Calibration device, calibration apparatus, bond test apparatus, and method of calibration for a bond test apparatus

Abstract

A calibration device for a bond test apparatus comprises a calibration contact, a flexure and a displacement sensor. The flexure has a fixed end and a free end, the free end of the flexure being coup

Description

Field of the Invention The present invention relates to a calibration device for calibrating a bond test apparatus for testing the strength of bonds on electrical circuitry, such as a PCB or semiconductor device. In particular, the invention relates to a calibration device for calibrating the force on a bond test apparatus test tool during a bond test, to a calibration apparatus containing such a calibration device, to a bond test apparatus, and to a method of calibrating a bond test apparatus. Background to the Invention Semiconductor devices are very small, typically from 5mm x 5mm square to 50mm x 50mm square, and typically comprise numerous sites for the bonding of electrical conductors to a semiconductor substrate. Each bond consists of a solder or gold ball deposit, known as a “bump”, a copper pillar, or a wire adhered to the substrate. It is necessary to test the mechanical bond strength of the bonds between connections or devices, in order to be confident that a particular bonding method is adequate. Because of the very small size of the bonds, tools used to test the bond strength of these bonds must be able to measure very small forces and deflections accurately. There are several different types of bond tests that are used to test bond strength. For example, shear testing tests the shear strength of a bond by applying a shear force to the side of the bond and shearing the bond off the substrate. Pull testing tests the pull strength of the bond by pulling a ball deposit, or a wire embedded in a ball deposit, away from the substrate. In a push test, a force, or load, is applied in the vertical plane directly downward onto a bond. The bond test apparatuses that perform these tests typically comprise a bond test tool, which may be either a shear test tool, a push test tool or a pull test tool, that can be positioned relative to the bond under test and then either the bond or the tool is moved in order to perform the test by measuring the force needed to break the bond. Bond test apparatuses, or bond testers, are machines that provide a means of mechanically loading features (for example bonds, bumps or wires) either until a failure occurs (destruct) or to a prescribed limit that is within the features safe loading limits (NDT or non-destruct testing). During the loading process the load on the test tool, and its motion profile, are monitored in real time by the bond test apparatus. Load measurements are performed using transducers which contain a flexure designed to suit a specific load. Elements of the flexure are fitted with strain gauges (either semiconductor or foil) that produce an electrical output proportional to the load applied to the flexure. The selection of strain gauge is based on the deflection characteristics and the required voltage output. Bond test transducers are normally fitted within a bond tester cartridge which enables a user to easily swap between different test types and forces by exchanging the cartridges fitted to the bond tester. Test types include shear (force applied horizontally), pull (force applied vertically upwards) &push (force applied vertically downwards). Transducers can have a low deflection (for example <0.5 mm) or high deflection (for example >2.0 mm) depending on the nature of the test to be performed. Maintaining the accuracy of bond test transducers traditionally involves calibrating each transducer with respect to a range of calibration weights. The calibration weights used are traceable to international standards. Calibration weights are either applied directly to the test tool of a particular transducer, or using a jig which amplifies the weight or changes the direction in which the weight acts with respect to gravity. The linearity of the transducer output is found by testing using multiple different weights within the transducer’s load capability, so weights must be exchanged and the same transducer tested multiple times before the calibration sequence is complete. Current bond tester calibration methods involve the use of a wide range of weights and jigs, meaning that current bond tester calibration methods are therefore extremely cumbersome and time consuming. Bond test apparatuses are increasingly being used in semi and fully automatic environments, for example to test products that are produced on semi or fully automated production lines. The traditional calibration methods are not suitable for either semi or full automatic applications due to the need for frequent manual intervention to change calibration jigs and calibration weights. The present disclosure seeks to overcome these deficiencies in the prior art, and to provide an improved means of calibrating bond test apparatuses. Summary of the Invention The present invention is defined in the appended independent claims to which reference should now be made. Preferred or advantageous features of the invention are defined in the dependent claims. Calibration Device for a Bond Test Apparatus According to a first aspect of the present disclosure, there is provided a calibration device for a bond test apparatus. The calibration device may comprise: a calibration contact; a flexure having a fixed end and a free end, the free end of the flexure being coupled to the calibration contact; and a displacement sensor which is configured to measure a displacement of the free end of the flexure relative to the fixed end of the flexure and to produce an output signal indicating the magnitude of the displacement. The calibration contact is configured to be contactable with a test tool of the bond test apparatus, in use, so that application of a test force between the test tool and the calibration contact displaces the free end of the flexure relative to the fixed end of the flexure, such that the displacement sensor measures the resulting displacement of the flexure and produces an output signal indicating the magnitude of the displacement caused by the test force. The conventional way of calibrating a bond test apparatus involves applying a calibration weight with a known mass to a test tool, and measuring the load indicated by the transducer of the bond test apparatus. For calibrating some pull test transducers, this may be done simply by lifting the calibration weight using the test tool. For higher loads, and for calibrating push and shear transducers, then it is necessary to use calibration jigs - also known as ratio jigs - to apply the load between the calibration weights and the test tools. This process involves applying the calibration weight to the calibration jig, applying a target test force between the test tool and the calibration jig, which test force should displace the calibration weight by a predicted distance, and measuring the load indicated by the transducer of the bond test apparatus. This must be repeated for a series of calibration weights of known masses, so that the apparatus can be calibrated to compensate for any inaccuracies in the measured force. The present calibration device is used to calibrate a bond test apparatus by bringing the test tool of the bond test apparatus into contact with the calibration contact, applying a test force (which during calibration may alternatively be termed a calibration force) of a target magnitude between the test tool and the calibration contact, and measuring the resulting displacement of the flexure. The stiffness of the flexure in the calibration device has a known fixed value, so the displacement of the flexure in the calibration device is usable to calculate very accurately the actual test force being applied between the calibration contact and the test tool. This actual test force can be compared to the target test force which the bond test apparatus is aiming to apply, and the control system of the apparatus can be adjusted to compensate for any difference between the actual and target test forces. The test force may be applied by instructing the bond test apparatus to apply a particular target test force. The force transducer (load cell) of the bond test apparatus will apply what it thinks is the target test force to the calibration contact. In a perfectly calibrated system, the actual test force experienced and measured by the calibration device will exactly match the target test force. In an imperfectly calibrated apparatus, however, the actual test force applied to the calibration contact and measured by the calibration device will not be equal to the target test force. This difference between expected and actual test force values can be used to calibrate the bond test apparatus by adjusting the control system of the bond test apparatus so that the actual test force produced by the apparatus does match the target test force that the apparatus means to apply. This calibration process may be repeated at a plurality of target test forces within the operating load range of the force transducer used by the bond test apparatus, using the same calibration device, without any need to exchange calibration weights or adjust the calibration device. The flexure displacements caused by each applied test force may be plotted to check the linearity of the force applied by the bond test apparatus. This calibration device therefore allows a bond test apparatus to be calibrated without the need for manual intervention by an operator, as it is no longer necessary to exchange weights on a jig in order to calibrate the apparatus at different loads. This makes the present calibration device particularly suitable for integration into partially or fully automated bond testing systems in a way that would not be possible using conventional calibration jigs. The calibration device is preferably configured to calibrate test forces within a predetermined calibration force range. The calibration force range may correspond to the stiffness and maximum displacement of the flexure in said calibration device. The calibration contact provides a point of contact which provides a consistent load point between the test tool and the calibration device. The flexure is preferably configured so that flexure displacement varies linearly with the applied test force, such that the output signal from the displacement sensor provides a measurement of the magnitude of the test force. The calibration device may comprise a data connection which is configured to deliver the output signal to a processor external to the calibration device. For example, the data connection may be configured to deliver the output signal to a processor in the bond test apparatus. The calibration device preferably comprises a power and data connection through which information may be transferred between the calibration device and a processor, for example a processor in a computer or in the bond test apparatus. The data connection may preferably be configured so that information may be read or written between the calibration devices and a connected processor. The calibration device may preferably contain a non-volatile memory which is usable to store data and parameters associated with calibration and usage. The calibration device preferably comprises a housing which contains the flexure and the displacement sensor. The fixed end of the flexure is fixed relative to the housing, and the calibration contact is external to the housing, or exposed within the housing, so that the calibration contact is contactable by the test tool of the bond test apparatus. The housing may advantageously protect the flexure, displacement sensor, and internal electronics of the calibration device. In a preferred embodiment, the displacement sensor is a linear encoder which produces a digital output signal. The linear encoder displacement sensor may be configured to measure a displacement of the free end of the flexure relative to the fixed end of the flexure, and to deliver a digital output signal to a processor which may optionally be external to the calibration device. The processor may be programmed to convert the digital output signal from the linear encoder into a measurement of the magnitude of the test force between the test tool and the calibration contact during calibration. In the present calibration device, a linear encoder may preferably be configured to measure the deflection of the flexure, and to deliver a digital output signal from the calibration device. This provides a distinct advantage over solutions relying on strain gauges to measure forces, as the output signals produced by strain gauges are analogue voltages which typically require amplification and conversion before they can be recognised and processed by the control electronics of a bond test apparatus. Linear encoders provide distinct advantages over strain gauge transducers by providing a higher-magnitude output signal which removes the need for an amplification step, as well as providing a digital output signal that can be delivered directly to a processor without first requiring an analog-to-digital conversion step. By providing a digital output signal directly from the linear encoder, the risk of noise or electrical interference is greatly reduced compared to the sensor options typically used in bond testing. A linear encoder is a sensor which comprises two parts: an encoder readhead and a detectable member which is detectable by the encoder readhead. The readhead acts as the sensor, and the detectable member acts as a scale which is readable by the readhead. The linear encoder readhead detects linear movement of the detectable member and encodes the position of the detectable member relative to the readhead. The linear encoder converts the encoded position of the detectable member into a digital output signal which can be decoded into a position measurement by the processor. One of the detectable member or the encoder readhead may be coupled to (or fixed relative to) the free end of the flexure, while the other of the detectable member or the encoder readhead is fixed relative to the fixed end of the flexure. When a test force is applied between the bond and the test tool, and the flexure is deflected as a result, the resulting deflection will thus cause the detectable member to be displaced relative to the encoder readhead. In a preferred embodiment, either the encoder readhead or the detectable member is removably couplable to the free end of the flexure, while the other of the encoder readhead or the detectable member is removably couplable to an encoder mount which is fixed relative to the fixed end of the flexure. By removably mounting both portions of the linear encoder to the apparatus, it is advantageously straightforward to replace one or both of the readhead or the detectable member without disassembling the entire apparatus. This makes service and repair much more straightforward than it is with strain gauges which are adhered directly onto flexures. The linear encoder may be an absolute linear encoder or an incremental linear encoder. The linear encoder is preferably a non-contact encoder, in which the readhead does not contact the detectable member in operation, which ensures that friction between the readhead and detectable member does not alter the measurement accuracy. In a first preferred embodiment, the linear encoder is a magnetic linear encoder, and the detectable member is a magnetic detectable member. In this embodiment, the magnetic detectable member is a magnetic encoder code strip comprising a plurality of alternating magnetic poles. The magnetic linear encoder readhead is configured to sense changes in magnetic field as the magnetic encoder code strip moves linearly past the readhead. Suitable magnetic linear encoders are commercially available for use in other fields, for example the LM10 linear magnetic encoder sold by Renishaw (TM), or the RLB Miniature Incremental Magnetic Encoder sold by RLS (TM). Magnetic linear encoders based on the anisotropic magneto resistive (AMR) effect may also be used in preferred embodiments, for example the Bogen Magnetics IKP11. In a second preferred embodiment, the linear encoder is an optical linear encoder, and the detectable member is a distance scale marked at regular intervals. Suitable optical linear encoders are commercially available for use in other fields, for example ATOM, QUANTiC, TONiC and RESOLUTE optical encoder readheads available from Renishaw (TM), and corresponding optical scales such as RTLF-S, RTLC-S and RTLA-S available from Renishaw (TM). A linear optical encoder may comprise an interpolator chip, an optical encoder chip, and an optical scale. The interpolator chip acts as a digitiser which digitises sin / cosine signals which it receives from the optical encoder chip as it moves along the optical scale. By selecting an interpolator chip with a suitably high bit resolution, and an optical scale with a suitably high resolution, small flexure deflections may be measured to a high degree of accuracy. As the deflections experienced in bond shear testing are very small (typically in the range of 0.090-0.100 mm), it is challenging to measure this displacement accurately. The present inventors have found, however, that by using a linear optical encoder with an interpolator chip having a resolution of at least 18-bit, or 24-bit, or 26-bit, the measurement resolution of the encoder is high enough to measure shear test forces to a high degree of accuracy. Calibrating Different Bond Tests The calibration device may be used to determine test forces for a number of different bond test types, in which different types of test tool are used, and different test forces are applied. For example the calibration device may be configured to calibrate a pull force transducer which is used for pull testing, or the calibration device may be configured to calibrate a push force transducer which is used for push testing, or the device may be configured to calibrate a shear force transducer which is used for shear testing. The arrangement of the flexure may vary depending on the type of bond test that the calibration device is configured to calibrate, but the principles of operation of the calibration device are the same regardless of the type of bond test transducer to be calibrated. The shape and form of the calibration contact may also vary depending on the type of bond test that the calibration device is configured to calibrate. A variety of calibration contacts may be used, depending on the type of bond testing to be calibrated, and the type of test tools in use on the bond test apparatus. For example the calibration contact may be provided as a metal ball, or a partial ball, which is shaped like a solder bump. Providing a calibration contact shaped like the solder bumps to be tested may advantageously mean that test forces applied between test tools and the calibration contact during calibration reflect the forces applied to solder bumps during testing. Alternatively the calibration contact may be provided as a bar or wire, so that test forces applied between test tools and the calibration contact during calibration reflect the forces applied to wire connections during testing. In one preferred embodiment, the calibration device is a pull test calibration device. In this embodiment, the calibration contact is a pull test calibration contact configured to cooperate with a pull test tool so that in use, a pull test force is applied to the calibration contact by the pull test tool. In the pull test calibration device, the flexure may be oriented to deflect in a vertical direction in response to the application of a vertical test force to the calibration contact. The pull test calibration contact may comprise a crossbar which is engageable by a hooked-tip of a pull test tool, or a recess which is engageable by a hooked-tip of a pull test tool, or a rounded ball which is clampable by jaws of a pull test tool. In another preferred embodiment, the calibration device is a push test calibration device. In this embodiment, the calibration contact is a push test calibration contact configured to cooperate with a push test tool so that in use, a push test force is applied to the calibration contact by the push test tool. In the push test calibration device, the flexure may be oriented to deflect in a vertical direction in response to the application of a vertical test force to the calibration contact. The push test calibration contact may comprise a flat pad, or a partial ball, which may be pushed against by a push test tool. In yet another preferred embodiment, the calibration device is a shear test calibration device. In this embodiment, the calibration contact is a shear test calibration contact configured to cooperate with a shear test tool so that in use, a shear test force is applied to the calibration contact by the shear test tool. In the pull test calibration device, the flexure may be oriented to deflect in a horizontal direction in response to the application of a lateral test force to the calibration contact. The shear test calibration contact may comprise a flat pad, or a ball, or a partial ball, against which a shear tool can apply a lateral shearing force. The calibration device may preferably be mountable to an XY table of a bond test apparatus. By mounting the calibration device to a movable XY table, the calibration process may be partially or fully automated, as the XY table may be controlled to move the calibration device to a position underneath the test tool when calibration is required, and to move the calibration device out of the way after calibration is complete. Calibration Apparatus A particular advantage of the calibration device of the present disclosure is that instead of manually swapping weights on a calibration jig, each calibration device may be set up to calibrate a specific bond test transducer without requiring manual intervention. A separate calibration device may be provided for each bond test transducer that is to be calibrated, such that each calibration device is configured to calibrate both the type of test, and the correct load range, for its associated bond test transducer. In a second aspect of the present disclosure, there is provided a calibration apparatus for a bond test apparatus. The calibration apparatus preferably comprises a plurality of calibration devices according to the first aspect. In the calibration device, the calibration apparatus may comprise a power and data connection configured to transmit power to each of the calibration devices, and to receive output signal data from each of the calibration devices. In the calibration apparatus, multiple calibration devices may, for example, be provided in a shared housing, with separate calibration contacts that are usable to operate the separate calibration devices. In a particularly preferred embodiment, the calibration apparatus is a modular calibration apparatus. In this embodiment, the calibration apparatus is provided as a modular system, in which a plurality of calibration devices are connected together. Each calibration device according to the first aspect described above may act as a module of the modular calibration apparatus. Once multiple calibration device modules are connected together to form a modular calibration apparatus, the functionality of the modular apparatus is preferably the same as a calibration apparatus in which the same calibration devices are integrated into a shared housing. Providing the system in a modular fashion may advantageously allow a user the flexibility to create a bespoke calibration apparatus which contains only the calibration devices which that user needs to calibrate the transducers which they use in their bond test apparatus. Where a calibration system is required for a bond test apparatus that has three bond test transducers, for example, a calibration apparatus which combines three calibration devices may be provided. For example, the three bond test transducers of the bond test apparatus may be: a pull test transducer with a specific load rating; a push test transducer with a specific load rating; and a shear test transducer with a specific load rating. In order to calibrate this bond test apparatus, a calibration apparatus may be provided in which a first calibration device is a pull test calibration device, a second calibration device is a push test calibration device, and a third calibration device is a shear test calibration device. It is important that the flexures of the calibration devices have stiffnesses that correspond to the load ratings of the transducers to be calibrated. During the calibration process, each transducer will apply a range of test forces to the associated calibration device, so that the transducer is calibrated across its working load range. The flexures of the respective calibration devices must be such that the applied test forces displace the flexure over a suitable range of travel within the apparatus, and do not displace the flexures beyond their elastic limits. In order to calibrate a system in which separate bond test transducers have different load ratings, but the same type of test tool, the calibration apparatus may contain multiple calibration modules of the same type (push / pull / shear), but with the calibration devices having different load ratings corresponding to the transducers to be calibrated. The calibration apparatus may for example comprise a plurality of pull test calibration devices, in which each of the pull test calibration devices contains a flexure having a different stiffness corresponding to a different load rating. The calibration apparatus may for example comprise a plurality of push test calibration devices, in which each of the push test calibration devices contains a flexure having a different stiffness corresponding to a different load rating. The calibration apparatus may for example comprise a plurality of shear test calibration devices, in which each of the shear test calibration devices contains a flexure having a different stiffness corresponding to a different load rating. The calibration apparatus may contain a mixture of push, pull and shear calibration devices. The calibration apparatus may for example include at least two of a push test calibration device, a pull test calibration device, and a shear test calibration device. In a particularly preferred embodiment the calibration apparatus may include a push test calibration device, a pull test calibration device, and a shear test calibration device. Bond test Apparatus According to a third aspect of the present disclosure there is provided a bond test apparatus comprising: a calibration device according to the first aspect of the disclosure, and a test tool; in which the test tool is controllable to contact the calibration contact, in use, and to apply a test force between the test tool and the calibration contact which deflects the flexure of the calibration device, so that the displacement sensor of the calibration device measures the resulting displacement of the flexure. The calibration device may be provided as a device that is connected to the bond test apparatus but moveable relative to the test tool. For example, the calibration device may be mounted on a moveable XY stage of the bond test apparatus. The combination of the bond test apparatus and the calibration device enables fully automated calibration of the bond test apparatus, as described above in relation to the first aspect of the disclosure. The bond test apparatus may comprise a force transducer, also known as a load cell or simply a transducer, which is configured to apply the test force to the test tool. A wide variety of bond tester force transducers are known in the art, and may be calibrated using the aspects of the present disclosure. In a preferred embodiment, the test tool and force transducer may be provided in a cartridge which is removable attachable to the bond test apparatus. In a particularly preferred embodiment, the bond test apparatus may comprise a plurality of force transducers, which may be provided in a plurality of exchangeable cartridges. The bond test apparatus may comprise a processor which is configured to receive the output signal from the displacement sensor. The processor may be programmed to convert the output signal from the calibration device into a measurement of the magnitude of the test force between the test tool and the calibration contact. As the stiffness of the flexure in the calibration device is known, this may be calculated using Hooke’s law according to conventional methods. The processor may be configured to compare the measured test force with stored calibration values. The processor may be configured to compare the measured test force with the target test force which the bond test apparatus is aiming to apply to the calibration contact. The processor may be configured to adjust control parameters of the bond test apparatus in response to a difference between the actual test force measured by the calibration device, and the target test force. The bond test apparatus may comprise a plurality of calibration devices. In a preferred embodiment, the bond test apparatus may comprise a calibration apparatus according to the second aspect of the disclosure. In a particularly preferred embodiment, the bond test apparatus may comprise a modular calibration apparatus in which each calibration device is one of a plurality of calibration device modules in the modular calibration apparatus. In embodiments comprising a plurality of calibration devices, for example as modules of a calibration apparatus, the test tool is preferably controllable to contact any one of the calibration devices. The bond test apparatus may thus be controlled to perform the calibration process with the specific calibration device that corresponds to the force transducer and test tool on the bond test apparatus at that time. As described above in relation to the second aspect, each of the calibration devices of the calibration apparatus may be configured to calibrate test forces within a predetermined calibration force range. The calibration force range may correspond to the stiffness and maximum displacement of the flexure in said calibration device. The calibration apparatus preferably comprises a plurality of calibration devices which are configured to calibrate test forces within different calibration force ranges. The calibration apparatus preferably comprises a plurality of calibration devices which are configured to calibrate different types of test force, for example some or all of push, pull or shear test forces. The bond test apparatus may be connectable to a plurality of bond test cartridges, each of the bond test cartridges containing a test tool and a force transducer (load cell). Each bond test cartridge is configured to perform one or more of a push, pull, or shear bond test. The force transducer of each bond test cartridge is preferably configured to apply test forces within a predetermined transducer force range. in a preferred embodiment, the bond test apparatus comprises a calibration device for calibrating each bond test cartridge, and the respective calibration force ranges of the calibration devices correspond to the transducer force ranges of the bond test cartridges. Method of Calibration According to a fourth aspect of the present disclosure there is provided a method of calibrating a bond test apparatus using a calibration device, for example using a calibration device according to the first aspect described above. The method may be a method of calibrating the bond test apparatus of the third aspect of the disclosure. The method of calibration may comprise the steps of: bringing the test tool of the bond test apparatus into contact with the calibration contact of the calibration device; applying a test force between the test tool and the calibration contact, so that the test force displaces the free end of the flexure relative to the fixed end of the flexure, measuring the resulting displacement of the flexure using the displacement sensor; providing an output signal from the displacement sensor to a processor; converting the output signal into a measurement of the magnitude of the test force between the test tool and the calibration contact. The method may comprise the step of comparing the measured test force with a calibration value to determine a calibration characteristic of the bond test apparatus. The method may comprise the first step of instructing the bond test apparatus to apply a target test force to the calibration contact. In a perfectly calibrated bond test apparatus, the test force applied between the test tool and the calibration contact will exactly match the target test force that the apparatus is instructed to apply. In an imperfectly calibrated bond test apparatus, the target test force will not match the magnitude of the actual test force that is applied between the test tool and the calibration contact. The actual test force experienced by the calibration contact is the same as the measured test force that is measured by the calibration device and found by the processor. The step of comparing the measured test force with a calibration value to determine a calibration characteristic of the bond test apparatus may involve calculating a difference between the measured test force and the target test force. The method may comprise the additional step of adjusting a control parameter of the bond test apparatus to compensate for the difference between the measured test force and the target test force. The method may comprise the step of adjusting a control parameter of the bond test apparatus so that the measured test force is equal to the target test force. Features described with reference to one aspect of the invention may equally be applied to other aspects of the invention. In particular, it should be clear that features described in relation to the first aspect of the invention may be applied to the second and third aspects of the disclosure, and features described in relation to the fourth aspect of the disclosure may be applied to any other aspect. Brief Description of the Drawings Embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1A is a side view of a shear test calibration jig forming part of the prior art; Figure 1B is a side view of a push / pull / shear test calibration jig forming part of the prior art; Figure 2 is a perspective view of an exemplary three-module modular calibration apparatus according to an aspect of the present disclosure; Figure 3 is a schematic illustration of the modular arrangement of a calibration apparatus according to an aspect of the present disclosure; Figure 4A is a front view of a three-module calibration device according to an aspect of the present disclosure; Figure 4B is a right-end view of the modular calibration device of Figure 4A; Figure 4C is a top view of the modular calibration device of Figure 4A; Figure 5A is a perspective view of a three-module calibration device according to an embodiment of the present disclosure, mounted on the XY stage hole matrix of a bond test apparatus, with a prior art shear calibration jig mounted on the calibration boss of the stage; Figure 5B is a perspective view of a three-module calibration device according to an embodiment of the present disclosure, mounted on the XY stage hole matrix of a bond test apparatus, with an additional shear calibration module mounted on the calibration boss of the stage; Figure 6A is a side view of a shear calibration module according to an embodiment of the present disclosure, with the housing sidewall removed; Figure 6B is an isometric sectional view of the shear calibration module shown in Figure 6A, with the cross-section taken through the centre of the module; Figure 6C is a side-on sectional view of the shear calibration module shown in Figure 6A and 6B; Figure 7A is a side view of a push calibration module according to an embodiment of the present disclosure, with the housing sidewall removed; Figure 7B is an isometric sectional view of the push calibration module shown in Figure 7A, with the cross-section taken through the centre of the module; Figure 7C is a side-on sectional view of the push calibration module shown in Figure 7A and 7B; Figure 8A is a side view of a pull calibration module according to an embodiment of the present disclosure, with the housing sidewall removed; Figure 8B is an isometric sectional view of the pull calibration module shown in Figure 8A and 8B; and Figure 8C is a side-on sectional view of the pull calibration module shown in Figure 8A. Detailed Description As described above, conventional methods for calibrating bond test load transducers involve applying a known load to the transducer using calibration weights, the masses of which are precisely known. To calibrate a pull transducer, a calibration weight is placed on a surface, the hook-shaped pull test tool is connected to the weight, and the bond tester then lifts the weight from the surface. The load sensed by the bond tester (for example the load measured by a strain gauge on the flexure of the bond tester transducer) is measured and compared to the known weight of the calibration weight. Only when the load to be calibrated exceeds a threshold value such as 5 kg or 10 kg would ratio jigs be used between the test tool and the calibration weights. Prior art calibration jigs for calibrating the load cell transducers of bond test apparatuses are shown in Figures 1A and 1B. When calibrating a shear transducer, a calibration jig such as the jig shown in Figure 1A is used to change the direction in which the load is applied to lift the calibration weight. Figure 1A shows a shear test calibration jig 100, in which a cantilever arm 110 is arranged to pivot on a leg 120. Removable calibration weights 130 are suspended from the cantilever arm 110. In use, the calibration weight 130 is placed on a surface, a shear test tool (not shown) mounted on a bond tester is urged laterally against a calibration contact point 140 on the jig, and the shear force between the tool and the calibration contact point 140 causes the cantilever arm to pivot so that the calibration weight is lifted off the surface. The shear calibration jig has an “L” shaped cantilever arm the pivot point, or fulcrum, in the corner of the “L”. The ratio of vertical to horizontal distances define the ratio between the weight of the calibration weight 130 and the calibration load at the calibration contact point 140. When the calibration weight 130 lifts off the surface, then the user knows that the bond test transducer is applying the calibration load for that calibration weight 130. The exact mass of the calibration weight 130 is known, so the actual shear force calibration load required to lift the calibration weight 130 off the surface is known. The shear test force which the bond test apparatus thinks is needed to lift the weight 130 to that predetermined height is monitored and the control system of the bond test apparatus is calibrated to correct the force readout to match the actual shear force being applied. The calibration weight 130 must then be removed and replaced with another calibration weight with a different mass, with the process being repeated multiple times with different calibration weights before the transducer is fully calibrated. In jigs designed to calibrate push test transducers, the cantilever arm effectively becomes a see saw where the fulcrum position defines the ratio between the weight of the calibration weight and the calibration load at the calibration contact point. When the weight lifts from the surface, then the user knows that the bond test transducer is applying the calibration load for that calibration weight. Figure 1B shows a push / pull / shear test calibration jig 150. Weights are suspended from a cantilever arm end 155. The structure and operation of the push / pull calibration jig 150 are similar to that of the shear calibration jig 100, except that the jig 150 is configured to pivot and lift the calibration weight (not shown) in response to either a push force applied vertically downwards onto a push calibration contact 160, or a pull force applied vertically upwards on a pull calibration contact 170, or a shear force applied horizontally against a shear calibration contact 180. Ratio jigs are typically used to keep the weights to a reasonable size, so that they fit the calibration tooling, and so users can handle the calibration weights safely. Figure 2 shows an exemplary modular calibration apparatus 200 according to an aspect of the present disclosure. The modular calibration apparatus 200 is made up of a interface module 210 and a row of three calibration device modules: a shear test calibration device 600; a push test calibration device 700; and a pull test calibration device 800. The interface module 210 provides power and data to the row of calibration modules 600, 700, 800. The interface module is preferably connected to an external processor such as the internal processor of a bond tester, or a processor in a computer, so that data may be exchanged between the processor and the individual calibration modules. Each of the calibration device modules has its own housing, with vertical housing sidewalls which allow adjacent modules to be stacked together in a continuous row. Corresponding power and data connectors 610, 710, 810 are positioned on the housing sidewalls, so that these power and data connectors connect adjacent modules together. This provides a power and data connection from the interface module 210 through the entire row of calibration modules. This continuous power and data connection allows separate calibration devices to be connected in a stacked arrangement as shown in Figure 3. The number and arrangement of the modules is flexible, so a user may combine however many calibration modules they need, so that the connected calibration modules and interface module act as a modular calibration device. The power and data connector of the end-most module may be covered, in use, by an end cap 220. In a preferred embodiment, each module will have a power and RS485 data connector that passes through the module from one side to the other, enabling modules to be stacked side by side and permitting daisy chaining of communications. The interface module is connectable to the side of the housing, providing a USB interface to a PC for example. Instead of an end-mounted interface module 210 as shown, the power and data connection may be provided through, for example, a base plate with connections to the underside of the calibration modules, or as a connector strip which contacts each individual module. The illustrated end-mounted interface module 210, however, is extremely compact and effective. Although shown and described as a modular system, the modular calibration apparatus 200 could alternatively be provided as a non-modular calibration device, for example by integrating all three calibration devices into a shared housing. The operation of such a non-modular calibration apparatus would be the same as the modular calibration apparatus 200. Providing each calibration device as a separate module in its own housing, however, gives a user the flexibility to combine the specific selection of calibration devices which are useful to calibrate their bond test apparatus and its transducers. Figures 4A-4C show further details of the three-module calibration apparatus 200. Although the Figures show a preferred embodiment in which the calibration apparatus contains three modules (one push, one pull, one shear), it is entirely possible to connect a different combination of calibration modules together. The apparatus need not include a module for each test type. The apparatus may contain multiple modules configured to calibrate the same type of test, for example, but those modules may contain flexures of different stiffnesses so that the modules can calibrate different transducers having different transducer force ranges. The modular nature of the calibration apparatus 200 advantageously allows the apparatus to be constructed from a combination of modules to match the load cell transducers which a given user needs to calibrate. The calibration apparatus 200 may for example contain a separate calibration module corresponding to each transducer which is to be calibrated, with the test tool type and transducer force range matching the configuration of the calibration module. Each of the calibration device modules 600, 700, 800 has its own calibration contact 620, 720, 820. The calibration contacts and the internal arrangements of the different modules are described in more detail below in relation to Figures 6A-8C. Inside each of the calibration device modules 600, 700, 800 there is a flexure arranged with a fixed end that is fixed to the housing of the module, and a free end that is connected to the calibration contact. A displacement sensor is arranged in each module, with the displacement sensor configured to measure the distance by which the free end of the flexure is displaced relative to the fixed end. In some preferred embodiments, an optical encoder scale is fixed relative to the free end of the flexure and displacement measurements are performed by a circuit comprising a readhead optical device connected to a digitizing integrated circuit. The output of the digitizing integrated circuit may be connected to a microprocessor containing any required firmware providing the necessary software feature set. In use during calibration, a test force is applied to a calibration contact 620, 720, 820, and this test force deflects the free end of the flexure in the calibration module. The distance by which the flexure is deflected (displaced) is proportional to the test force on the calibration contact. This displacement is measured by the displacement sensor in the calibration module. The displacement measured by the displacement sensor is communicated out of the calibration module through the data connection. This data is communicated to a processor which is pre-programmed with the stiffness of the flexure, and which converts the measured displacement into a measured test force using Hooke’s Law. In an alternative embodiment, the calibration modules could be provided with internal processors so that this calculation of measured test force is done inside each individual calibration module. As shown in Figures 4A-4C, the calibration modules have consistent housing dimensions, and the calibration contacts are positioned so that the loading point is consistent regardless of whether a particular module is a push, pull or shear calibration device. The calibration contacts of each module are positioned so that loads applied to the calibration contacts are loaded evenly onto the flexure through a central loading plane. This module construction advantageously simplifies the programming required for the modular calibration apparatus 200 to be used in a highly automated bond testing system, where the bond test apparatus is programmed to automatically carry out calibration processes based on known coordinates of the calibration contacts. Automated calibration processes can be implemented by installing the calibration apparatus 200 so that it is movable relative to the test tool of the bond test apparatus. The calibration apparatus 200 is preferably movable in a lateral plane relative to the test tool, so that the calibration apparatus can be moved automatically into position under the test tool when needed, and then automatically removed once calibration is complete. Figure 5A illustrates the three-module calibration device 200, mounted on the XY stage hole matrix 500 of a bond test apparatus (not shown). For comparison, a prior art shear calibration jig 100 is shown mounted in a conventional position on a calibration boss 510 of the stage 500. Mounting the calibration apparatus 200 on a movable XY stage provides a straightforward way of transferring the calibration apparatus 200 in and out of position when required. As the XY stage may be programmed to move automatically, this may advantageously allow the calibration apparatus 200 to be incorporated into an automated workflow. Figure 5B shows an alternative example in which the calibration apparatus 200 is mounted on the XY stage hole matrix 500 of a bond test apparatus, with an additional shear calibration module 600 mounted on the calibration boss 510 of the stage. Various adapters may be provided to enable the calibration device to be fitted to the XY stage in various locations. On some bond testers the hole matrix may be used, or alternatively the calibration device may be mounted to a vacuum plate. The calibration apparatus of the present disclosure can be adapted to use any existing mounting location on a variety of bond test apparatuses. A variety of bond tester designs and XY stages are available in the art, and the versions shown in Figures 5A and 5B are intended for illustration only. Shear Calibration Device Figures 6A-6C show a shear calibration module 600 according to a preferred embodiment of the present disclosure. The shear calibration module 600 is designed to calibrate shear transducers which apply a lateral shear force to test features such as solder bumps. The shear calibration module 600 has a rigid housing 630 that is releasably attachable to a surface such as an XY stage by base bolts 605. The housing has removable sidewalls, through which power and data connectors 610 protrude for connection with adjacent modules. Dowel slots (not shown) may be formed in the base of the housing to provide additional connection points and increased stability. A double flexure 640, also known as a load beam, is positioned inside the housing 630. A first end of the double flexure is connected to the housing 630 by flexure screws 645. The other end of the double flexure 640 is connected by a flexure screw 645 to a flexure tool 650 which is suspended inside the housing but free to move relative to the housing. A calibration contact 620 is mounted on one end of the flexure tool 650. The calibration contact 620 is shaped to mimic a round solder bump of the type that is testable by a bond test apparatus. The calibration contact is preferably formed from hardened material, for example a hardened alloy, and may be mounted to or partially embedded within the flexure tool. The use of a hardened ball shape for the calibration contact allows consistent positioning and point loading during calibration. The calibration contact protrudes out of an opening in the housing 630, so that the calibration contact can be contacted by a shear test tool. One part of a linear encoder displacement sensor is mounted to the other end of the suspended flexure tool 650. In the illustrated embodiment, an optical encoder scale 660 is mounted on the flexure tool 650, while a corresponding optical encoder readhead 665 is positioned on a printed circuit board (PCB) 670 that is fixed to the housing 630. In the illustrated embodiment, the PCB includes the encoder readhead interpolator IC and a microcontroller. Other types of displacement sensor may be used with the calibration devices of the present disclosure, but the present inventors have found that optical linear encoders are highly accurate, high resolution and low cost options for measuring small displacements. The shear calibration module 600 is used to calibrate the shear transducer of a bond test apparatus by bringing a shear test tool (not shown) into contact with the shear calibration contact 620. This may be done either my moving the calibration module 600 to a position adjacent the test tool, or moving the test tool to a position adjacent the calibration module. The bond test apparatus may be instructed to perform a shear calibration operation at a target shear test force. This may be done either by a user inputting a command, or as part of an automated calibration protocol. The shear transducer of the bond test apparatus will apply a test force to urge the shear test tool against the shear calibration contact 620. The transducer will apply the force that it expects is required to produce the target test force between the test tool and the calibration contact 620. In response to the shear test force applied at the calibration contact 620, the flexure 640 will deflect to the right as shown. The flexure tool 650 will therefore be displaced towards the right, causing the optical encoder scale 660 to be displaced relative to the optical encoder readhead. The optical encoder readhead 665 senses the displacement of the scale 660, and measures the distance of the deflection by counting the number of scale line pairs that pass the readhead. The readhead produces a digital output signal identifying the magnitude of the displacement between the encoder scale 660 and the readhead. This output signal is transmitted through the data connection 610 to a processor which uses the known stiffness of the flexure 640 to convert the measured displacement into a measured test force on the flexure 640. This measured test force indicates the magnitude of the actual shear force that has been applied between the shear test tool and the calibration contact 620. The processor can compare this measured test force with the target test force that the bond test apparatus was instructed to apply. If the measured test force is equal to the target test force, then the transducer is perfectly calibrated. If the measured test force is not equal to the target test force, then the processor may adjust a control parameter of the bond test apparatus to compensate for the difference between the measured test force and the target test force. The processor may adjust a control parameter of the bond test apparatus so that the measured test force becomes equal to the target test force. In order to check that the transducer is properly calibrated across its operating range, this calibration process may be carried out at multiple target test forces within the transducer force range. Unlike the systems of the prior art, however, there is no need for manual intervention in order to swap weights or adjust the calibration module between calibration measurements. Push Calibration Device Figures 7A-7C show a push calibration module 700 according to a preferred embodiment of the present disclosure. The push calibration module 700 is designed to calibrate push test transducers which apply a vertical force downwards to test features such as solder bumps. The push calibration module 700 has a rigid housing 730 that is releasably attachable to a surface such as an XY stage by base bolts 705. The housing has removable sidewalls, through which power and data connectors 710 protrude for connection with adjacent modules. A double flexure 740 is positioned inside the housing 730. A first end of the double flexure is connected to the housing 730 by flexure screws 745. The other end of the double flexure 740 is connected to a flexure tool 750 which is suspended inside the housing but free to move relative to the housing. As the push test calibration module 700 is intended to measure test forces applied in a vertical direction, the double flexure 740 is oriented horizontally, at right angles relative to the flexure arrangement in the shear module 600. A push calibration contact 720 is mounted on one end of the flexure tool 750. The push calibration contact 720 is shaped to mimic a round solder bump of the type that is testable by a bond test apparatus. The calibration contact protrudes out of an opening in the housing 730, so that the calibration contact can be contacted by a push test tool. In the same way as the shear module 600, an optical encoder scale 760 is mounted on the flexure tool 750, while a corresponding optical encoder readhead 765 is positioned on a printed circuit board 770 that is fixed to the housing 730. The push calibration module 700 is used to calibrate the push transducer of a bond test apparatus similarly to the operation of the shear calibration module 600 described above. The only difference between the operations of these modules is the direction in which the test forces are applied to the calibration contacts, and therefore the direction of flexure displacement. Pull Calibration Device Figures 8A-8C show a shear calibration module 800 according to a preferred embodiment of the present disclosure. The pull calibration module 800 is designed to calibrate pull test transducers which apply a vertical force upwards to test features such as wire bonds. The pull calibration module 800 has a rigid housing 830 that is releasably attachable to a surface such as an XY stage by base bolts 805. The housing has removable sidewalls, through which power and data connectors 810 protrude for connection with adjacent modules. A double flexure 840 is positioned inside the housing 830. A first end of the double flexure is connected to the housing 830 by flexure screws 845. The other end of the double flexure 840 is connected to a flexure tool 850 which is suspended inside the housing but free to move relative to the housing. As the pull test calibration module 800 is intended to measure test forces applied in a vertical direction, the double flexure 840 is oriented horizontally, in the same orientation as the flexure arrangement in the push module 700. A pull calibration contact 820 is mounted on, or formed from, one end of the flexure tool 850. A variety of shapes of pull calibration contact 820 are usable to suit a variety of different pull tests. In the illustrated embodiment, the pull calibration contact 820 projects upwards from the housing and contains an opening through which a hook-shaped pull test tool can engage the calibration contact. The calibration contact protrudes out of an opening in the housing 830, so that the calibration contact can be contacted by a pull test tool. In the same way as the shear module 600 and the push module 700, an optical encoder scale 860 is mounted on the flexure tool 850, while a corresponding optical encoder readhead is positioned on a printed circuit board 870 that is fixed to the housing 830. As mentioned above, alternative displacement sensors could be used in a similar way, but this linear encoder embodiment has been found to be particularly compact and accurate. The pull calibration module 800 is used to calibrate the pull transducer of a bond test apparatus similarly to the operation of the shear calibration module 600 and the push calibration module 700 described above. The only difference between the operations of these modules is the direction in which the test forces are applied to the calibration contacts, and therefore the direction of flexure displacement. Although Figures 6A-8C are illustrated as “modules” of a modular calibration apparatus, the calibration modules could instead be provided as standalone calibration devices with housings that do not fit together in modular fashion.

Claims

1. A calibration device for a bond test apparatus, the calibration device comprising:a calibration contact;a flexure having a fixed end and a free end, the free end of the flexure being coupled to the calibration contact; anda displacement sensor which is configured to measure a displacement of the free end of the flexure relative to the fixed end of the flexure and to produce an output signal indicating the magnitude of the displacement;in which the calibration contact is contactable with a test tool of the bond test apparatus so that, in use, application of a test force between the test tool and the calibration contact displaces the free end of the flexure relative to the fixed end of the flexure, such that the displacement sensor measures the resulting displacement of the flexure and produces an output signal indicating the magnitude of the displacement caused by the test force.

2. A calibration device according to claim 1, in which the calibration device comprises a data connection which is configured to deliver the output signal to a processor external to the calibration device.

3. A calibration device according to claim 1 or 2, in which the displacement sensor is a linear encoder, and in which the linear encoder produces a digital output signal.

4. A calibration device according to claim 3, in which the displacement sensor comprises an encoder readhead and a detectable member which is detectable by the encoder readhead, in which one of the detectable member or the encoder readhead is coupled to the free end of the flexure, and the other of the detectable member or the encoder readhead is fixed relative to the fixed end of the flexure.

5. A calibration device according to claim 3 or 4, in which the linear encoder is a magnetic linear encoder; and in which the detectable member is a magnetic detectable member.

6. A calibration device according to claim 5, in which the magnetic detectable member is a magnetic encoder code strip comprising a plurality of alternating magneticpoles.

7. A calibration device according to claim 3 or 4, in which the linear encoder is an optical linear encoder; and in which the detectable member is a distance scale marked at regular intervals.

8. A calibration device according to any preceding claim, in which the calibration device comprises a housing which contains the flexure and the displacement sensor, in which the fixed end of the flexure is fixed relative to the housing, and in which the calibration contact is external to the housing, or exposed within the housing, so that the calibration contact is contactable by the test tool of the bond test apparatus.

9. A calibration device according to any preceding claim, in which the calibration device is a pull test calibration device, and in which the calibration contact is a pull test calibration contact configured to cooperate with a pull test tool so that in use, a pull test force is applied to the calibration contact by the pull test tool.

10. A calibration device according to claim 9, in which the pull test calibration contact comprises a crossbar which is engageable by a hooked-tip of a pull test tool, or a recess which is engageable by a hooked-tip of a pull test tool, or a rounded ball which is clampable by jaws of a pull test tool.

11. A calibration device according to any of claims 1 to 8, in which the calibration device is a push test calibration device, and in which the calibration contact is a push test calibration contact configured to cooperate with a push test tool so that in use, a push test force is applied to the calibration contact by the push test tool.

12. A calibration device according to any of claims 1 to 8, in which the calibration device is a shear test calibration device, and in which the calibration contact is a shear test calibration contact configured to cooperate with a shear test tool so that in use, a shear test force is applied to the calibration contact by the shear test tool.

13. A calibration device according to any preceding claim, in which the calibration device is mountable to an XY table of the bond test apparatus.

14. A calibration apparatus for a bond test apparatus, the calibration apparatus comprising a plurality of calibration devices according to any preceding claim; in which the calibration apparatus comprises a power and data connection configured to transmit power to each of the calibration devices, and to receive output signal data from each of the calibration devices.

15. A calibration apparatus according to claim 14, in which the calibration apparatus is a modular calibration apparatus, in which the plurality of calibration devices are connectable in a modular fashion.

16. A calibration apparatus according to claim 14 or 15, in which the calibration apparatus comprises a plurality of pull test calibration devices, in which each of the pull test calibration devices contains a flexure having a different stiffness corresponding to a different load rating;and / or in which the calibration apparatus comprises a plurality of push test calibration devices, in which each of the push test calibration devices contains a flexure having a different stiffness corresponding to a different load rating;and / or in which the calibration apparatus comprises a plurality of shear test calibration devices, in which each of the shear test calibration devices contains a flexure having a different stiffness corresponding to a different load rating.

17. A calibration apparatus according to claim 14, 15 or 16, in which each of the calibration devices of the calibration apparatus is configured to calibrate test forces within a predetermined calibration force range, the calibration force range corresponding to the stiffness and maximum displacement of the flexure in said calibration device.

18. A calibration apparatus according to any of claims 14 to 17, in which the calibration apparatus comprises a plurality of calibration devices which are configured to calibrate test forces within different calibration force ranges.

19. A calibration apparatus according to any of claims 14 to 18, in which the calibration apparatus includes at least two of a push test calibration device, a pull test calibration device, and a shear test calibration device.

20. A bond test apparatus comprising:a calibration device according to any of claims 1 to 13; anda test tool;in which the test tool is controllable to contact the calibration contact, in use, and to apply a test force between the test tool and the calibration contact which deflects the flexure of the calibration device, so that the displacement sensor of the calibration device measures the resulting displacement of the flexure.

21. A bond test apparatus according to claim 20, in which the bond test apparatus comprises a processor configured to receive the output signal from the displacement sensor, and in which the processor is programmed to convert the output signal into a measurement of the magnitude of the test force between the test tool and the calibration contact.

22. A bond test apparatus according to claim 20 or 21, in which the processor is configured to compare the measured test force with stored calibration values.

23. A bond test apparatus according to claim 20, 21 or 22, in which the bond test apparatus comprises a calibration apparatus according to any of claims 14 to 19, and in which the calibration device is one of a plurality of calibration devices of the calibration apparatus.

24. A bond test apparatus according to claim 23, in which the test tool is controllable to contact any one of the calibration devices of the calibration apparatus.

25. A bond test apparatus according to any of claims 20 to 24, in which the bond test apparatus is connectable to a plurality of bond test cartridges, each of the bond test cartridges containing a test tool and a force transducer, in which each bond test cartridge is configured to perform one or more of a push, pull, or shear bond test, and in which the force transducer of each bond test cartridge is configured to apply a force within a predetermined cartridge force range.

26. A bond test apparatus according to claim 25, in which the bond test apparatus comprises a calibration device for calibrating each bond test cartridge, the respective calibration force ranges of the calibration devices corresponding to thecartridge force ranges of the bond test cartridges.

27. A method of calibrating a bond test apparatus using a calibration device according to any of claims 1 to 13, comprising the steps of:5 bringing a test tool of the bond test apparatus into contact with the calibrationcontact of the calibration device;applying a test force between the test tool and the calibration contact, so that the test force displaces the free end of the flexure relative to the fixed end of the flexure,10 measuring the resulting displacement of the flexure using the displacement sensor;providing an output signal from the displacement sensor to a processor; converting the output signal into a measurement of the magnitude of the test force between the test tool and the calibration contact; and comparing the measured test force with a calibration value to determine a calibration characteristic of the bond test apparatus.15s

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