GB2358466A

GB2358466A - Testing interconnections in an electronic circuit assembly by thermal conduction detection.

Info

Publication number: GB2358466A
Application number: GB0001075A
Authority: GB
Inventors: Cindy Maria Marcella Motmans
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-01-18
Filing date: 2000-01-18
Publication date: 2001-07-25
Also published as: GB0001075D0

Abstract

In a system for testing whether the pins (5) of electronic components (2, 3) are correctly connected to a circuit such as a printed circuit board (1), thermal energy is applied to the conductive area in question, <I>eg</I> using a laser beam, and the resulting thermal profile between the heated areas and the rest of the test object is detected, <I>eg</I> using a thermal imaging camera. A control system evaluates the thermal image to determine what proportion of heat from a hot spot created on one side of a pin and pad connection has been conducted to the other side of the connection, in order to determine whether pins which are intended to be connected to the board are correctly connected (short circuit) and pins that are not intended to be connected are correctly isolated (open circuit).

Description

2358466 VERIFICATION OF THE INTERCONNECTIONS BETWEEN ELECTRONIC DEVICES OF

AN ELECTRONIC CIRCUIT ASSEMBLY BY MEANS OF THERMAL CONDUCTION DETECTION

FIELD OF THE INVENTION

This invention relates to the testing of the connections by which individual electronic components are connected together, and in particular for a printed circuit board, the testing of the connections between the components and the printed circuit board on, which are they are mounted.

BACKGROUND OF THE INVENTION

A major source of expense during the production of electronic modules and assemblies such as printed circuit boards is the diagnosis and localisation of connection defects such as short circuit and open circuit connections which occur during the manufacture of the board or assembly. A functional test of the circuit board or circuit assembly which detects that the overall functionality is incorrect does not usually deliver sufficient diagnostic information to enable production personnel to easily localise manufacturing defects such as bad solder contacts and faulty electrical connections.

Electrical in-circuit methods were developed to locate such defects on a circuit board. The in circuit technique requires electrical access to every node on the circuit under test, either by means of a bed of nails fixture, or by means of a moving probe mechanical system. An electrical in-circuit test system can detect short circuits between points on the circuit under test to which the system has nodal access. This test is known in the art as a shorts test, and a shorts tests is intended to detect connections which should not be present between circuit nodes on a correctly manufactured circuit board. A test for the detection of poor or missing connections between device pins and the the rest of the circuit is known in the art as an opens test. The effectiveness of such an open pin test and the problems associated with its implementation depend on the device to which the pin belongs, and on surrounding circuitry.

The first in-circuit techniques were based on treating each component in the circuit individually, and carrying out a stimulus-response action using an appropriate functional parameter on each component in turn. Such a technique can be used on analog and digital circuits, if the test system can provide a functional stimulus the component's response to which the system can in turn measure. Such tests give not only an indication of the value or type of component fitted, but also by implication detect whether the component under test is correctly connected to the circuit, and thus manufacturing defects can be localised.

The inability of such individual functional tests to be effectively carried out in-circuit on increasingly complex digital circuits lead to the adoption of parametric techniques. Parametric techniques involve an electrical stimulus and response which are intended only to verify that the electrical contact of the pins of the device under test is good, and do not verify the intrinsic functionality of the device. This type of test, in which the aim is to detect open connections between component pins and between component pins and the circuit board at points where there should be a good electrical contact in the case of a correctly manufactured circuit board assembly, is known in the art as an opens test or open pin test.

Whilst such parametric techniques can theoretically be used for testing the connections of digital devices, many discrete analog devices such as low value capacitors cannot be effectively electrically tested even for presence, due to the physical limitations of test fixtures and wiring which load the circuit under test. Furthermore, device geometries and circuit boards are being constantly further miniaturised, and the ability to directly mechanically probe metallic points such as pins and pads on circuit boards and other electronic assemblies is becoming more difficult.

Optical inspection techniques are widely used for seeking connection faults on printed circuit assemblies and boards. Optical techniques rely on the comparison of stored optical image data derived from a connection orjoint model deemed as good, which are applied to the image data of the connection which is to be checked. Such optical techniques suffer from numerous drawbacks, one such being the fact that a seemingly optically correct connection is not a guarantee that the electrical connection is good.

There is clearly a requirement in the art for a device and method which more directly verifies by means of a physical test the suitability of a connection of any type as a medium for electrical conduction, without relying on the measurement or detection of any electrical or electronic parameter or function, and by implication, without relying on any form of direct electrical or mechanical contact with the unit under test.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus and method to test for a connection between a pin of a component and a printed circuit board. An additional object of the invention is to detect short circuits between pins, pads, or other conducting elements or points on a circuit board or assembly. Another object of the invention is to provide such an apparatus and method that does not rely on either the stimulus or measurement of any electrical parameter of any components on the unit under test. A further object of the invention is to verify the correctness of a connection on the circuit board without reliance on any kind of mechanical access. Yet another object of the invention is to overcome the limitations of purely optical techniques. A benefit of the invention is that no electrical power is applied to the board.

The foregoing and other objects of the present invention are embodied in a system which overcomes the limitations of the prior art.

In accordance with the present invention, the thermal conduction which occurs at a metal to metal junction is substantially indicative of the degree to which electrical conduction occurs at the junction. The present invention detects whether thermal conduction occurs between metal parts constituting component pins, and metallic parts of the circuit board. The present invention determines whether metallic parts which are intended to be connected together are connected together. Conversely, the present invention determines whether metallic parts which are intended to be isolated from each other are actually isolated.

A controlled amount of thermal radiation in the form of a laser beam or other radiation source capable of being targeted at an individual pad or component pin is targeted at the localised area of the circuit board at which point the connectivity is to be tested. This can be a component pin or a component pad on the circuit board. The resulting thermal image of the area in question is analysed for thermal gradients and used to determine the degree of the temperature gradient existing between the points between which the connectivity is to be tested.

If a test for short circuits is to be carried out between given component pins or component pads on the circuit board, the thermal image covering adjacent metal parts - pins and pads - is analysed. If no short circuits are present - the correct condition - there is a suitably sharp thermal gradient. between the heated pin and surrounding metal parts. If a short circuit is present - the fault co ' ndition - no sharp thermal gradient exists between the pin under test and between one or more of the adjacent pins or pads.

If an open pin test is to be carried out, the heat applied to the pin under test will conduct to the component pad or pin to which there should be conduction in the case of a correctly manufactured circuit board. The low thermal gradient between the pin and the part to which the pin is intended to be connected indicates for a correctly connected pin that thermal conduction takes place, which implies that the electrical connection is good.

The advantage of the system is that it measures an analogue of the electrical conductivity parameter of a connection, and obviates the need for complex and sensitive electrical or electronic measurement equipment and fixturing which is required when making direct electrical verification of the contact.

The present invention can detect the two most common manufacturing defects, the first being that two or more pins or pads are wrongly shorted together, and the second that two points are electrically unconnected when they should actually be connected.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. I shows an overview in schematic form of an implementation of the device of the present invention.

Fig.2 shows a view of an optically correct solder joint Fig.3 shows a sectional view of a device pin seated in a conducting connector Fig.4 is a flowchart which shows the process of the invention DESCRIPTION OF THE PREFERRED EMBODIMENT

The following describes a mode of carrying out the present invention, being the best presently. contemplated mode. This description is intended to illustrate the general principles of the invention, but not irf a limiting sense.

Fig. 1 shows an overview of an implementation of the present invention. The printed circuit board I has a plurality of analog components 2 and digital components 3 and connectors 4 connected by way of component pins 5 to a plurality of conductors 6 which make up a circuit on the circuit board 1. A controlled amount of energy from the projection system 7 is directed at the point which is to be heated 8 on the connection under test. The thermal imaging device 9 captures a thermal image of the predetermined area 10 associated with the connection under test. The thermal image is processed by the test system I I which includes the necessary control, measurement, and evaluation equipment to carry out the operations and control of the stimulus and measurement sequence and to provide a quantification of the status of the connection under test.

Referring to Fig.2, which shows a typical solderjoint connection 15, if thermal energy is applied to the pin 12 of the component 13 and the connection between 13 and the component pin pad 14 is good, the pad 14 will undergo a significant temperature rise which the thermal imaging device 9 will detect. Alternatively, when thermal energy is applied to the pad 14 the pin 12 will undergo a significant temperature rise which the thermal imaging device 9 will detect.

Referring to Fig.3, which shows a typical connection of device pins to a connector receptacle 18 by means of spring-loaded contacts 17, if thermal energy is applied to the pin 16, and the connection between the pin 16 and its connector receptacle 17 is good, the connector receptacle 17 will undergo a significant temperature rise which the thermal imaging device 9 will detect. Alternatively, when thermal energy is applied to the receptacle 17 the pin 16 will undergo a significant temperature rise which the thermal imaging device 9 will detect.

Fig.4 shows a flowchart of the present invention in terms of a single test. Block 19 selects the point on the circuit which is to be heated and selects an area on the circuit which is to be tested and Block 20 applies the energy source to the predeten-nined point on the area. Block 21 captures the thermal image of the area under under consideration. Block 22 determines whether the thermal profile of the adjacent area indicates whether the connection is conducting or non-conducting, i.e. Block 22 determines from the thermal image data whether the connection exhibits significant conductance. Block 23 uses the information from Block 22 to pass control further, depending on whether a conducting or non-conducting connection is detected. Block 24 determines in the case of an open test in which the connection under test should exhibit conductance - whether the detected connection condition is correct. Block 25 determines in the case of a short test - in which there should not be conductance between the pins or areas or points under test - whether the detected connection condition is correct. Block 26 ensures that a fault condition is flagged. Block 27 represents the end of the test sequence for a single test. A test program for an entire electronic assembly contains a plurality of such tests.

Having thus described a presently preferred embodiment of the present invention, it will be clear to those having knowledge of the art that differing implementations of the present invention can be envisaged without departing from the scope and method of the present invention. The description thus disclosed is therefore not be taken as limiting in respect of the invention.

To summarise, in one aspect the present invention provides:

A system for testing the connections of a circuit board or assemoty to determine whether short circuits are present between pins or pads on the said circuit board or assembly and whether pins or pads on the said circuit board or assembly which which should be connected are suitably connected for subsequent proper electrical operation, said system being comprised of- thermal energy source capable of being directed at a predetermined point on the said circuit board or assembly in order to provide a local hot spot on the connection under test; thermal imaging device to capture the thermal image of the connection under test which is adjacent or near to said hot spot; control system which evaluates the data from the thermal imaging device against previously determined limits in order to determine whether said connection under test is correct or faulty.

Claims

1 A system for testing the connections of a circuit board or assembly to determine whether short circuits are present between pins or pads of said circuit board or assembly and whether pins or pads on said circuit board or assembly which should be connected are suitably connected for proper electrical operation, said system being comprised of:

a thermal energy source capable of being directed at a predetermined point on said circuit board or assembly in order to provide a local hot spot on the connection under test; a thermal imaging device to capture the thermal image of the connection under test which is adjacent or near to said hot spot; and a control system which evaluates the data from the thermal energy device against previously determined limits in order to determine whether said connection under test is correct or faulty.

2. A system as claimed in claim 1, wherein the directable thermal energy source is a laser.

3. A method for testing the connections of a circuit board or assembly to determine whether short circuits are present between pins or pads on said circuit board or assembly and whether pins or pads on said circuit board or assembly which should be connected are suitably connected for subsequent proper electrical operation, said method comprising the steps of:

providing a system as claimed in claim 1 and directing the thermal energy source at one side of the connection, such as at a pin or at a pad of a pin to pad connection; and directing the thermal imaging device to capture the thermal image of the other side of the connection under test, such as at the other of the pin and pad of a said pin to pad connection.

4. A method as claimed in claim 3 which further comprises the further step of operating the control system to determine whether a sufficient proportion of the heat from the hot spot on the first side of the connection has been conducted to the second side of the connection to indicate that the connection under test is suitably connected for subsequent proper electrical operation.

5. A method as claimed in claim 4, wherein the control system is operated to compare the thermal image of the second side of the connection with a thermal image of the first side of the connection or with an earlier thermal image of the second side of the connection from prior to provision of the local hot spot on the first side of the connection.

6. A system substantially as hereinbefore described with reference to any suitable combination of the accompanying drawings.

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