WO2004090560A2

WO2004090560A2 - Automatic test machine for testing printed circuit boards

Info

Publication number: WO2004090560A2
Application number: PCT/GB2004/001523
Authority: WO
Inventors: Colin Barker
Original assignee: Colin Barker
Priority date: 2003-04-10
Filing date: 2004-04-08
Publication date: 2004-10-21
Also published as: WO2004090560A3; GB0308550D0

Abstract

An automatic test machine for testing printed circuits boards (1) comprising a rotation test-head (3, 4); means for track capacitance measurement (51-70); dual-purpose test pins (70-75); and a module construction (13, 14, 83-85,87-90) based on adapter cards (83, 90).

Description

DESCRIPTION

Improvements to an automatic test machine.

Field This patent describes a number of techniques for improving the test of printed circuit boards.

Background A key component in many electronic systems, especially in the

Telecomms industry, is the backplane. This is a special type of printed circuit board (pcb's) which serves to provide interconnections between the other printed circuit boards that comprise the electronic system. Typically a backplane has rows of connectors on it and the other printed circuit boards plug into it.

Because it is at the heart of the system it is difficult to replace if a fault is found when the system is fully assembled, and this means that quality is paramount. Since it is has far more connections than typical printed circuit boards but has relatively few components on it (apart from connectors), test equipment designed for "normal" boards is of limited use for testing backplanes.

Typically all machines designed for testing pcb's fall into one of two types, they either require hard tooling of some sort to interface with a particular pcb or are "robotic" in nature and the test capability, usually called a "test head" is moved under computer control, in X, Y and Z to suit the particular pcb being tested. This robotic type of tester is usually known in the pcb industry as a "flying probe" machine. This patent application principally deals with improvements to the latter category.

A machine specifically designed for testing backplanes is described. Figure 1 shows some of the key features. The backplane (1) is attached by a fixture to the frame (2) of the machine. A test head (3) can be moved under computer control relative to the backplane in order to conduct specific tests at particular locations. (4) shows the test head viewed from the other side. Movement in the X and Y directions can be achieved by linear motors (5,6). The test head can also be moved in the Z direction. This complete mechanism may be duplicated on the reverse side of the backplane. ^■

The test head can incorporate a number of separate test units designed to perform optical, electrical or mechanical tests. This test head can be moved to a magazine (7) located at the side of the machine where it can pick up any one of a number of adapter modules, designed to tailor the test units to suit a particular application on the backplane being tested.

It is clearly desirable that the test time is reduced as much as possible. Factors which can increase the test time are (a) the need to keep changing adapter modules (b) the need to perform lots of mechanical tests as these are much slower than optical tests and (c) the need to make contact with a pin (or group of pins) more than once in order to perform say a mechanical test, followed by an electrical test.

This patent describe a number of features which could be incorporated into the test head and the adapter modules of such a machine, (but are not limited to this application) and which are designed to improve the capability with which the test head can perform various types of test. The Invention

There are four separate aspects of the invention.

The invention - Rotating head

A test head containing test electronics and to which the adapter module is attached can be rotated under computer control to any angle in the plane of the board under test (see Fig 2). The rack (11) containing the test electronics is attached to a circular plate (12). On the front of this is an adapter module (13) which has spring-loaded pins (14) which make contact with the component being tested. The circular plate (12) is mounted in a rotary bearing (15). The drive unit (16) can rotate the whole of the test assembly to any position.

This has two advantages :— a) A backplane may have similar connectors mounted in different orientations. Since connectors are usually mounted parallel to the main axes of the board there are two possible orientations for a symmetrical connector (Fig 3 a) and four for an asymmetric one (Fig 3b). With a fixed head each of these orientations will require a separate test module. With a rotating head only one test module is needed. A rotating head can also deal easily with connectors orientated at intermediate angles whereas the design of a module to suit an intermediate angle would be cumbersome. b) It is possible that the connector may be at a slight angle to the expected orientation because either (Refer Figs 3c, 3d):—

Although the board axes (22) are parallel to the machine axes (23) the connector (21) is not parallel to the board axes (22) due to inaccuracies in the basic printed circuit board.

OR

Although the connector (24) is parallel to the board axes (25) the board itself

(25) is slightly misaligned with respect to the machine axes (26). c) Any such misalignment be measured and the head rotated so that the test module is exactly aligned with the connector being tested. d) These advantages apply to any component requiring to be probed or tested and which can be oriented in different directions. By using a module with two probes whose separation can be varied any two leaded component in any orientation can be tested with a single module.

The invention Track capacitance measurement a) There are a variety of methods in use for testing the integrity of the tracks on a PCB by measuring the capacitance between the track and a nearby power plane. [The capacitance of a track is directly related to its surface area. If the capacitance of a track differs significantly from the capacitance of the same track on a "known good" board, then the track is suspect.]

All of these methods require the use of two probes. One of these probes applies an AC voltage to a large plane or track (usually a ground plane or power plane). The second probe, which makes contact with the track being tested, is connected to a sensitive detection circuit to detect the signal passed to this track by capacitive coupling from the large plane or track. b) The outline of a conventional circuit is shown in Fig 4a. An AC signal (51) is applied between earth (52) and a large track or power plane on the PCB (53). The detector circuit consists of a pair of diodes (55,56) together with a pair of resistors (57) and a capacitor (58). The voltage appearing across the pair of diodes is fed to a simple detector (59), giving an output voltage (60) which is a measure of the capacitance between the track (54) and the power plane (53). c) The method proposed is unique in that it requires only a single probe. d) It achieves this by having the functions of the "exciting electronics" and the "sensing electronics" combined into a single circuit. It then requires only that a ground plane or power plane (or several) be connected to ground with a simple piece of wire. e) The design of the new circuit is shown in Fig 4b. The capacitor (58 in Fig 4a) becomes two capacitors (62,63 in Fig 4b). The AC source instead of being connected to the large plane (53 in Fig 4a) is connected to the junction between the two capacitors (62,63 in Fig 4b). The large plane is connected directly to earth by a wire (61), so that only a single probe (70 in Fig 4b) is needed. f) The signal generated across the two diodes in Fig 4b is fed via low-pass filters (64, 67 and 65, 66) to a low-noise instrumentation amplifier (69). g) In this method the exciting electronics is charging up a small capacitance (the track being tested) rather than a large capacitance (the ground plane) which is being charged up by the conventional circuit as in Fig 4a. This reduces waveform distortion and noise by at least an order of magnitude thereby improving the accuracy of the measurement that can be achieved by this new circuit.

The invention Dual-purpose test pins

As described above, capacitance characterisation is a convenient way of testing track integrity. However, in a small number of circumstances within a particular pcb, this technique may not be possible or adequate and conventional measurements may be necessary (for example but not exclusively resistive or high voltage measurements). In order not to need duplicate adapters and test moves, a group of test channels with both measurement capabilities is desirable. a) One method of implementing this is shown in the circuit of Fig 5. The points A, B in Fig 5 correspond to points A, B in either Fig 4a, or Fig 4b. The two diodes (55, 56) in Fig 5 correspond to the two diodes shown in Fig 4a or 4b. b) The circuit incorporates two switches (71, 72) activated by two signals (73, 74). Although semiconductor switching devices are shown in the diagram any kind of semiconductor or electro-mechanical switch can be used. c) When the movement of the adapter card towards the connector being tested starts the test circuitry is switched to capacitive mode. This is achieved by switching both switches (71, 72) "off, so the circuit of Fig 5 is having no effect. d) The circuit of Fig 4b will detect as soon as the test pin comes into contact with the track or the connector being tested. In the case of a connector pin this gives another method of measuring (as a by-product) the pin height since the position of the test head is known accurately. e) When contact has been made with the circuit still in capacitive mode the pin can measure the track capacitance to confirm (or fail) track continuity. f) The circuit can then switch to either

"Ground mode" by switching (71) "on" in which case the circuit will ground the pin allowing a connectivity measurement to be made from a probe elsewhere on the PCB OR

"Current mode" by switching (72) "on" in which case the circuit will supply a current determined by the DC supply voltage and the resistor (75), to allow a connectivity measurement to be made to a pin which has been grounded by another probe. g) A further feature of this circuit is that by switching it to "ground" mode when the test pin is NOT in contact with another pin or track, the effects of any stray capacitance in each measuring circuit may be determined and any further measurement adjusted to allow for this stray capacitance.

The invention Module construction

A key requirement for using the capacitance measurement technique described above is to minimise the stray capacitance due to the equipment itself. This can be achieved by locating the two diodes (55, 56 in Fig.4a, 4b) as close as possible to the feature being measured. In practice this means locating them within the adapter modules immediately behind the pin used for contacting the pcb. A method of achieving this result is described. a) This method of module construction (as shown in Fig 6) allows easy interfacing from the array of contact probes on a fixed pattern (81), situated on the test-head (3) to a pattern (82) which will suit the connector (or other component) under test and could therefore be different for each connector / component type. b) A key component of the module is the adapter card (83 in Fig 6). There could be more than one in a module. These are illustrated in Fig 7. c) Fig 7a shows the adapter card (83) in side view. d) At one end of the adapter card are pins (84, 89) at the standard spacing to interface with the test head pin layout, shown at (81). e) At the other end are pins (85) at the custom spacing to interface with the connector or other component being tested (86). Such pins can be replaced by the mating half of the connector under test. f) The conductive tracks (87) on the adapter card allow connection between the two sets of pins on these different spacings and the insertion of the diodes (88) close to the custom pins. g) Fig 7b shows the adapter cards viewed from above. At one side are pins (84, 89) at the correct spacing to interface with the pins on the test head (81). Note that in the configuration shown two test head pins are required to support each test pin because there are two diodes in the capacitance measuring circuit described earlier. h) At the other end are pins (85) at the correct spacing to interface with the connector

(or other item) under test (86). i) Any variation in spacing between the pins at either end is accommodated by assembling the boards into the module at an angle as shown (90). j) The effects of any residual stray capacitance may be further reduced by varying under software control the pattern of test pins which are excited simultaneously, k) Where the test points on the component under test are very close together the effective spacing between test pins may be increased by arranging them in a

"chequerboard" pattern. 1) The adapter cards in the module may incorporate additional test electronics for any purpose. One example would be RF circuitry which sends test data to the system so RF signals are confined to the testhead. m) Modules can incorporate a compliant base with inbuilt sensing. Any significant force on the module can be detected and reported to the control system. This feature can be used for measurement purposes or for the prevention of damage.

Summary

In summary the invention is a series of inventions which taken either together or separately improve the scope, speed and accuracy with which backplanes (or other printed circuit boards) can be tested using an automatic machine.

Claims

CLAIMS A series of improvements to an automatic test machine which may be used together or separately to improve the scope, speed and accuracy of the tests, and comprise a rotating test-head, an improved method of track capacitance measurement, dual- purpose test pins and a method of module construction based on adapter cards. A rotating test-head whereby components mounted at any angle may be tested using a single adapter module. An improved method of track capacitance measurement which allows the testing of PCB tracks using a single probe. Dual-purpose test pins whereby a single pin may be switched between different test modes. A method of module construction which allows interfacing between a fixed array of contacts, such as but not limited to, the array on the test-head to a variable contact array, such as but not limited to, the contacts on the component being tested. A method of module construction which can increase the effective spacing between test pins. A method of module construction which allows additional electronic circuitry to be located very close to the component under test.