Multiple mode buckling beam probe assembly

A multiple mode buckling beam probe is formed by top and bottom mating locating guides interposed between a space transformer die bearing exposed wire contact ends and an underlying substrate having correspondingly positioned conductive pads. Wire probes pass slidingly through aligned holes within the top and bottom mating locating guides. At least one center locating guide having correspondingly aligned holes with the top and bottom locating guides slidably receives the wire probes and is spaced at different distances from the top and bottom locating guides. Interposed between the center locating guide and the top and bottom locating guides are respective slotted guides having elongated slots through which the wire probes pass, which slots are offset relative to the top, bottom and center locating guide holes. This permits an increase in axial deflections of the wire probes over a standard buckling beam probe, adequately confines the directionality of the buckling wire probes, achieves greater probe density, and permits the controlled sequence and magnitude of each buckle in multiple mode buckling with the characteristic force versus deflection curve for the buckling wire probes being tailorable to a specific user's need.

BACKGROUND OF THE INVENTION 
Buckling wire or beam probes have come into vogue for facilitating the 
testing of electrical characteristics of integrated circuits connected to 
pads on a semiconductor chip wherein the flexing or buckling wire probes 
engage at opposite ends, the pads and contact points on the tester space 
transformer die by relative movement of the substrate toward the space 
transformer die. A typical buckling beam probe assembly is set forth in 
U.S. Pat. No. 3,806,801 issued Apr. 23, 1974, to Ronald Bove and assigned 
to the common corporate assignee. 
The application of axial forces on the wire probes causes the wire probes 
or beams to buckle laterally under axial compressive force, the result of 
which is to insure low ohmic contact of each current conducting wire 
probe, at respective ends, to the space transformer die contact and the 
chip carried pad. If the force of the probe engaging the pad exceeds that 
for which the pad or chip has been designed, then the pad and/or chip may 
be damaged. Such systems are complicated by the fact that it is necessary 
to space the probes sufficiently from each other to enable such deflection 
without the probes contacting each other and shorting out the probes. 
Referring to FIG. 1 of the drawings, in a typical buckling beam probe 
assembly 10, the underlying substrate 18 is placed some distance from the 
overlying space transformer die 12 carrying the potted wire contacts 14 
whose ends 16 are exposed at the bottom of the space transformer die 12. 
Typically, a top mating locating guide 22 and a bottom locating guide 34 
are provided in juxtaposition to the space transformer die 12 and a 
substrate 18, respectively, but spaced somewhat therefrom, and wherein 
both of these locating guides 22, 34 are provided with small diameter 
holes 24 and 36, respectively, sized slightly larger than the diameter of 
the probes passing therethrough, so as to slide therein. Thus the wire 
probes 38, absent deflection, are aligned throughout their axes with the 
wire contacts 14 of the space transformer die 12, and pads 20 on the 
substrate 18. In addition, there is normally provided an offset die 26 
whose holes 28 through which the wire probes 38 pass, are offset relative 
to the holes 24 of the locating guides to thus bias the wire probes to 
buckle laterally, in a given direction. Additionally, a floating die 30 is 
employed intermediate of the offset die and the lower locating guide to 
isolate and insulate the probe wires such that during axial applied force, 
the wires are axially deflected, and under the applied axial force 40, the 
lateral deflections result in a controlled manner and in preset directions 
defined by the offset die. Typically, all guides and dies are connected to 
the space transformer die 12 and movable as an assembly relatively toward 
and away from the underlying substrate 18. 
FIGS. 2 and 3 are plots which show applied force versus axial deflection, 
and lateral deflection versus applied axial deflection for such typical 
buckling beam probe assemblies. In FIG. 2, under initial applied axial 
force, the wire probes deflect axially to a limited degree during initial 
lateral deflection and then to a greater degree when the axial force 
reaches a predetermined value. In FIG. 3, the lateral versus axial 
deflection curve shows that, for small axial deflections, that there are 
relatively large lateral deflections. This characteristic limits the 
application of buckling beam probes for two main reasons. First, when 
probing a complex (dense) pattern, only very small probe axial deflections 
can be handled. The wire probes tend to come into lateral contact with 
each other and short out. Even if floating die is used to isolate the 
probes and to thus insulate them electrically, it too is limited due to 
the floating die causing some of the probes to buckle without achieving 
contact. Secondly, even if the lateral deflections are not critical for 
contact reasons, a large buckle will be constricted due to the increase in 
frictional force of the dies (binding effect on the wire probes). In 
addition, too much axial deflection will cause the beams, i.e. the probe 
wires, to permanently buckle. Therefore, there are substantial limits in 
the use of buckling beam probes, as conventionally fabricated, since 
complex (dense) patterns need very uniform probing surfaces and the 
overall axial deflections of conventional buckling beams are limited to 
about five mils maximum. 
Some attempts have been made to provide buckling beam probes whose probe 
wires operate under multiple mode buckling. Such concepts are set forth in 
IBM Technical Disclosure Bulletin, Vol. 17, No. 2, July 19, 1974, pages 
444 and 638. In both these disclosures, the multiple buckling modes of the 
connectors/contactors are independent, i.e. separate, which while being a 
step in the right direction, require isolation of the wire probes into two 
halves, one to the connector side and the other to the contactor side. 
It is, therefore, a primary object of the present invention to provide a 
buckling beam probe assembly which permits increase in the axial 
deflections of the wire probes while maintaining desired force deflection 
characteristics, and in which lateral deflections are decreased, 
permitting an increase in density of the probing pattern and allowing the 
probing of more varied probing surfaces, and wherein the buckling beam 
assembly may be tailored to a user's specific needs by use of multiple 
mode buckling. 
SUMMARY OF THE INVENTION 
The invention is basically directed to a buckling beam assembly in which 
the wire probes are buckled in multiple modes, and wherein each separate 
buckle increases the axial deflection while maintaining a set maximum 
lateral deflection. The amount of lateral deflection is controlled by the 
use of laterally slotted planar locating guides, these slotted guides are 
used to bias the initial buckling mode, control the buckling direction and 
isolate individual wire probes to prevent the probe wires from contacting 
each other. The assembly includes a middle locating guide intermediate of 
the slotted guides whose slots are offset with respect to aligned holes 
within the middle guide, and the top and bottom locating guides in 
juxtaposition to wire contacts borne by the space transformer die at the 
top of the assembly and the underlying substrate bearing the chip pads at 
the bottom of the assembly. Thus, the wire probes are allowed to slide 
freely through the middle locating guide with the middle locating guide 
permitting free transfer of the axially applied force from the lower 
buckle to the upper buckling mode. Additionally, the middle locating guide 
is purposely spaced at a different distance from the lower locating guide 
than it is to the upper locating guide. As a result, since the effective 
free length of one buckling mode is larger than the effective free length 
of the other buckling mode, the wire probes will buckle in a first mode 
where the distance is greater between the middle locating guide and the 
adjacent locating guide when the probe wires are subjected to their 
initial axial deflections. Once the buckling mode's lateral deflection 
butts up against the slotted guide slot end, its effective free length is 
halved. Increased load (axial deflection) will cause the buckling of the 
probe wires in the second buckling mode between that middle locating die 
and the closer locating die. Dependent upon the relative effective free 
lengths of the buckling modes, a force deflection curve may thus be 
tailored for a user's specific needs. 
In a preferred form, multiple center locating guides are interposed between 
top and bottom locating guides, all of which have holes of common diameter 
and axial alignment both with themselves and the substrate pads and space 
transformer die contacts. In each instance, slotted guides are employed 
intermediate of the center locating guides, or the center locating guides 
and the top and bottom locating guides, to control and direct the 
deflections for the sequential modes with the slots of the slotted guides 
having their centers offset with respect to the axes of the holes within 
the various locating guides.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 4a through 4d inclusive, there is shown specifically in 
FIG. 4a, a controlled multiple buckling beam probe assembly indicated at 
50, with each separate buckling acting to increase the axial deflection of 
the probe while maintaining a set minimum lateral deflection of the wire 
probes, controlled by the length of the slots within multiple slotted 
guides employed in the assembly 50. Many of the elements making up the 
assembly 50 have counterparts within the prior art buckling beam probe 
assembly 10, illustrated in FIG. 1. In that respect, a space transformer 
guide 52, functions as the tester interface and incorporates at 54 and 54' 
wire contacts whose lower ends 56 and 56' are exposed beneath the bottom 
of the transformer guide 52 and permit the supply of electrical current to 
the substrate 58 at the lower end of the probe assembly 50. The substrate 
58 may be supported for movement upwardly in the direction of the 
transformer die 52 in the manner of U.S. Pat. No. 3,806,801 referred to 
previously. On the top of the substrate 58, there are provided at spaced 
positions, electrically conductive pads 60, 60' at exact correspondingly 
spaced positions to, and oriented in axial alignment with the contacts 54, 
54' of the relatively fixed space transformer die 52. 
The probe assembly 50 is comprised, in order, from the space transformer 
die 52 in the direction of substrate 58, of a top locating guide 70, a 
first slotted guide 86, a first center locating guide 82, a second slotted 
guide 90, a second center locating guide 78, a third slotted guide 94, and 
a bottom locating guide 74. The first and second center Locating guides 82 
and 78, respectively, are similar to and function in the manner of the top 
and bottom locating guides 70 and 74, respectively. In that respect, the 
top locating guide 70 is provided with relatively small diameter holes as 
at 72 which are of a diameter slightly larger than that of the wire probes 
62, 62' which pass therethrough to permit relative sliding of the wire 
probes within those holes. In like fashion, similarly sized holes are 
provided at 84 within the first center locating guide 82, at 80 for the 
second center locating guide 78, and at 76 for the bottom locating guide 
74. Wire probes or beams 62 and 62' are of a length such that when the 
substrate 58 and the space transformer die 52 forming the tester interface 
use at their maximum spacing, the wire probes 62 are straight and 
undeflected and without any applied axial force applied thereto. There 
headed ends 62a cause the wire probes to be maintained suspended in 
assembly 50 and straight, absent applied axial compression forces thereto. 
Important to an appreciation of the present invention, is the fact that the 
first, second and third slotted guides 86, 90 and 94, include slots as at 
88, 92 and 96, respectively, within the same whose centers are offset to 
the axes of the holes 72, 84, 80 and 76 within respective top locating 
guide, first and second center locating guides, and bottom locating guide. 
The slots therefore control the extent of and nature of the localized 
deflections or buckling. It is the effective free lengths of the portions 
of the probe wires between the top 70 and center 82 locating guides, the 
first 82 and second 78 center locating guides, and the second center 
locating guide 78 and the bottom locating guide 74, respectively, which 
determine the mode sequence in buckling of the wire probes 62, 62' due to 
axial applied force on the probe wires. Such axial force is derived by 
relative movement of the substrate 58 towards the space transformer die 
52. All of the guides are at fixed positions and at fixed vertical 
spacings with respect to each other by multiple, spaced mounting rods or 
bars 98 extending downwardly from the space transformer 52 to the bottom 
locating guide 74, through all the other guides and being fixed thereto to 
achieve that purpose. Again, such structural arrangement is conventional 
and may be readily ascertained from prior art patents and publications, 
such as U.S. Pat. No. 3,806,801. 
In the illustrated embodiment of FIGS. 4a through 4d inclusive, the 
effective free Lengths for the successive deflecting portions of the wire 
probes 62, 62' differ from each other and decrease from the top of 
assembly 50 towards the bottom. As shown, therefore, EFL.sub.1 is larger 
than EFL.sub.2, which, in turn, is larger than EFL.sub.3. This means that 
the distance between the top locating guide 70 and the first center 
locating guide 82 is somewhat greater than the spacing or distance between 
the first center locating guide 82 and the second center locating guide 78 
while, in turn, that spacing is somewhat larger than the spacing between 
the second center locating guide 78 and the bottom locating guide 74. The 
relationship is as follows: 
EQU EFL.sub.1 &gt;EFL.sub.2 &gt;EFL.sub.3 &gt;(EFL.sub.1 /2) 
Thus, each separate buckle increases the axial deflection of the wire 
probes 62, 62', while maintaining a set maximum lateral deflection defined 
by the length of the slots as at 88, 92 and 96 within slotted guides 86, 
90 and 94, respectively. The slotted guides 86, 90 and 94 are used to bias 
the initial buckling mode, control the buckling direction, isolate 
individual probes 62 from contacting each other, and limit the lateral 
deflections of the probes 62. The probes are allowed to freely slide 
through the center locating guides 82, 78 as well as the top locating 
guide 70 and bottom locating guide 74 of the probe assembly 50. The center 
locating guides or dies 82, 78 determine the effective free length of each 
buckling mode, while permitting free transfer of force along the length of 
the probe. 
As seen in the sequence from FIGS. 4a through 4d, due to the different 
effective free lengths (EFL.sub.1 EFL.sub.2 and EFL.sub.3), a wire probe 
as at 62 will buckle in mode No. 1, FIG. 4b, upon the application of an 
initial axial force indicated by arrow 97, when the substrate 58 is 
shifted vertically upwards by a distance .DELTA.. In the illustrated 
embodiment, this vertical rise of the substrate 58 is shown as being 0.008 
inch. Once this buckling mode's lateral deflection causes the wire probe 
62 to abut against the left end of slot 88, its effective length is 
halved. Increased load axial deflection) as evidenced by arrow 99 is a 
result of the substrate 58 being raised another increment as at .DELTA.', 
the effect of which is to cause wire probe 62 to buckle where its 
effective free length is now the longest, i.e. to the extent of EFL.sub.2. 
This causes a second localized buckling of the wire probe 62 between the 
first center locating guide 82 and the second center locating guide 78 to 
the extent again where this localized second buckling portion of the wire 
probe 62 contacts the left end of slot 92 of the second slotted guide 90, 
FIG. 4c. The incremental vertical movement of the substrate 58 again is 
shown as being 0.0008", i.e. as indicated at .DELTA.' in FIG. 4c. 
This cycle is repeated over and over, buckling where the effective free 
length is greatest, depending upon the relative effective free length of 
each of the buckling modes. FIG. 4d shows the third mode, in which case 
the effective free length (EFL.sub.3, is the greatest effective free 
length portion of the wire probe 62 under axially applied force. A further 
increase in applied vertical force by raising the substrate incrementally 
at .DELTA.," in the equal amount of an 0.0008" increment causes lateral 
buckling to occur for the portion of the wire probe 62 between the second 
center locating guide 78 and the bottom locating guide 74. The buckling is 
limited between the second center locating guide 78 and the bottom 
locating guide 74. Buckling again is limited by the length of slot 96 
within the third slotted guide 94 and when the buckled portion of the wire 
probe 62 abuts the left end of slot 96, further buckling of the wire probe 
62 ceases. Depending upon the relative effective free lengths of each of 
the buckling modes, this design can be extended to use more guides and 
increased buckling modes to allow larger axial deflections and/or complex 
force deflection characteristics. 
Reference to FIG. 5 shows a plot of applied force in grams against axial 
deflection in mils for the multiple wire probes 84 of the multi-mode 
buckling beam probe assembly 50. As may be appreciated by reference to 
FIGS. 4a through 4d inclusive, while two wires probes 62, 62' are 
illustrated in FIG. 4a, the succeeding FIGS. 4b, 4c and 4d show only the 
leftmost wire probe 62 under first, second and third mode deflection to 
illustrate in sequence the multiple mode deflections of the various wire 
probes, each being identical. 
In correlating FIG. 5 to FIGS. 4b, 4c and 4d, the applied force evidenced 
by arrow 97 in FIG. 4b increases to the extent of 12 grams prior to 
initiation of probe lateral deflection. The axial deflection at the point 
of initial lateral deflection is twelve mils. Axial force application of 
12 grams, under axial deflection of 8 mils, causes lateral deflection to 
point P on the plot. This is the lateral limit for the first mode buckle 
and the point where the wire probe 62 abuts the left end of slot 88 in the 
first slotted guide 86, FIG. 4b. Continued increase in applied axial force 
to thirteen grams causes the wire probe 62 to start buckling in the second 
mode, FIG. 4c, and this buckling continues under axial deflection of 
approximately 17 mils to point P', at which the wire probe 62 terminates 
its second mode buckling since the buckled wire probe 62 now abuts the 
left end of slot 92 of the second slotted guide 90. Upon an increase in 
applied axial force from 13 to 14 grams, the wire probes such as 62 buckle 
in the third mode between second center locating guide 78 and bottom 
locating guide 74. That third mode buckling continues during axial 
deflection from 16 to 24 mils at which point P" is reached which is the 
lateral limit for the third buckle. Axial deflection ceases, in spite of 
the increase of applied axial force which is shown on the plot as rising 
from 14 to 16 grams. The wire probes such as probe 62 cannot buckle to any 
greater extent because the buckling is limited laterally by the length of 
the slots as at 88, 92, 96 within first, second and third slotted guides 
86, 90 and 94 respectively. 
In the illustrated embodiment, as exemplified by the force versus axial 
deflection curve of FIG. 5, the material forming the wire probes 84 is 
Paliney 7 alloy, and the probe wires have a diameter of 0.005 inch. 
While the invention has been partially shown and described with reference 
to a preferred embodiment thereof, it will be understood by those skilled 
in the art that various changes in form and details may be made therein 
without departing from the spirit and scope of the invention.