Broad band contactor assembly for testing integrated circuit devices

A test contactor assembly for integrated circuit (IC) electronic devices and the like has at least one row of flexible contacts that are each secured at a lower end to a base. A conductive plate is also secured to the base and extends in a generlly parallel, closely spaced relationship to each row of contacts. The dimensions of the plate and its spacing from the associated contacts produce a distributed capacitance with respect to each contact in the row such that a fast-rising test signal launched in a contact encounters a generally "characteristic" or purely resistive impedance that is frequency independent. A flexible insulating material is preferably located between the plate and its associated row of contacts to maintain the desired spacing as the plate and the contacts are flexed into electrical connection with associated pins of the device. For use with dual-in-line packaged (DIP) IC'S and Kelvin contacts, a pair of "inner" plates closest to the device carry pin contacts that can supply a large test current surge to the device. The inner plates are preferably connected to one another through a small capacitor and substantially reduce ground noise generated by current surges.

BACKGROUND OF THE INVENTION 
This invention relates in general to testing apparatus for electronic 
devices. More specifically it relates to a contactor assembly that is 
generally frequency insensitive to allow broad band testing with 
fast-rising signals. 
In the manufacture and use of integrated circuits (IC's) and similar 
electronic devices it is important to test the devices accurately, 
reliably and at a high rate. Automatic testing and handling apparatus 
machines that can perform this task are available. Such apparatus suitable 
for testing dual-in-line packaged (DIP) IC's are sold by the Daymarc 
Corporation, Waltham, Mass., under the trade designation Type 1156 and 
1157. In a DIP device, the circuit is contained in a molded plastic body 
having a generally rectanguar, box-like configuration. Two rows of 
generally parallel connecting pins are arrayed along parallel sides of the 
body with each pin extending in a direction generally normal to the main 
faces of the body. 
In each of the aforementioned apparatuses the IC's are momentarily brought 
to rest at a test station where a set of contacts, typically double Kelvin 
contacts, are flexed through a cam action into electrical connection with 
the pins of the device. The contacts establish an electrical connection 
between testing circuitry and the device. The contacts are usually part of 
a probe or contactor assembly which includes an insulating base member 
that mounts the contacts. The contacts are typically narrow strips of a 
resilient and highly conductive material. The contacts typically make 
electrical connection with an associated connecting pin at a free end 
opposite the base. The cross-sectional dimensions of the contacts are 
relatively small due to (1) the requirement that all of the contacts 
simultaneously make connection a set of closely packed pins and (2) the 
requirement that the contacts flex for millions of cycles of operation 
without material fatigue. The length of the contacts is determined by the 
spacing between the test station of the IC handling apparatus and the test 
circuitry. 
Frequently the testing of the integrated circuits requires that the testing 
signal be "fast-rising", that is, a signal which is a very steep, 
step-like increase in potential. A typical fast-rising signal is 
characterized by a voltage change of 5 volts per nanosecond. Such a signal 
can be represented through Fourier analysis as being composed of a 
multitude of superimposed sine waves having a very high frequency, 
typically on the order of 300 mHz. The fast-rising signal launched by the 
test circuitry and carried by the contacts to the device therefore 
contains components with a very high frequencies. 
A major problem with this testing arrangement is that due to the inherent 
inductance of the contacts themselves, the signal encounters an inductive 
reactance X.sub.L. This reactance produces distortions and reflections 
which degrade the quality and accuracy of the test. The inductance L of 
the contact is a function of the cross-sectional area of the conductor and 
its length. It increases directly with the length and inversely with the 
cross-sectional area. Since the inductive reactance X.sub.L =2.pi.fL, for 
the very high frequencies f associated with a fast-rising signal, the 
inductive reactance associated with even the relatively short contacts in 
normal use becomes a significant source of distortion and limits the 
accuracy of measurements. 
One possible solution would be to increase the cross-sectional area of the 
contacts. However, the physical constraints of the test environment limit 
the useful dimensions of the contacts. For example, the contacts must be 
separated laterally from adjacent contacts while still maintaining a 
unique association with one pin on the IC. Also, the contacts must be 
sufficiently thin to flex repeatedly without exhibiting fatigue. Another 
possible solution is to make the contacts shorter. This solution works 
well if the IC can be placed manually into the test circuit. However, with 
high speed automated operation (e.g. 6,000 units per hour), the test 
circuitry must be physically separated from the device handling mechanisms 
with electrical connection made over some short distance spanned by a 
probe or contactor assembly of the type described above. In short, modern 
production economics require contacts having a length which is troublesome 
for fast-rising signals. Another solution is surrounding each contact with 
a shield in the manner of a coaxial cable. The shield, however, would 
interfere with the flexure of the enclosed contact. Still another possible 
solution is simply to test each device more slowly to wait for distortions 
and reflections to die out. With many modern IC's however, the speed of 
operation of the device itself is so fast that if the testing operation 
were to extend over a sufficient period of time to allow distortions and 
echoes induced by the fast-rising testing signal to subside, then the 
speed rating of the devices cannot be determined. In short, the testing 
operation must have a speed comparable to that of the device function 
being tested. At present, there is no known contactor assembly for use 
with automated IC testing and handling apparatus which can provide a 
reliable electrical connection between the IC device and the testing 
circuitry while at the same time avoiding the distortions, reflections and 
the resulting uncertainty of the measurement when the IC is tested with 
fast-rising signals. 
Another consideration is minimizing "ground noise", that is, changes in the 
reference voltage due to current surges during the test procedure 
simulating operation of the device. A typical situation is a test where a 
change in the device state causes a current surge in the range of 20 
milliamperes per nanosecond. Such a surge can cause the ground reference 
to move 1 volt or more thereby distorting measurements referenced to 
ground by 20% or more. The end result is that good devices may not pass 
the test and are downgraded. 
It is therefore a principal object of this invention to provide a contactor 
assembly for testing electronic devices, particularly high speed IC's that 
presents substantially no inductive reactance to a fast-rising signal 
launched in any contact of the assembly. 
Another object of the invention is to provide a contactor assembly with the 
foregoing advantage which is characterized by low signal distortion and 
reflection and a substantially resistive or "characteristic" impedance. 
Yet another object of the invention is to provide a contactor assembly with 
the foregoing advantages which allows testing at high production speeds 
and with extremely high degrees of accuracy and hence certainty. 
A further object of the invention is to provide a contactor assembly that 
can produce a test current surge originating very close to the device, 
presents a low inductance path for the surge to the device, and 
substantially reduces ground noise normally attendant such a surge. 
A still further object of the invention is to provide a contactor assembly 
with the foregoing advantages that has a generally simple, low cost and 
highly durable construction. 
SUMMARY OF THE INVENTION 
A contactor assembly for electronic devices, particularly dual-in-line 
packaged (DIP) IC's with two parallel rows of connecting pins, has a base 
that supports at least one row of resilient electrical contacts secured at 
their lower end to the base and extending generally perpendicular to the 
base to a free end. Typically the free end is angled toward an associated 
pin to make electrical connection with the pin when the contact is fixed 
toward the device. Each contact has a small cross section and is designed 
to conduct an electrical signal along its length between test circuitry 
and the device. Each contact is structured to flex resiliently from a 
first non-testing position where the free end of the contact is spaced 
from the associated pin to a second testing position where the free end is 
forced into electrical connection with the pin. 
The base also supports a conductive plate associated with and oriented in a 
generally parallel, closely spaced relationship with respect to each row 
of the contacts. The spacing is preferably uniform. In the preferred form, 
the plate is continuous and extends substantially the full length of the 
associated row. The plate is also preferably secured at one edge to the 
base and is resilient. The dimensions and spacing of the plate produce a 
distributed capacitance as seen by a signal transmitted along the 
contacts. The value of the resulting capacitive reactance substantially 
offsets the conductive reactance produced by the self inductance of the 
contacts. As a result, a fast-rising signal launched in a contact 
encounters a substantially characteristic impedance. Stated in other 
terms, the contactor assembly is substantially frequency independent and 
therefore capable of operating over a broad band. 
The contactor assembly preferably includes a layer of flexible insulating 
material which is inserted between each of the upstanding plates and the 
associated row of contacts. The insulating layer maintains the desired 
dimensional separation between the plate and the associated contacts 
through the flexural movement of the contacts into and out of electrical 
connection with the pins. Also in the preferred form, the value of the 
characteristic impedance is generally in the range of 50 to 100 ohms. 
Another feature of this invention is that at least one plate is located 
generally between the associated row of contacts and the device being 
tested. This "inner" plate carries a contact element structured and 
located to make electrical connection with one of the pins. In a preferred 
form of the invention adapted for use with devices having two parallel 
rows of pins, there are two such "inner" plates that make electrical 
connection with two different pins of the device. These plates and 
contacts provide a means located close to the IC for applying a large test 
current surge to it. The dimensions of the plates present a low 
inductance. One inner plate is connected to a source of a large current 
surge; the other plate is connected to ground. Both inner plates are 
connected to one another through a small capacitor that can be located 
either above or below the base. 
These and other features and objects of the invention will be more clearly 
understood from the following detailed description of the preferred 
embodiments which should be read in light of the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1-4 show a test contactor assembly 12 that has a base 14, four rows 
16a, 16b, 16c and 16d of contacts 16, and a set of capacitive plates 18a, 
18b, 18c and 18d that will be referred to collectively as plates 18. For 
the purpose of this description the base is taken as being horizontally 
oriented. It is also insulating and has a generally plate-like 
configuration. Each plate 18 is associated with one of the rows of 
contacts. Each contact 16 has a lower end 20 that is secured in the base 
14 and extends to a free or upper end 22 that is angled toward an 
associated pin 24 of an integrated circuit device 26. The cross section of 
each contact 16 is preferably generally rectangular with the broad face of 
each contact parallel to the pins 24. As shown, the contacts in the rows 
16a and 16b are arranged in Kelvin pairs with two contacts, 16' from the 
row 16b and 16" from the row 16a, adapted to make electrical connection 
with the same pin 24 at the same time. The contact 16" from the "outer" 
row 16a (the one more distant from the device) generally surrounds the 
contact 16 with the free end of the contact 16" overlying the free end 22 
of the contact 16. Similarly, one contact from the row 16c and an 
associated, "overlying" contact from the row 16d form a Kelvin pair that 
maked an electrical connection with a pin on the opposite side of device 
26. The pairs of contacts in the rows 16a and 16b are generally opposed to 
a like set of pairs of contacts in the rows 16c and 16d which allows the 
contact assembly 12 to make a reliably electrical connection to all of the 
pins of a DIP IC 26 of the type shown in FIGS. 3 and 4. The contacts in 
the rows 16a-16d are generally parallel to one another and to the rows of 
connecting pins 24 of the device 26. 
The lower ends 20 of the paired contacts are spaced by insulating strips 
28,28. In their normal unflexed position shown in FIGS. 1 and 2 the 
contacts are generally perpendicular to the base 14. In this position the 
ends 22 of the contacts are spaced from the pins 24. Besides being formed 
of a highly conductive material, the contacts 16 are also formed of a 
material which is resilient and resistant to material fatigue during 
cycled flexures to and from a testing position shown in FIGS. 3 and 4. In 
the flexed state, the end surfaces 22a of the free ends 22 make electrical 
connection to the associated pins 24. Spacer rods 30,30 are secured 
between the rows 16a and 16b and rows 16c and 16d. The rods 30,30 insure 
the desired spacing between the contacts during the flexural movement 
developed by a lateral force (typically delivered by a cam, not shown) 
applied in a direction indicated by the arrows 32,32 in FIG. 3. 
A principal feature of the present invention is the set of conductive 
plates 18a-18d each of which is associated with one row of the contacts 
16. The plates 18a-18d are preferably generally rectangular and are 
secured at a lower edge 34 to the base 14 with the edge passing through a 
suitable opening in the base 14. A flanged portion 36 of each plate 
extends under and is generally parallel to the base 14 provides an 
electrical connection site to ground (for plates 18a and 18d). The inner 
plates 18b and 18c also preferably have a flange-like portion 37 that is 
generally similar to the flange 36 except that it extends along the upper 
surface of the base 14. The flanges 37,37 provide electrical connection to 
ground for the plate 18c and to a source of a large current surge for the 
plate 18b. The plates are formed of a conductive sheet material that is 
resilient and has a substantially uniform thickness. Each of the plates is 
also oriented so that it is substantially parallel to the associated row 
of contacts and is closely spaced therefrom at a substantially constant 
distance. The plates extend vertically over substantially the full length 
of the contacts with a small clearance between the upper edge of the 
"inner" plates 18b and 18c and the overlying upper portions 22 of the 
"inner" rows of contacts 16b and 16c. Each plate extends laterally at 
least the full length of the row of contacts with which it is associated. 
For any contact, the associated plate is an approximation to a parallel 
plate with unlimited dimensions. 
In the preferred form shown, a thin layer 38 of a flexible insulating 
material such as a fluoroplastic maintains each plate at the preselected 
spacing from the associated row of contacts. The layers 38 should have a 
sufficient Shore hardness that the lateral flexural force (in direction of 
the arrows 32,32) does not compress the layers 38 to any significant 
degree and thereby substantially reduce the spacing between the plates and 
their associated rows of contacts. On the other hand, the layers 38 should 
be sufficiently flexible not to impede significantly the flexural 
movement. Similarly, the plates themselves should be sufficiently flexible 
that they do not significantly impede flexural movement under the lateral 
flexing force applied through insulating rods 40,40 held at the outer face 
of the plates 18a and 18d at a point aligned with the rods 30,30. The 
layers 38 preferably extend substantially the full height of the plates to 
provide a generally uniform dielectric constant between the plates and the 
associated rows of contacts. The plate-contact spacing can also be 
maintained by other means such as small, non-resilient spacer elements 
with air as the principal insulating medium. 
Each contact conducts a signal along its length from a connection point to 
a testing circuit at its lower end 20 adjacent the spacer 28 to its end 22 
in contact with the associated pin 24 of the IC 26. As with any conductor, 
each contact has a characteristic inductance L which is a function of its 
cross-sectional area and length. For a conductor having a generally 
rectangular cross section such as the contacts 16 the inductance is a 
function of the thickness of the contacts (measured generally in the 
direction of the arrows 32,32), the width W of the contacts (measured 
along the direction of the rods 30,30) and the length l of the contact. 
When a fast-rising signal is launched the contact contacts having any 
significant length (a typical length being approximately one inch) will 
generate an inductive reactance X.sub.L which is equal to 2.pi.fL. The 
precise inductive impedance generated will, of course, depend upon the 
configuration of the conductor and the exact nature of the fast-rising 
signal. Since, as noted above, a fast-rising signal can be analyzed as a 
collection of waves having an extremely high frequency (a typical 
frequency being 300 mHz) even the extremely small inductance of a contact 
16 can produce a sufficient inductive reactive to introduce distortions 
and reflections in the transmitted signal. 
The plates of the present invention neutralize this inductive reactance by 
providing a distributed capacitance along the length of the contacts. This 
equivalent circuit for a single contact is shown in FIG. 5 where a contact 
16 is represented as a series of inductors 42 and the distributed 
capacitance between the contact and the associated plate is represented as 
a series of capacitors 44 connected between the inductors 42 and ground. 
The signal source or test circuitry 46 is connected at one end of the 
circuit and the IC 26, represented as a load 48, is connected at the 
opposite end. Utilizing conventional transmission line theory and formulas 
for calculating inductance and capacitance of the contacts 16 and 
associated plates 18, for a given contact and a given test signal, it is 
possible to calculate the plate-contact spacing and that will produce a 
distributed capacitance whose capacitive X.sub.c offsets the phase shift 
of the inductive reactance X.sub.L. As a result, the fast-rising signal 
transmitted by the contacts 16 encounter a "characteristic" impedance 
Z.sub.o, that is substantially resistive. That is, there is no net phase 
shift between the current and voltage due to capacitive or inductive 
elements in the transmission line. The impedance Z.sub.o is, therefore, 
substantially independent of the frequency of the signal and the test 
contactor assembly 12 is broad band. 
Typically the IC's 26 are tested in systems where the impedance of the 
contactor assembly is either 50 ohms or 100 ohms. The contactor assembly 
12 of the present invention therefore preferably has plates 18a-18d that 
are structured and located with respect to the contacts 16 to produce a 
characteristic impedance with one of these values. The value of Z.sub.o in 
ohms is given by the square root of L/C. Of course, the inductance of the 
contacts or the capacitance introduced by the plates 18 can be designed to 
produce a value for the characteristic impedance encountered by the signal 
which is less than 50 ohms, more than 100 ohms, or at some intermediate 
value. 
While calculations can provide a reasonably good indication of performance, 
particularly when the elements are rigid and fixed and the insulating 
medium is air, for the contactor assembly of this invention trial and 
error adjustments will usually be required to arrive at a configuration 
that produces the desired characteristic impedance. It should be noted 
that because pairs of contacts forming a Kelvin contact are connected to 
the same pin and carry the same signal, as a practical matter the 
capacitance between the pair is not significant. However, the capacitance 
between laterally adjacent contacts is important. To reduce this 
capacitance, it may be preferable for certain applications to use a single 
contact for each pin rather than a Kelvin pair. For a typical Kelvin 
contact assembly of the type used in testers/handlers manufactured by 
Daymarc Corporation, and for use with fast rising signals that have a 
change in potential of 5 volts over 1 nanosecond, it has been found that a 
plate-contact spacing of approximately 0.014 inch produces the desired 
response. 
Another significant feature of the contactor assembly according to the 
present invention is that the inner plates 18b and 18c include pin contact 
elements 50 and 52, respectively, mounted on their faces adjacent the IC 
26 and positioned to make contact with an associated pin 24 of the IC. The 
contacting elements 50 and 52 are L-shaped conductors which are welded or 
brazed onto the inner plates. They are useful for testing the operation of 
the IC in response to a large current surge. One of the inner plates, such 
as the plate 18b, is preferably connected to a source of a suitable large 
current surge (such as a capacitor, not shown) which can be applied to the 
IC through the plate 18b and the contact 50. It should be noted that due 
to the large dimensions of the plate 18b as compared to a single contact 
16, the inductive impedance problems usually associated with a large 
current surge are minimized. The contact 52 is connected through the plate 
18c to ground, as are the plates 18a and 18d. 
The plates 18b and 18c are also connected to one another through a small 
capacitor 54. The function of the capacitor 54 is best seen with reference 
to FIG. 6. The source of the large current is represented by the reference 
numeral 56, the source of the surge by the capacitor. The IC appears as a 
load with the capacitor 54 connected in parallel with the load. Because 
the impedance is low for high frequencies, the capacitor presents a very 
low impedance to ground. As a result, even though the plate 18b is 
connected to a high voltage source, for fast-rising signals it acts as 
though it is at ground. Another advantage of this arrangement is that it 
provides a source for a large current surge that is located very close to 
the device being tested. This also results in reductions in distortions in 
the transmission and increases the accuracy of the test results. 
The capacitor 54 is shown in FIGS. 1 and 3 as connected between the flanges 
37 and located at the upper surface of the base 14. This location has the 
advantage of placing the capacitor 54 very close to the IC 26. However, in 
certain test situations it is necessry to plate the IC and components near 
the IC in a temperature environment that adversely affects the capacitor. 
In these instances, the capacitor 54 can be connected between the flanges 
36,36 of the plates 18b, 18c located at the under side of the base 14 
(shown in phantom). This location provides temperature isolation for the 
capacitor 54 without significantly increasing the distance between the IC 
and the capacitor. 
While the invention has been described with reference to its preferred 
embodiment, we understood that various modifications and alterations will 
occur to those skilled in the art from the foregoing detailed description 
and the drawings. For example, while the invention has been described with 
respect to continuous plates the capacitive function of the plates can be 
performed by a series of conductive strips which are each associated with 
the contacts or by non-rectangular plates which are not necessarily 
continuous. Further, while the plates 18 have been described generally 
resilient members that are secured at one edge to the base 14, it is 
possible to produce the desired operating characteristics of the present 
invention utilizing substantially generally inflexible plates and plates 
which are not secured to the base. One embodiment could utilize a plate 
which is fastened by an adhesive to the associated layer 38 but is not 
directly secured to the base 14. These and other modifications and 
variations which will occur to those skilled in the art are intended to 
fall within the scope of the appended claims.