Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
1999-05-28
2001-01-30
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
C073S15000R
Reexamination Certificate
active
06178823
ABSTRACT:
This invention relates to an apparatus and method for testing the mechanical integrity of connections in miniature electrical circuits.
The physical size of electronic devices, particularly microprocessors, is rapidly reducing, and as a consequence the size of electrical connections must also reduce. A current microprocessor may have an array of edge connections comprising copper strands having a width of 40 &mgr;m and a spacing of 250 &mgr;m. These edge connections may typically extend around all sides of a rectangular circuit component and are in use bonded to other parts of the device, for example by ultrasonic or thermocompression techniques.
In mass production it is necessary to periodically test the mechanical strength of these bonds, both to ensure that the connection method remains adequate and to give assurance that the electronic device will be reliable in service.
A tensile test can be performed by restraining the device on a horizontal workholder, and hooking the connection strand with a tool. Gripping the strand with mechanical tweezers is also known. A shear test can be performed by pushing the bond site sideways with a tool. In each case the breaking force is measured using appropriate strain gauge methods.
Owing to the very small size of the connection strands it is essential to view the workpart through a magnifying device. The tool is typically mounted on a 3 axis motor drive. Great care is required to minimise friction forces which would otherwise mask the very low breaking forces measured. Precision is a prerequisite since the tool must be guided into engagement with a strand, and perform the test without damaging adjacent strand connections. Speed is also important since the number of connections to be tested is rather high, and thus the test duration has a significant effect on testing costs.
A problem arises in the case of connections which incline relative to the plane of the electrical circuitry. This occurs in the case of printed circuits formed on an insulating substrate, and having connection strands pushed through apertures in the substrate for connection to an electrical device. Typically such strands may lie at about 45°.
In a tensile test a hook tool tends to slip up the strand to a greater or lesser extent, and in a manner which is neither consistent nor repeatable. The position of the hook on the strand is however important since the proportion of tensile load taken at opposite ends of the strand will change if the hook position moves. This effect is especially noticeable in short strands of the kind found in miniature devices. The variation in measured force due to slipping of the hook is more than sufficient to mask the test result.
One solution to this problem is to place the device at an angle so that the connection strand is generally perpendicular to the direction of pull. This can give a consistent result because the tool is not prone to slipping, but placing the device at an angle brings other problems. Firstly if the device is wide, it may interfere with free movement of the test head. Secondly, operator view of the device may be poor, and thus positioning of the tool imprecise. Thirdly movement of the device between adjacent rows of strands requires unclamping, indexing, reclamping and refocusing of the magnifying device; this has significant manipulation difficulties, and is time consuming.
Another solution provides a stiffer hook tool with support bearings adjacent the tip thereof. However the inherent tool stiffness cannot be greatly increased without increasing the section thereof, and thus risking contact damage to the device during tool manipulation. Furthermore, the additional bearings introduce an additional source of friction, restrict the range of free movement of the test head, and may also restrict operator visibility. A better solution is required.
According to the invention there is provided an apparatus for testing the bond strength of strand connections in a miniature electrical component, the apparatus including a workholder, a test head movable over the workholder and having a 2-axis drive, one drive axis being operable simultaneously with the other drive axis, a test tool on the test head, a strain gauge adapted to measure force at the tool along one of said axes, and control means adapted to drive the test head at an acute angle to one of said axes.
Preferably the workholder is movable in a plane perpendicular to one of said axes, in translation and/or rotation. In the preferred embodiment the workholder is horizontal. A 3-axis drive is preferably provided for optimum tool manipulation; this provides greater freedom in choosing the direction of pull, and consequently greater freedom of workpart placement and the likelihood of optimum operator visibility.
According to a further aspect, the invention provides a method of testing the bond strength of strand connections in a miniature electrical component, and in which the strand direction lies at an angle to the plane of the component substrate, the method comprising the steps of:
orientating said substrate in a first plane perpendicular to a first axis;
hooking said strand with a test tool;
moving said test tool perpendicular to the direction of said strand, by simultaneous movement of said test tool along a first axis and a second mutually perpendicular axis;
calculating the angle of movement of said test tool with respect to the plane of said substrate;
measuring the breaking force of said strand along one of said first and second axes; and
calculating by reference to said angle the breaking force of said strand in the direction of movement of said test tool.
This test apparatus and method is both elegant and counterintuitive. Conventionally the skilled man would look for a solution to the existing problem by providing an improved test head better able to resist lateral forces, or an improved workholder, or more rapid refocusing techniques, or some other improvement directly addressing one of the prior test difficulties. The present invention relies on the realisation that a computerised 2-axis motor drive for this kind of test apparatus must inherently be very precise since accurate positioning of the test tool is necessary. Simultaneous operation of the drive motors along two or three axes does not affect the workholder or the test head. Furthermore the breaking loads to be measured are very small, and thus no special upgrading or adaptation of the drive motor perpendicular to the normal test axis is required; in other words the positioning drive motor has sufficient capacity to exert the necessary test force, without adaptation.
Finally, the use of geometrical techniques to calculate the actual breaking force means that a known low friction test head can be used, and this avoids the problem of introducing additional frictional forces by adaptation of the mechanism to overcome problems in a conventional manner.
The invention has the particular advantage that the test machine can be used both for vectored tests according to the invention, and unidirectional tests without significant adaptation. Importantly, the device is mounted on a generally horizontal workholder, and thus facilitates indexing and rotation without requiring refocusing of the magnifying device. In particular the device can easily be oriented to give the operator a good view of the array of angled connection strands, without refocusing.
Another advantage of the invention is that the test head can be driven at any desired angle and velocity by adjusting the relative speeds of the drive motors. In this way a truly perpendicular pull is assured, and a fixed angle is not required for a particular array of strands. Thus the most appropriate manufacturing method can be adopted, and the direction of pull selected to suit the actual angle of the strands. Variation of the strand angle is easily accommodated by reprogramming of the drive motors, and no adjustment of the workholder is required. In particular, specialised jigs to hold different electrical devices at suitable angles are not required, and thu
Dage Precision Industries Ltd.
Noori Max
Stockton Kilpatrick
LandOfFree
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