AUTOMATIC TEST EQUIPMENT

Abstract: An in-circuit automatic test equipment has several channel control arrays (20), each of which can supply test signals to or receive output signals from a unit under test (16) via any one of 8 test nails (14) per array, selected by relays (18). In addition, second and third relays (44, 46) connected to each nail permit currently unselected nails to be used to apply guard signals elsewhere in the UUT.

Description: Automatic Test Equipment This invention relates to automatic test equipment, and particularly to in-circuit automatic test equipment. In-circuit automatic test equipment is used for testing the integrity of individual electronic components after they have been assembled on a printed circuit board. To this end, a test fixture known as a bed-of-nails, comprising typically several hundred spring-loaded pins or nails, is placed against the circuit board (unit under test or UUT) so that each nail contacts a predetermined circuit junction or node on the UUT. Each component in turn is subjected to a programme of test signals appropriate to that component. applied by stimulus circuits via the nails contacting the nodes connected to the input terminals of the component. The resulting output signals are monitored by sensing circuits, via other nails contacting the output terminals of the component, to check whether the component is correctly inserted and operating in accordance wlth its specification. To isolate the component electrically and avoid perturbations caused by the operation of other interconnected components in the UUT, so-called guard signals are applied to selected nodes elsewhere in the circuit to suppress signal changes that would otherwise affect the component being tested. To economise, there are usually fewer stimulus and sensing circuits than there are nails, with each stimulus or sensing circuit being multiplexed amoung a group of several (for example eight) nails. Conventionally, the guard signals are applied by the same stimulus circuits as supply the test signals. Thus, in any one group of nails, only a nail selected by the multiplexing circuitry for connection to the stimulus circuit is active at any one time, to supply either a test signal or a guard signal. This is inefficient and can cause particular problems if, in designing a test fixture, a need for an additional guard signal is belatedly found when all multiplexed stimulus circuits having nails near the relevant node are already committed to other nodes; this necessitates an expensive modification to the test fixture. According to one aspect of this invention there is provided an in-circuit automatic test equipment comprising a plurality of nails for contacting circuit nodes in a unit under test: and a circuit for either applying a test signal to said unit or sensing a signal in said unit, via a selected one of said nails; including means for applying a guard signal to said unit via any one or more of said nails other than said selected nail. With this arrangement. any nail not committed to supplying an actual stimulus signal or to sensing an output signal can be used to supply guard signals, thereby increasing the flexibility of design of the test fixture. An in-circuit automatic test equipment in accordance with this invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a simplified block schematic diagram of an automatic test equipment: and Figure 2 is a block diagram of a channel control array forming part of an automatic test equipment according to this invention. Referring to Figure 1. an in-circuit automatic test equipment (ATE) 10 includes a bed-of-nails test fixture 12 having a large number (for example 1024) of spring-loaded nails 14 arranged to contact selected circuit junctions or nodes in an electrical circuit 16 (the unit under test or UUT) when the fixture 12 is urged towards the UUT 16 by means not shown. The nails 14 are arranged in groups of eight. and any one nail in a group can be selected by an eight-way switch or relay network 18 for connection to a respective channel control array (CCA) 20 in a module 22 containing eight such arrays. The channel control arrays 20 can generate and supply test signals to the selected nail 14, for application to the UUT 16, or receive and monitor output signals from the UUT via the selected nail. The operation of a pair of CCA modules 22 is controlled by a common control board 24, which is in turn operated by a master processor 26. For clarity, only some of the nail groups and switches 18 for one only of the CCA modules 22 have been shown in Figure 1. A typical ATE might have 8 sets of control boards 24 and pairs of modules 22, giving a total of 128 CCA's 20 and 1024 nails. The general principles of construction and operation of an in-circuit ATE 10 are well known in the art and form no part of this invention. and thus need not be described in detail here. The design and operation of one CCA 20 will now be described, with particular emphasis on the output buffer circuitry. with reference to Figure 2. Referring to Figure 2. the CCA 20 generates I/O and H/L signals, as represented by block 30. indicating whether the channel is currently operating as an input or output channel, and, in the latter case, whether it is to supply a high or low signal for the current step of the test programme. These signals are combined with an enable signal by NkND-gates 32 and 34 and a WOR-gate 36 to control a high-current output buffer 38, to generate high or low forcing signals on a line 40 for application to the UUT, or to adopt a high-impedance state to permit sensing of signals from the UUT. Such signals from the UUT are supplied by the line 40 to a comparator 42 which supplies an output signal on a sense line to the block 30 in accordance with the value of the sensed signal relative to a reference signal REF. This output signal is analysed by the CCA 20 and the control board 24 in the process of verifying the integrity of the component currently under test The line 40 is coupled to the nail 14 appropriate to each step of the test programme by energising the corresponding one of eight relays 18 (only two of which are shown in Figure 2 for the sake of clarity). The test programme is so arranged that at any given time one and only one relay 18 associated with a CCA 20 is closed. Each nail 14 also has two further relays 44 and 46 connected to it. All the relays 44 are connected to a current-limited buffer amplifier 48 supplying a low voltage signal, and all the relays 46 are connected to a current-limited buffer amplifier 50 supplying a high voltage signal. The test programme is arranged so that when any relay 18 is closed the associated relays 44 and 46 for that nail 14 cannot be closed. However, for the remaining seven nails 14 in the same group, either one of the relays 44 and 46 can be closed, irrespective of the setting of the relays for the other nails in the group. The precise combination of relay settings is determined by relay control logic 52 in accordance with the requirements of the test programme. Thus, for example, while one of the nails 14, say #3, is applying a set of test signals to a component in the UUT 16, or sensing the responses of the component to such a set of signals from a nail 14 in another group. two other nails. say #2 and #7, may be applying static low voltage guard signals to the UUT lÏ: and three other nails, say *1, #5 and #6, might be applying static high voltage guard signals. with this arrangement, much flexibility can be achieved in the assignment of nails to specific functions in each step of a test programme: also, the nails which are not currently involved in applying test signals or sensing output signals in a given step can still be put to use, instead of being temporarily redundant. Thus, for example, if an unforeseen need for an additional guard signal at a particular node arises during test programme development, it may well be found that this node already has a nail assigned to it for input or output in a test step elsewhere in the programme. Accordingly, this existing nail can be used to apply the required new guard signal. simply by arranging for the appropriate relay 44 or 46 to be closed for the relevent steps in the test programme. This avoids the need to modify the test fixture 12, or even completely re-arrange the nail assignments.

Claims: CLIB 1. An in-circuit automatic test equipment comprising a plurality of nails for contacting circuit nodes in a unit under test; and a circuit for either applying a test signal to said unit or sensing a signal in said unit. via a selected one of said nails: including means for applying a guard signal to said unit via any one or more of said nails other than said selected nail. 2. An in-circuit automatic test equipment as claimed in claim 1 and wherein the nails are arranged in groups, one nail within each group being selectable to apply a test signal to said unit or to sense a signal in said unit; the other nails within a group being arranged to apply a guard signal. 3. An in-circuit automatic test equipment as claimed in claim 2 and wherein a guard signal may be applied via nails within a group where no nail is selected. 4. An in-circuit automatic test equipment as claimed in any preceding claim and wherein relay means is connected to a nail such that in a first relay state the nail znay be selected and in a second relay state a guard signal may be applied. 5. An in-circuit automatic test equipment as claimed in claim 4 and where further relay means is connected to the nail such that alternative guard signals may be applied. 6. An in-circuit automatic test equipment substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: BRAZIER, KEVIN EDWARD, LELIEVRE, DAVID HOWARD
Applicants: MEMBRAIN LTD
Application year: 1984
Main classification: G01R31
IPC codes: G01R31