel. The lowest reading of the five measurements was
`25 recorded to a "normalized file" used as a base resistance
`reading on gold. The contact resistance on the vertical scale
`is based on measurements taken on the aluminum pads
`minus the resistance measured on the "normalized file", i.e.,
`on the gold pads.
`While the invention has been described in terms of a
`30 single preferred embodiment, those skilled in the art will
`recognize that the invention can be practiced with changes
`and modifications, which fall within the scope of the
`appended claims.
`What is claimed is:
`1. A buckling beam contactor for contacting an array of
`pads positioned on a device under test, comprising:
`an upper, an offset and a lower plate with holes therein,
`each of the plates being distally separated from the next
`plate by a spacer, wherein the distance that separates
`the bottom of the offset plate and the top of the lower
`plate determines a critical buckling force that is depen(cid:173)
`dent on a critical buckle length LcR separating the
`offset plate from the lower plate, LcR being determined
`by the equation:
`
`wherein
`N is a coefficient dependent on end conditions of the
`wire,
`E is a modulus of elasticity of the probe material,
`I represents a moment of inertia of the probe, and
`P cR is a critical force that buckles a long wire; and
`an array of flexible wires positioned transversally to the
`plates, wherein each of the flexible wires cross a
`corresponding hole in the upper, offset and lower
`plates, each wire respectively contacting a pad of the
`device under test, wherein by shifting the center of
`the hole of the lower plate relative to the center of the
`offset plate, the tip of the wire exiting from the lower
`plate at an angle with respect to the plane formed by
`the pads of the device under test provides a control(cid:173)
`ling wiping movement.
`2. The buckling beam contactor as recited in claim 1,
`wherein the exit angle of the wire tip is controlled by the
`relative displacement of the offset plate relative to the lower
`plate.
`
`FF1011
`Formfactor v. Feinmetall
`Page 8 of 9
`
`

`

`US 6,529,021 Bl
`
`8
`pads of the device under test, and wherein by further
`changing the angle in a reciprocating motion of the
`tips of the flexible conductive, a scrubbing action on
`the pads of the device under test is obtained.
`10. A method of wiping an array of pads positioned on a
`device under test, the method comprising the steps of:
`providing a buckling beam contactor comprising an
`upper, an offset and a lower plates with holes therein,
`each of the plates being distally separated from the next
`plate by a spacer, the distance separating the bottom of
`the offset plate and the top of the lower plate deter(cid:173)
`mining a critical buckling force that is dependent on a
`critical buckle length LcR separating the offset plate
`from the lower plate, LcR being determined by the
`equation:
`
`10
`
`15
`
`7
`3. The buckling beam contactor as recited in claim 1,
`wherein a change of direction of the exit angle of the tip of
`the wire at the lower plate changes when the probe wire is
`under compression.
`4. The buckling beam contactor as recited in claim 1, 5
`wherein the critical buckle distance separating the upper
`plate from the offset plate is determined by the amount of
`offset between the center of the hole positioned in the upper
`plate relative to the center of the corresponding hole in the
`offset plate.
`5. The buckling beam contactor as recited in claim 1,
`wherein a reciprocating motion of the tip of the wire against
`a surface of the device under test provides a scrubbing
`motion that lowers the resistance between the pad of the
`device under test and the tip of the wire.
`6. The buckling beam contactor as recited in claim 5,
`wherein a scrubbing motion of the tip of the wire is obtained
`by a compressive force exerted by device under test on tip
`of the wire.
`7. The buckling beam contactor as recited in claim 5, 20
`wherein the scrubbing motion of the tip of the wire cleans
`the surface of the pad contacted by the wire tip.
`8. The buckling beam contactor as recited in claim 5,
`wherein the scrubbing motion of the tip prevents contami(cid:173)
`nants from adhering to the tip of the wire.
`9. A buckling beam contactor for contacting an array of
`pads positioned on a device under test, comprising:
`an array of flexible conductive wires arranged in a for(cid:173)
`mation that matches a footprint of the device under test,
`the flexible conductive wires positioned transversally 30
`to the device under test;
`an upper, an offset and a lower insulating plate with holes
`therein, each of the plates being distally separated from
`the next plate by a spacer, the distance separating the
`bottom of the offset plate and the top of the lower plate 35
`determining a critical buckling force that is dependent
`on a critical buckle length LcR separating the offset
`plate from the lower plate, LcR being determined by the
`equation:
`
`25
`
`wherein
`N is a coefficient dependent on end conditions of the
`wire,
`E is a modulus of elasticity of the probe material,
`I represents a moment of inertia of the probe, and
`P cR is a critical force that buckles a long wire; and
`providing an array of flexible wires positioned trans-
`versally to the plates, each of the flexible wires
`crossing a corresponding hole in each of the upper,
`offset and lower plate to allow each wire respectively
`contact a pad of the device under test, wherein by
`shifting the center of the hole of the lower plate
`relative to the center of the offset plate, the tip of the
`wire exits from the lower plate at an angle with
`respect to the plane formed by the pads of the device
`under test.
`11. The method as recited in claim 10, wherein the exit
`angle of the wire tip is controlled by the relative displace(cid:173)
`ment of the offset plate relative to the lower plate.
`12. The method as recited in claim 10, wherein a change
`of direction of the exit angle of the tip of the wire at the
`lower plate changes when the probe wire is under compres-
`4o SlOn.
`13. The method as recited in claim 10, wherein a recip(cid:173)
`rocating motion of the tip of the wire against a surface of the
`device under test provides a scrubbing motion that lowers
`the resistance between the pad of the device under test and
`45 the tip of the wire.
`14. The method as recited in claim 13, wherein the
`scrubbing motion of the tip of the wire is obtained by a
`compressive force exerted by device under test on tip of the
`Wlfe.
`15. The method as recited in claim 13, wherein the
`scrubbing motion of the tip of the wire cleans the surface of
`the pad contacted by the wire tip.
`16. The method as recited in claim 13, wherein the
`scrubbing motion of the tip prevents contaminants from
`adhering to the tip of the wire.
`
`wherein
`N is a coefficient dependent on end conditions of the
`wire,
`E is a modulus of elasticity of the probe material,
`I represents a moment of inertia of the probe, and
`P cR is a critical force that buckles a long wire; and
`each of the flexible conductive wires crossing a corre-
`sponding hole in each of the upper, offset and lower 50
`plate to allow each of the flexible conductive wires
`respectively contact a pad of the device under test,
`wherein by shifting the position of the center of the
`hole of the lower plate relative to the position of the
`center of the offset plate, the tip of each of the 55
`flexible conductive wires exits from the lower plate
`at an angle with respect to the plane formed by the
`
`* * * * *
`
`FF1011
`Formfactor v. Feinmetall
`Page 9 of 9
`
`