U.S. patent application number 10/389630 was filed with the patent office on 2004-01-29 for low-current pogo probe card.
Invention is credited to Cowan, Clarence E., Tervo, Paul A..
Application Number | 20040017214 10/389630 |
Document ID | / |
Family ID | 25357776 |
Filed Date | 2004-01-29 |
United States Patent
Application |
20040017214 |
Kind Code |
A1 |
Tervo, Paul A. ; et
al. |
January 29, 2004 |
Low-current pogo probe card
Abstract
A low-current pogo probe card for measuring currents down to the
femtoamp region includes a laminate board having a layer of
conductive traces interposed between two dielectric layers. A
plurality of probing devices, such as ceramic blades, are
edge-mounted about a central opening so that the probing needles or
needles included therein terminate below the opening in a pattern
suitable for probing a test subject workpiece. A plurality of pogo
pin receiving pad sets, each including a guard pad, occupy the
periphery of the board. Each guard pad is electrically connected to
a trace from the layer of conductive traces. The pad sets may be
connected to the probing devices by low noise cables or traces. Air
trenches separate the pad sets for reducing cross talk and signal
settling times.
Inventors: |
Tervo, Paul A.; (Vancouer,
WA) ; Cowan, Clarence E.; (Newberg, OR) |
Correspondence
Address: |
Kevin L. Russell
Suite 1600
601 SW Second Ave.
Portland
OR
97204-3157
US
|
Family ID: |
25357776 |
Appl. No.: |
10/389630 |
Filed: |
March 13, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10389630 |
Mar 13, 2003 |
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08871609 |
Jun 10, 1997 |
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6034533 |
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10389630 |
Mar 13, 2003 |
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09490264 |
Jan 24, 2000 |
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6559668 |
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Current U.S.
Class: |
324/755.11 ;
324/756.03 |
Current CPC
Class: |
G01R 1/06733 20130101;
G01R 1/06772 20130101; G01R 1/07342 20130101; G01R 1/067 20130101;
G01R 1/06711 20130101 |
Class at
Publication: |
324/754 |
International
Class: |
G01R 031/02 |
Claims
1. A pogo probe card for probing a test device comprising: (a) a
laminate board including a top dielectric layer, a bottom
dielectric layer and a set of auxiliary guard traces interposed
between said top dielectric layer and said bottom dielectric layer,
said board forming an opening and having a top major surface and a
bottom major surface; (b) a plurality of probing devices for
probing a plurality of probing sites on a test device, each probing
device including an elongate probing needle and an electrical
connection point electrically connected to said probing needle,
said probing devices being mounted to said top major surface in
radial arrangement about said opening and extending below said
opening, so that said probing needles terminate in a pattern
suitable for probing said sites; (c) a plurality of pogo pin
receptive pad sets; (d) a set of first conductors for electrically
connecting each pad set to a said auxiliary guard trace; and (e) a
second conductor for electrically connecting a said electrical
connection point to a said pad set.
2. The pogo probe card of claim 1 wherein said second conductor is
in the form of traces on the first surface of said laminate
board.
3. The pogo probe card of claim 1 wherein said second conductor is
in the form of a coaxial cable center conductor.
4. The pogo probe card of claim 3 wherein each said pad set
includes a signal pad and a guard pad and said coaxial cable center
conductor connects said signal pad to said electrical connection
point and is encircled by a peripheral conductor that is
electrically connected to a said guard line pad.
5. The pogo probe card of claim 1 wherein said first conductors are
in the form of plated vias.
6. The pogo probe card of claim 1 wherein said first conductors are
in the form of plated channels.
7. The pogo probe card of claim 1 wherein a trench is formed
between each neighboring pair of said pogo pin receptive pad
sets.
8. The pogo probe card of claim 1 wherein said second dielectric
layer has a first and a second major surface and wherein said first
major surface is joined to said conductive layer and said probe
card further includes a chuck guard conductive layer substantially
covering said second major surface of said second dielectric
layer.
9. A probing device for probing a probing site on a test device,
comprising: a dielectric substrate having first and second sides,
an elongate conductive path on said first side and a replaceable
elongate probing needle connected to one end of said elongate
conductive path so as to extend in a cantilevered manner beyond
said substrate.
10. The probing device of claim 9 including an adjustment mechanism
for adjusting the position of said probing terminus of said probing
needle relative to the position of the dielectric substrate.
11. The probing device of claim 10 wherein said adjustment
mechanism is a set of set screws.
12. A method for custom assembling a pogo probe card for testing a
predetermined set of probing sights resident on a predetermined
integrated circuit type, to fully utilize the number of pogo pin
receptive pad sets available, comprising: determining the number,
location and relative sensitivity providing a pogo probe card
workpiece, including: (a) a laminate board including a first
dielectric layer, a second dielectric layer and a conductive layer
interposed between said first dielectric layer and said second
dielectric layer, said board forming an opening and having a first
and a second major surface; (b) a plurality of probing devices for
probing a plurality of probing sites on a test device, each probing
device including an elongate probing needle and an electrical
connection arm electrically connected to the probing needle, said
probing devices being mounted to the first major surface in radial
arrangement about said opening, extend into said opening and
traverse a plane defined by the second major surface, so that said
probing needles terminate in a pattern suitable for probing said
sites; (c) a plurality of pogo pin receptive pad sets each pad set
including a force line pad, a sense line pad and a guard line pad
that surrounds both said sense line pad and said force line pad;
and (d) a set of first conductors for electrically connecting each
guard line pad to said conductive layer; and electrically
connecting both the sense line pad and the force line pad of a
first pogo pin receptive pad set to a first electrical connection
arm; electrically connecting only the force line pad of a first
pogo pin receptive pad set to an electrical connection arm adapted
to be connected to a first test site; and electrically connecting
only the sense line pad of a second pogo pin receptive pad set to
an electrical connection arm that is adapted to be connected to the
first test site.
13. The method of claim 12 wherein both said sense line pad of said
first pogo pin receptive pad and said force line pad of said second
pogo pin receptive area are electrically connected to the same
electrical connection arm.
14. The method of claim 12 wherein said sense line of said first
pogo pin receptive pad is electrically connected to a second
electrical connection arm, which is adapted to be connected to the
first site and said force line pad of said second pogo pin
receptive pad is electrically connected to a third said electrical
connection arm, which is adapted to be connected to the first site.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to pogo probe cards used for
probing test subject workpieces, such as wafers of integrated
circuit dies ("IC wafer dies"), and in particular relates to pogo
probe cards suitable for use in measuring current as low as the
femtoamp order of magnitude (ultra-low current).
[0002] Typically, a pogo probe card includes a dielectric board
having top and bottom major surfaces and forming a base for other
elements. A plurality of probing devices are mounted in radial
arrangement about the top rim of a round central opening in the
board. A probing needle for each of these devices terminates below
the opening in a pattern suitable for probing contact sites,
otherwise referred to as test sites, of an IC wafer die. For ease
of description in this application the portion of the pogo probe
card which in operation is closest to the IC wafer is denoted as
the bottom of the card, although other geometries of test are
possible.
[0003] Around the exterior periphery of the pogo probe card there
are typically 48, 64 or 96 pogo pin receptive pads or pad sets,
each of which is electrically connected to a respective one of the
probing devices by a signal trace or set of traces. During testing,
a mating pogo test head with a matching set of 48, 64 or 96 pogo
pins or pin sets, touches the pogo probe card so that the pogo pins
make electrical contact with the receptive pads. In this manner the
probing devices are individually connected to respective channels
of a test instrument by the pogo pin sets and further cabling.
[0004] In one conventional type of setup for testing IC wafer dies,
the pogo probe card is mounted by a supporting rig above the IC
wafer, and a chuck supports and moves the IC wafer so that each
die, or region to be tested, is consecutively brought into contact
with the probing needles.
[0005] As integrated circuitry has been made smaller, a need has
developed for test devices which can measure ultra-low current. The
typical use for this type of device is to measure IC leakage
currents. These are currents that flow away from the intended
current path within the IC, typically due to design flaws or
fabrication artifacts.
[0006] Low-current measurements are typically performed with two
conductive paths ("force" and "sense") either reaching the test
site independently ("true Kelvin" connection) or joining together
in the proximity of the test site ("quasi Kelvin" connection) to
form a "signal path." The force path, whose test equipment terminus
has a relatively low impedance, is provided with a particular
current. The sense path, whose test equipment terminus has a very
high impedance, measures the voltage at the test site. As such, the
current versus voltage characteristics of the test device can be
obtained using the force and sense paths.
[0007] This test configuration is desirable because although small
variations in current are being measured, the amount of current
directed to the test site might be large enough so that there is a
significant voltage drop through the signal line leading to the
test site. Because this signal line typically includes solder
connections and pogo pin contacts, its resistance is impossible or
impractical to predict using current technology. Therefore, the
distance from the test site to the point at which the signal path
splits into force and sense path is a determinant of test quality,
referred to in the low-current test industry as the degree to which
the test configuration approaches the ideal "true kelvin"
configuration in which the force and sense paths are connected by
the conductive test site itself.
[0008] Collectively, the force and sense paths are referred to as
the signal path(s). On pogo probe cards the force and sense paths
are typically in the form of conductive traces, both of which are
on the top surface of the card.
[0009] Designers of ultra-low current probe cards have been
concerned with reducing probe card leakage currents. These are
unwanted currents that can flow into a first force or sense path
from nearby conductive path sets, thereby distorting the current
measured in the first force or sense path. The amount of leakage
current between two conductive path sets is dependant on the
resistivity of the insulating material that separates the paths.
When measuring in the femtoamp order of magnitude, even materials
which are generally thought of as being completely insulative, such
as rubber or glass-epoxy, may permit a detrimental flow of leakage
current.
[0010] To protect a test station from electromagnetic interference,
elaborate shielding has been developed. U.S. Pat. No. 5,345,170,
which is assigned to the same assignee as the present application,
describes one such design.
[0011] One technique that has been used for suppressing
interchannel leakage currents on pogo probe cards is providing
"guard" traces on both sides of a force or sense trace on the top
surface of the card which is maintained at the same potential as
the signal trace on the top surface of the card by a feedback
circuit in the output channel of the test instrument. Because the
voltage potentials of the outer guard traces on both sides of a
force or sense trace on the top surface of the card and the inner
signal trace are made to substantially track each other, negligible
leakage current will flow across the dielectric material of the
card that separates these traces.
[0012] Although leakage current can still flow between neighboring
guard traces, this is typically not a problem because these guard
conductors, unlike the inner signal trace, are at low impedance. By
using this guarding technique, significant improvement may be
realized in the low-level current measuring capability of pogo
probe cards.
[0013] Low current pogo probe cards that have force, sense, and
guard traces have a "pad set" for each signal channel consisting of
a force, sense, and guard pad and a corresponding "trace set"
consisting of a guard trace and a combined force and sense
trace.
[0014] To further improve low-current measurement capability, pogo
probe cards have been constructed so as to minimize leakage
currents between the individual probing devices that mount the
probing needles or other needles. In these devices,
higher-resistance ceramic insulating materials have been
substituted for lower-resistance materials and additional guard
channel conductive surfaces have been added.
[0015] In one type of assembly, for example, each probing device is
constructed using a thin "blade" of ceramic material, which is a
material known to have a relatively high volume resistivity. An
elongate conductive trace is provided on one side of the "blade" to
form the signal line and a backplane conductive surface is provided
on the other side of the "blade" for guarding purposes.
[0016] The probing element of this device is formed by a slender
conductive needle, such as of tungsten, which extends in a
cantilevered manner away from the signal trace. Such devices are
commercially available, for example, from Cerprobe Corporation
based in Tempe, Ariz. During assembly of the probe card, the
ceramic blades are edge-mounted in radial arrangement about the
opening in the card so that the needles terminate below trace sets
results in a delay in charging and discharging the trace sets to a
predetermined potential. In essence, the dielectric absorption
forms a capacitor that must be charged or discharged to reach a
different voltage. A test sequence for a particular IC wafer may
include hundreds of brief tests. A delay of 1 second or a fraction
thereof in the performance of each test, may substantially increase
the total test time. By reducing the settling time of the trace
sets, it may be possible to run the same number of test sequences
in less time and with fewer test stations.
[0017] An additional problem encountered in prior art probe cards
is the problem of probing needle damage. Probing needles
occasionally break, or are otherwise damaged during testing,
requiring replacement or repair. In currently available probe
cards, when a probing needle breaks, the entire probing device must
be replaced which is time consuming and expensive.
[0018] What is desired, therefore, is a pogo probe card with
increased isolation between traces resulting in reduced leakage
currents, reduced cross-talk between trace sets, and reduced
settling time for each trace set.
SUMMARY OF THE INVENTION
[0019] The present invention overcomes the aforementioned drawbacks
of the prior art by providing in a first aspect a pogo probe card
that has auxiliary guard traces interposed between top and bottom
dielectric layer of the card. Preferably, a set of conductors
formed through the top dielectric layer electrically interconnects
each guard trace on the top surface of the card to the auxiliary
guard trace. By interposing the auxiliary guard traces between the
top and bottom dielectric layers it is possible to place the
auxiliary guard traces in close proximity to the guard trace on the
top surface of the card, thereby reducing leakage current by
reducing the cross-section for the leakage current path, but
nevertheless providing a card with sufficient structural
integrity.
[0020] In another aspect of the present invention, the pogo pin
receptive pad sets of the pogo probe card are connected to the
probe devices by coaxial or triconductor ("triax") cables, rather
than by traces on the card. This permits far greater isolation and
complete guarding of each signal channel.
[0021] In a further aspect of the present invention, the probe card
includes probing device holders that permit probing devices to be
easily replaced when broken or damaged.
[0022] The foregoing and other objectives, features, and advantages
of the invention will be more readily understood upon consideration
of the following detailed description of the invention, taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a top sectional view of a prior art low-current
pogo probe card.
[0024] FIG. 2A is an isometric sectional view of a low-current pogo
probe card according to the present invention.
[0025] FIG. 2B is a cross-sectional view of the low-current probe
cards of FIGS. 2A and 6A taken along line 2B-2B of FIG. 6A.
[0026] FIG. 3 is a sectional top view of the pogo probe card of
FIG. 2.
[0027] FIG. 4 is a sectional isometric view of the pogo probe card
of FIG. 2.
[0028] FIG. 5 is a transverse cross-sectional view of the low noise
coaxial cable shown in FIGS. 2, 3, 4, 6A, 6B, 6D and 6F.
[0029] FIG. 6A is top sectional view of the low-current pogo probe
card of FIG. 2 showing a first method of connecting the pogo pin
receptive pad sets to the probing devices.
[0030] FIG. 6B is top sectional view of the low-current pogo probe
card of FIG. 2 showing a second method of connecting the pogo pin
receptive pad sets to the probing devices.
[0031] FIG. 6C is top sectional view of the low-current pogo probe
card of FIG. 2 showing a third method of connecting the pogo pin
receptive pad sets to the probing devices.
[0032] FIG. 6D is top sectional view of the low-current pogo probe
card of FIG. 2 showing a forth method of connecting the pogo pin
receptive pad sets to the probing devices.
[0033] FIG. 6E is top sectional view of the low-current pogo probe
card of FIG. 2 showing a fifth method of connecting the pogo pin
receptive pad sets to the probing devices.
[0034] FIG. 7A is a cross-sectional view of the low noise
triconductive coaxial cable shown in FIG. 6C.
[0035] FIG. 7B is a cross-sectional view of a low noise
triconductive cable that may be substituted for the cable of FIG.
7A.
[0036] FIG. 8 is a cross-sectional view of an alternative
embodiment of a low-current probe card having a top view identical
with that of FIGS. 2A and 6A-6E and taken along line 2B-2B of FIG.
6A.
[0037] FIG. 9A is a top sectional view of a further alternative of
a low current pogo probe card.
[0038] FIG. 9B is a cross-sectional view of the low current pogo
probe card of FIG. 9A taken along line 9B-9B of FIG. 9A.
[0039] FIG. 10A is a top sectional view of an additional
alternative embodiment of a low-current pogo probe card according
to the present invention.
[0040] FIG. 10B is a cross-sectional view of the low-current pogo
probe card of FIG. 10A taken along line 10B-10B of FIG. 10A.
[0041] FIG. 11 is a pictorial view of an exemplary embodiment of a
probe housing with a probe connector, and probing device for use in
the present invention.
[0042] FIG. 12 is a cross-sectional view of the probe housing and
probing device engaged within the probe connector of FIG. 11.
[0043] FIG. 13 is a cross-sectional view of the probe connector and
probing device of FIG. 11.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] FIG. 1 shows a section of a prior art low-current pogo probe
card 110. Pogo pin receptive pad set 112 is comprised of a
conductive sense line pad 114, a conductive force line pad 116 and
a conductive guard line pad/trace 118 formed on a dielectric board
113. Each sense line pad 114 and force line pad 116 join together
to form a signal trace 119, which connects with a probing device
120. An auxiliary guard trace (not shown) on the bottom surface of
card 110, is electrically connected to each guard trace 118 through
a series of plated vias 121. Each guard trace 118 is electrically
connected into a conductive land 122 which is electrically
connected to a guard plane side of probing device 120 by solder
point 124. Each signal trace 119 is electrically connected, via
wire 127, to an electrical connection point 126 on a respective
probing device 120, which in turn is connected to a respective
probing needle 128. Unfortunately, when used for making ultra-low
current measurements card 110 hampers the testing effort because of
excessive charge-up time, cross talk, poor shielding of the test
subject workpiece and incomplete signal guarding.
[0045] Referring to FIGS. 2-6A a low-current pogo probe card 152 is
constructed in accordance with the present invention. This card
includes a plurality of probing devices 154 (only one shown in FIG.
2), arranged in a circular arrangement, supported on the top of an
inner rim 156 of an annular laminate board 158.
[0046] Each probing device 154 includes a probing needle 164.
Probing needles 164 are typically composed of tungsten and extend
generally radially inwardly in a cantilevered manner into a
circular opening 166, which is defined by inner rim 156. Each
probing needle 164 and probing device 154 are carefully adjusted by
a technician so that the point of the probing needle 164 will
contact one out of a set of test contact sites on a predetermined
test subject workpiece (not shown), which is to be placed beneath
card 152 for testing.
[0047] Near the outer rim of annular laminate board 158 are a
plurality of pogo pin receptive pad sets 170, each of which
includes a conductive force line pad 172, a conductive sense line
pad 174 and a conductive guard line pad/trace 176 that surrounds
both sense line pad 174 and force line pad 172.
[0048] A set of coaxial cables 180 (only one shown in FIG. 2)
connect each pad set 170 to a respective probing device 154 as
described herein. An annular electromagnetic shield 182, protects
the signals being carried by cables 180 from electromagnetic
interference. The shield 182 may be connected to the shield of the
test probe station.
[0049] FIG. 2B is a cross-sectional view taken along line 2B-2B of
FIG. 6A. Laminate board 158 comprises a top or first dielectric
layer 200, a bottom or second dielectric layer 202 (in this
instance layer 202 is composed of two laminae), and an auxiliary
guard trace 204 interposed between layer 200 and layer 202. A set
of plated vias 206 electrically connect guard line pad/trace 176 to
auxiliary guard trace 204, which is maintained at the guard
voltage. In this embodiment plated vias 206 perforate laminate
board 158, although this it is only necessary for vias 206 to
breach top layer 200 in order to connect with corresponding
auxiliary guard trace 204. Alternatively, wire wraps could extend
around the exterior periphery to interconnect guard line pad/trace
176 to guard trace 204. The shorter distance between pad set 170
and auxiliary guard trace 204, made possible by the interposition
of auxiliary trace 204 within laminate board 158, results in a
greater isolation of pad sets 170 from each other and from outside
electromagnetic noise. This primarily reduces the dielectric
absorption and thus settling time for signals.
[0050] A trench 210 separates pad sets 170 from one another. Trench
210 reduces the capacitance between pad sets 170, thereby reducing
cross-talk between pad sets 170 and further reducing the settling
time for each individual pad set 170. The reduction in capacitance
is primarily due to the much smaller dielectric constant of air
compared with any of the available solid dielectric materials, such
as polyimide, which has a dielectric constant approximately eight
times that of air. Trench 210, which is not plated preferably
extends through and therefore separates auxiliary guard traces 204
from one another. Typically in manufacture a solid conductive plate
is interposed between dielectric layers 200 and 202. This layer is
then divided into auxiliary traces 204 when trenches 210 are
machined therein. Alternatively, even without the trench 210, the
reduced cross section between the pad sets 170 and the auxiliary
guard traces 204 reduces leakage currents. In this variant, the
auxiliary guard traces 204 would be formed as separated metalized
areas under each pad set 170.
[0051] Referring to FIGS. 3 and 4, each cable 180 is electrically
connected to a proximal end of a probing device 154. Each probing
device 154 includes a dielectric substrate or "blade" 236
preferably formed of ceramic or a comparable high-resistance
insulating material.
[0052] Each blade 236 has a first and a second major surface, which
are parallel to each other. On the first surface of each blade is
an elongate conductive path 238 that connects probing needle 164 to
an electrical connection point 239. The second major surface bears
a conductive plate 240. Blade 236 is generally L-shaped in profile
and is edge-mounted on the top of inner rim 156 of annular laminate
board 158 so that its short arm extends through opening 166 thereby
permitting needles 164 to terminate below opening 166. Blades 236
having a construction of the type just described are commercially
available from Cerprobe Corporation of Tempe, Ariz.
[0053] A plurality of conductive lands 244 are formed on the
laminate board 158 about the opening 166 in circumferentially
spaced relationship to each other. A solder connection 248
electrically connects conductive plate 240 to a conductive land
244.
[0054] FIG. 5 shows a cross-sectional view of a preferred coaxial
cable 180, which includes an inner conductive core 250, an inner
dielectric 252, a buffer layer 254, an outer conductor 256 and an
insulative jacket 258. Buffer layer 254 is of suitable composition
for reducing triboelectric current generation between inner
dielectric 252 and outer conductor 256 to less than that which
would occur were inner dielectric 252 and outer conductor 256 to
directly adjoin each other. Inner layer 254 should have physical
properties similar to inner dielectric 252 so that layer 254 does
not rub excessively against inner dielectric 252 despite cable
flexing or temperature changes.
[0055] Moreover, inner layer 254 should have sufficient conductive
properties to dissipate any charge imbalances that may arise due to
free electrons rubbing off the outer conductor. A suitable material
for this purpose is a fluoropolymer such as TEFLON.TM. or other
insulative material such as polyvinylchloride or polyethylene in
combination with graphite or other sufficiently conductive
additive.
[0056] In the field of radio frequency (rf) cable technology,
cables that include a layer of the type just described are
generally referred to as "low-noise" cables. Commercial sources for
this type of cable include Belden Wire and Cable Company based in
Richmond, Ind. and Suhner HF-Kabel based in Herisau, Switzerland.
With regard to the preferred embodiment depicted, the cable which
was used was purchased from Times Microwave Systems based in
Wallingford, Conn.
[0057] It should be noted that some care must be exercised while
connecting cable 180 to probing device 154 in order to prevent
defects that would substantially degrade the low-current measuring
capability probe card 152. Referring to FIGS. 3-4, a solder
connection 260 connects the inner conductor 250 of each cable 180
to conductive path 238 at electrical connection point 239 of a
corresponding probing device 154.
[0058] Before making this connection, it is desirable to position
cable 180 so that the conductive and dielectric layers in cable 180
that surround inner core 250 are set back a certain distance 262
away from the proximal edge of the probing device 154. This reduces
the possibility that a fine strand of hair or other contaminant
will form a low-resistance or conductive bridge so as to cause a
low-resistance shunt or short across the signal line. Also, in
making this connection, it is important not to overheat the cable
so as not to impair the structural properties of inner dielectric
252, which material may comprise, for example, air-expanded
TEFLON.TM. for maximum temperature stability.
[0059] Finally, after the connection has been made, all solder flux
residue that remains should be removed from the board in order to
prevent undesired electro-chemical effects and to maintain the
surface resistivity of the laminate board 158 at a reasonable
level.
[0060] In order to further reduce the possibility of undesirable
shunting connections, outer conductor 256 (typically a metallic
braid) of cable 180 is connected indirectly to conductive surface
240 through conductive land 244.
[0061] Moreover, with respect to probing devices 154, each elongate
conductive path 238 is guarded by conductive path plate 240 on the
opposite side of the blade 236 and by the corresponding conductive
land 244 which is arranged below the path. Solder connection 264
electrically connects the outer conductor 256 to conductive land
244 and a second solder connection 248 electrically connects
conductive land 244 to the backplane conductive surface 240. Again,
care must be taken not to overheat cable 180 or to leave solder
flux residue on laminate board 158.
[0062] During use of probe card 152, the signal variation or
voltage is transmitted to the test site by means of inner conductor
250, elongate conductive path 238 and probing needle 164.
Preferably, the test equipment is connected so that a feedback
circuit in the output channel of the test equipment supplies a
"guard" voltage that matches the instantaneous signal voltage,
which guard voltage is applied to outer conductor 256 and to
conductive land 244. By use of the cable 180, additional shielding
is achieved by nearly eliminating the capacitance associated with
the prior art use or long traces on the probe card to route the
signal from the pad sets 170 to the probing devices 154.
Accordingly, the cable 180 reduces the leakage currents and
settling time.
[0063] Moreover, with respect to probing devices 154, each elongate
conductive path 238 is guarded by conductive plate 240 on the
opposite side of the blade 236 and by the corresponding conductive
land 184 which is arranged below the path. By minimizing leakage
currents into and out of each elongate path 238, this guarding
system reduces the levels of undesired background current and so
enhances the effect achieved by the use of cables in suppressing
leakage currents and reducing settling times.
[0064] FIGS. 6B-6E show alternative structures for connecting a pad
set 170 to a probing device 154. FIG. 6B shows a first coaxial
cable 180a and a second coaxial cable 180b connecting a pad set 170
to a probing device 154. A center conductor 250a of cable 180a
connects force line pad 172 to point 239 whereas a center conductor
250b of cable 180b connects sense line pad 174 to point 239. An
outer conductor 256a of first cable 180a and an outer conductor
256b of second cable 180b both connect guard line pad/trace 176 to
land 244. This structure provides separate guarding for the force
and sense signals until connection with the elongate conductive
path 238 of the probing device 154. This configuration a force and
sense connection that more closely approximates an ideal "true
kelvin" connection then the configuration of FIG. 6A.
[0065] FIGS. 6C and 7A show an alternative design in which a cable
with three conductors that share a common axis (a "triax") triax
276 connects a pad set 170 to a corresponding probing device 154.
Innermost conductor 250c of cable 276 connects sense pad 174 to
elongate path 238, middle conductor 256c connects force pad 172 to
electrical connection point 239, and exterior conductor 280
connects guard line pad/trace 176 to conductive land 244. FIG. 7A,
which is described later, shows a transverse cross-sectional view
of cable 276.
[0066] FIG. 6D shows a connection scheme in which a first and a
second pad set 170d and 170e are connected to the same probing
device 154 by two coaxial cables 180d and 180e. The force line pad
172d of first pad set 170d and the sense line pad 174e of a second
pad set 170e are both connected to electrical connection point 239
via a center conductor 250d of first coaxial cable 180d and a
center conductor 250e of second coaxial cable 180e respectively.
Guard line pad/trace 176d of a first pad set 170d and guard line
pad/trace 176e of second pad set 170e are both connected to a land
244 by an outer conductor 256d of first coaxial cable 180d and an
outer conductor 256e of second coaxial cable 180e, respectively.
This configuration approximates a "true kelvin" connection in the
same manner as the configuration shown if FIG. 6B.
[0067] FIG. 6E shows a connection scheme in which two adjacent pad
sets 170f and 170g are connected to two adjacent probing devices
154f and 154g by two coaxial cables 180f and 180g. The probing
needles 164 of both probing devices 154f and 154g are directed to
the same testing site on the test subject workpiece to create a
"true kelvin" connection between force and sense. The force line
pad 172f of first pad set 170f is electrically connected to an
electrical connection point 174g by way of center connector 250f of
cable 180f. The sense line pad 174g of second pad set 170g is
electrically connected to an electrical connection point 239g by
way of center connector 250g of cable 180g. Guard line pad/traces
176f and 176g of first and second pad sets 170f and 170g are
connected to lands 244f and 244g, respectively by outer conductors
256f and 256g, respectively.
[0068] FIG. 7A shows a transverse cross-sectional view of cable 276
of FIG. 6C. In similarity to cable 180, cable 276 includes an inner
conductor or core 250c, an inner dielectric 252c, an inner layer
254c, an outer conductor 256c and an insulative jacket 258c. Cable
276 further includes a second inner dielectric 278 and a second
outer conductor 280. Cable 276 is a low noise triax cable.
[0069] FIG. 7B shows a variant triconductive cable 281 which may be
used in the configuration of FIG. 6C instead of cable 276. In the
approximate center of cable 281 are a force conductor 282 (to be
connected in like manner to conductor 256c of FIG. 6) and a sense
conductor 284 (to be connected in like manner to 250c of FIG. 6). A
low noise dielectric layer 286 surrounds conductors 282 and 284. A
conductive guard layer 288, in turn, surrounds layer 286. Finally,
a dielectric insulative layer 290 surrounds layer 288.
[0070] FIG. 8 shows a cross-section taken along line 2B-2B of FIG.
6A of an embodiment of laminate board 158 in which a third layer of
conductive material 376 is interposed between a second layer of
dielectric material 378 and a third layer of dielectric material
380. Additionally, a forth layer of conductive material 384 is
attached to the bottom of layer 380. Third layer 380 is connected
to a first electrical feature 386 and forth conductive layer 384 is
connected to a second electrical feature 388. The first and second
electrical features 386 and 388 may be the same.
[0071] In one variant, feature 386 is the "chuck" or "return"
guard. This chuck or return guard is described in greater detail in
U.S. Pat. No. 5,345,170, which is assigned to the assignee of the
present application and is incorporated by reference into the
present application as if fully set forth herein. In the case where
the return signal path is through the wafer and into the chuck,
this allows for a return guard path from the chuck guard that is
the same as the guard of the probe card.
[0072] In a second variant, feature 386 is the same as shield 182,
so that layer 376 forms a bottom shield for card 152.
[0073] In a third variant feature 386 is an instrument channel set
to drive layer 380 to parallel the potential of whichever signal
channel was actively engaged in forming a measurement. In this
application layer 376 or 384 would be connected to a test
instrument channel that would drive layer 376 or 384 to the voltage
of whatever trace set was actively engaged in a measurement. This
would reduce noise and settling time. The second and third variants
perform the important additional function of shielding or guarding,
respectively, the test subject workpiece. This task is rendered
comparatively more difficult in the pogo probe environment because
of the 21 cm (8.5 in) aperture required to accommodate the
introduction of the pogo probe head into the shielded box that is
used in ultra-low current testing. By placing a guard or shield or
both in the bottom portion of the probe card 152, the test subject
workpiece may be shielded or guarded or both despite the presence
of this aperture.
[0074] In another set of three variants, feature 388 would
represent either the guard chuck, shield 182, or the test
instrument channel set to mimic the active measurement channel.
Typically, conductive layer 376 and 384 (either one of which may be
omitted) would be continuous over the area of board 158, except
they would be cut away to avoid contact with vias 206. In the third
variant the additional layer 376 or 384 may be divided into
additional auxiliary guard traces.
[0075] Dielectric layers 200, 378, and 380 are all about 1 mm (39
mils) thick. Dielectric layer 202 (FIG. 2B) is about twice that
thickness to provide sufficient structural strength to board 152.
Pad sets 170 and conductive layers 204, 376 and 384 are typically
about 75 microns (3 mils) thick. Dielectric layers 200, 202 (FIG.
2B), 378 and 380 are typically composed of polyimide or FR4. Pad
sets 170, plated vias 206 and conductive layer 384 are typically
made of gold over copper or some other highly conductive material.
Conductive layers 204 and 376 are typically made of copper.
[0076] FIGS. 9A and 9B show an embodiment in which a set of
continuous plated trenches 206a replaces vias 206 of FIG. 2A
Continuous trenches 206a more thoroughly isolate pad sets 170 than
do vias 206.
[0077] Yet another embodiment is shown in FIGS. 10A and 10B. In
this embodiment, a pogo probe card 410, in similarity to probe card
152, includes probing device 154. In contrast, however, conductive
traces replace the cables of the previous embodiments. This
embodiment includes a pogo pin receptive pad set 420 having a force
line pad 422, a sense line pad 424 and a guard line pad 426. Force
line pad and sense line pad merge into signal trace 428. Guard line
pad 426 is electrically connected with guard line trace 430 which
protects the force and sense line signals from electromagnetic
interference and is electrically connected through a series of
plated vias 432 to auxiliary guard line trace 434. Each signal
trace is connected to an electrical connection point 239 of
corresponding probing device 154 by a wire 438.
[0078] Each pair of auxiliary guard traces 434 is separated by a
set of trenches 436. Similar to trenches 210 trenches 436 separate
auxiliary traces 434 and reduces cross capacitance between pad sets
420 and traces 428 and 432 thereby reducing cross-talk and settling
time. This embodiment may be somewhat less expensive to produce in
large numbers than the previously described embodiments.
[0079] FIGS. 11-13 shows a probing device holder 502 holding a
probing device or blade 504, that includes a probing needle 505,
and further including coaxial cable connectors 506 and 508. Skilled
persons will appreciate that a number of holders 502 could be
attached to the top of rim 156 of laminate board 158 (FIG. 2).
Skilled persons will further appreciate that board 152b of FIG. 6b
can be modified so that cables 180a and 180b both terminate in
bayonet navy connectors that would screw onto connectors 506 and
508. This arrangement would render the advantage of replaceable
probing devices 504. With currently available test boards the
breakage or damage of a probing needle during test necessitates a
rather difficult and time consuming soldering or repair procedure.
By attaching holders 502 to a test board, the replacement procedure
is greatly simplified. In like manner any of the embodiments of
FIGS. 6A-6E, could be fitted with replaceable probing devices 504.
Skilled persons will note that a triax connector could be fitted to
holder 502 to accommodate the triax cable 276 of FIG. 6.
[0080] Skilled persons will further readily recognize that the
connection structure of connectors 506 and 508 could be cut away
and that the center connectors of 250a and 250b of cables 180a and
180b (FIG. 6B) could be soldered to connection point 576 (FIG. 13)
with outer conductors 256a and 256b being connected to the exterior
of holder 502.
[0081] With respect to the detailed structure of holder 502, an
elongate probe connector 552 is conductive and preferably has a
rectangular cross section. An insert 554 is sized to fit within the
probe connector 552. Insert 554 includes a ceramic insulator 556
and a conductive bent connector 558 attached to one side of the
insulator 556. Insulator 556 is in face-to-face abutment with the
interior upright surface 559 of probe connector 552. Probing device
504 is matingly and detachably engageable within the probe
connector 552.
[0082] Referring also to FIG. 13, device 504 preferably includes a
dielectric substrate 562 formed of a ceramic or a comparable
high-resistance insulating material. Device 504 has a pair of broad
parallel sides or faces interconnected by a thin edge. Formed on
one side of the device 504 is an elongate conductive path 564,
while the other side includes a backplane conductive surface 566. A
needle 568 is supported by dielectric substrate 562 and
electrically connected to elongate conductive path 564.
[0083] In the particular embodiment shown, the blade 504 is
generally L-shaped in profile and is edge-mounted within the probe
connector 552 so that the short arm of the L-shaped blade 504
extends downwardly making contact with the test subject device.
[0084] Referring also to FIG. 13, when the blade 504 is slidably
engaged within the probe connector 552, the backplane conductive
surface 566 is in face-to-face contact with the inner upright
surface 559 (FIG. 11) of the probe connector 552. Accordingly, a
guard signal path is provided from the guard conductors of the
force and sense cables 514 and 516, though the probe housing 550
and probe connector 552 to the backplane conductive area 566 of the
blade 560. This provides a guard path to a location near the end of
the needle 568. In addition, a conductive path is provided from
force and sense conductors 572 and 574 through a combined conductor
570 to the bent connector 558. It is to be understood that the
combined connector 570 may be any suitable type of coupler that
electrically connects the force and sense cables to the conductive
path 564 on the blade 560. Likewise it is to be understood that the
electrical connection between the backplane 566 on the blade 504
and the connectors 504 and 506 may be any suitable type of coupler.
The bent connector 558 is resiliently deformable as the blade 504
is inserted into the probe connector 552 and exerts pressure
between the backplane conductive surface 566 and the upright
surface 559 (FIG. 11) so a low loss connection is maintained. Also
the pressure maintains the position of the blade 504 during use.
Simultaneously, the bent connector 558 exerts pressure between the
conductive path 564 and the bent connector 558 to provide a low
loss connection. A signal path is thus provided through the needle
568, the conductive path 564, the bent connector 558, and the
combined conductor 570 to the force conductor 572 and sense
conductor 574. A threaded hole 580 accommodates set screw 582 for
rigidly retaining probing device 504, thereby allowing the
positional adjustment of probing needle 505.
[0085] The terms and expressions which have been employed in the
foregoing specification are used therein as terms of description
and not of limitation, and there is no intention, in the use of
such terms and expressions, of excluding equivalents of the
features shown and described or portions thereof, it being
recognized that the scope of the invention is defined and limited
only by the claims which follow.
* * * * *