This invention relates in general to testing apparatus for electronic devices such as integrated circuits ("IC's"). More specifically, it relates to: (i) an electrical contactor assembly that is generally frequency insensitive to allow broadband testing with fast rising signals, and (ii) an interface for interchangeably connecting such an electrically contacting assembly to testing circuitry.
In the manufacture and use of IC's packaged as surface mounted devices ("SMD's"), it is important to test the devices accurately, reliably, and at a high rate. Automatic testing and handling machines that can perform this task are available. Such apparatus suitable for testing SMD ICs are sold by the Daymarc Corporation, Waltham, Mass., under the trade designation Model 757. In one common type of SMD device, a PLCC packaged IC, the circuit is contained in a molded plastic body having a generally rectangular, planar, box-like configuration. SMDs typically include either two rows of contacting leads along opposite and parallel sides of the body, or four rows of contacting leads, one along each side of the body. The most common configurations of SMDs include four rows of connecting leads. In any event, the leads lie generally in a common connecting plane.
An SMD is designed to be mounted directly on the surface of a circuit board or within a suitable receiving socket. The SMD can be distinguished from dual-in-line packaged (DIP) integrated circuits in that DIP devices are intended for mounting with leads passing through the circuit board (or within a suitable socket) rather than for surface mounting. Additionally, DIP's typically include only two rows of parallel connecting leads, in contrast to the usual four-row SMD.
Prior-art SMD testing apparatus can generally be described as either manual or automatic. In a manual apparatus, an operator manually places each SMD into a socket, conducts the test, then removes the SMD. In addition to being an obviously slow and time-consuming procedure, the sockets tend to wear out rapidly. A typical life is only a few thousand devices. Replacing a socket requires de-soldering the old one and installing a new socket.
A manual testing apparatus, however, using a socket mounted directly the test circuit board is advantageous in that testing is done in the electro-magnetic environment of actual use. While automatic apparatus have proven to be fast, they test the SMD at a remote location from the test circuit, and thus in an inappropriate environment. As an example of the importance of proximity to the test circuit, it is known that merely changing the lead length in the test situation from the actual use situation by a quarter of an inch can lead to substantial changes in electrical response.
SMD leads tend to be very soft or delicate. A force component as small as a few grams in a direction parallel to the plane of the SMD can damage the leads. A single lead damaged in testing can render the entire SMD unsuitable for use. In several known automatic testing apparatus, the testing contacts exert a side-acting force which permanently displaces the lead causing lateral or longitudinal row misalignment. Other prior art devices used sloped surfaces to guide the SMD into test position. Depending on factors such as the extent of misalignment, such guidance mechanisms can also misalign the leads.
As described in U.S. Pat. No. 4,473,798 to Cedrone et al, the testing of integrated circuits frequently requires that the test signal be "fast-rising", that is, a signal which is a very steep, step-like increase in potential. A typical fast-rising signal may be characterized by a voltage change of 1 volt per nanosecond. Such a signal can be represented through Fourier Series 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 behave in the manner of a high frequency signal.
With such high frequency "components" in the signal, the inherent inductance of the contacts themselves becomes a problem. Inductive reactance X.sub.L 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 configuration of the conductor and its length. It increases directly with the length and inversely with the cross-sectional width. 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 width of the contacts. However, the physical constraints of the test environment limit the available dimensions of the contacts. For example, the contacts must be separated laterally from adjacent contacts while each still maintaining a unique association with one lead on the SMD. Also, the contacts elastically deform during a test and must be sufficiently thin to flex repeatedly without exhibiting fatigue. Another possible solution is to make the contacts shorter. This is difficult to execute in testing DIP ICs, and in testing SMDs while the contacts can be made short comparted to those in DIP contactors, the signal path from the contacts to the test circuit is long enough to affect signal integrity adversely.
Still another possible solution is simply to test each device more slowly to wait for distortions and reflections to die out. With many modern SMDs such as large gate arrays, 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 echos induced by the fast-rising testing signal to subside, then the speed rating of the devices could not be determined. In short, the testing operation must have a speed on the order of the device function being tested. At present, there is no satisfactory electrically contacting assembly for use with automated SMD testing and handling apparatus which can provide a reliable electrical connection between the SMD and the testing circuitry while avoiding the distortions, reflections and resulting uncertainty of the measurement when the SMD is tested with fast-rising signals, while at the same time avoiding lateral force components that can permanently misalign the SMD contacts.
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 one 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 the invention to provide a contactor assembly that maintains signal integrity when testing SMD devices even when the testing involves fast-rising signals or current surges.
Another principal object of the invention is to avoid harm to the delicate SMD leads.
Another object of the invention is to provide a virtual ground for selected leads close to the SMD.
Yet another object is to provide a short signal path between the DUT and the test circuit.
Still another object is to provide an electro-magnetic testing environment that closely simulates that of the intended use.
A further object is to provide an assembly that allows convenient replacement of the contacts and convenient attachment to the test circuit thereby providing enhanced operational flexibility and reducing the required inventory of test components.
It is yet a further object to provide a testing system with the foregoing advantages that is generally simple, low cost, and of highly durable construction.