WO2010020778A1

WO2010020778A1 - High speed shear test device and method

Info

Publication number: WO2010020778A1
Application number: PCT/GB2009/002026
Authority: WO
Inventors: John Melbourne Tingay; Robert John Sykes
Original assignee: Dage Precision Industries Ltd.
Priority date: 2008-08-19
Filing date: 2009-08-18
Publication date: 2010-02-25
Also published as: GB0815133D0

Abstract

A method of shear testing a ball deposit (19) adhered to a substrate (18) comprises the steps of mounting the substrate (18) on a support (11), relatively rotating the support (11) with respect to a shear tool (15) so as to bring the ball deposit (19) into contact with the shear tool (15), and detecting the shear force at the tool (15) which results from the shear tool (15) shearing the ball deposit (19) off the substrate (18). An apparatus for high speed shear testing of a ball deposit (19) adhered to a substrate (18) comprises a shear tool (15), a relatively rotatable support (11), means to secure a test substrate to the support (11), means to rotate the ball deposit (19) into contact with the shear tool (15) and means to detect the shear force of the tool (15) shearing the ball deposit (19) off the substrate (18).

Description

HIGH SPEED SHEAR TEST DEVICE AND METHOD

Field of the Invention

This invention relates to a test device for electrical bonds of semi-conductors, and particularly to a method and device suitable for high speed shear testing.

Background to the Invention

Electrical bonds of semi-conductor devices often comprise an array of solder or gold balls on a substrate having electrical pathways. These balls are used to connect individual wires to the pathways, or to connect to pathways on another substrate whilst the substrates are in aligned contact.

A solder ball is typically re-flowed and a gold ball is typically welded to a substrate. In order to confirm viability of a particular manufacturing technique, test devices are required to confirm that the balls have sufficient mechanical adhesion to the substrate. It is known to test in shear by driving a tool at the side of a ball, and in tension by gripping and pulling the ball orthogonally to the substrate.

The individual balls are generally arranged in an array, and are very small. A solder ball is typically in the range 1000-75 μm in diameter, whereas a gold ball is in the range 100-20 μm in diameter. These 'balls' or 'ball deposits' have generally hemispherical shapes when attached to a substrate. The very small size of some solder and gold balls means that the breaking forces are very low, and special measures are required to reduce the friction between testing tools and the substrate to a minimum so as to permit breaking force to be measured.

In the case of a shear test, the tool can be driven linearly against the respective ball from a distance, and thus the necessary impact speed can be generated. However, testing of the ball/substrate bond at a high shear strain is desirable. At low impact speeds many balls fail due to a break within the ball itself, rather than due to a break of the bond between ball and substrate. High speed shear testing of balls is desirable because at high speeds a greater proportion of bond failures occurs, rather than breaks within the balls. However, high speed shear testing is problematic because the linear path length required for accelerating a tool or substrate to the necessary speed and then decelerating it back to rest becomes unacceptably long. Summary of the Invention

The present invention is defined in the appended independent claims, to which reference should now be made. Preferred features of the invention are set out in the dependent claims.

In one aspect, the present invention provides a method of shear testing a ball deposit adhered to a substrate comprising the steps of mounting the substrate on a support, relatively rotating the support with respect to a shear tool so as to bring the ball deposit into contact with the shear tool, and detecting the shear force at the tool.

By using an arcuate shear test path, a relatively long acceleration path can be provided within a small space. For one complete revolution of a circular path, a path length more than three times longer than the equivalent linear path is provided.

To reach very high speeds, several relative rotations of the substrate may be used to accelerate the substrate to a desired speed before the shear test is conducted. In this case, the method further includes the step of moving the shear tool into the path of the ball deposit at the time of or just prior to performing the test.

In another aspect, the invention provides an apparatus for high speed shear testing of a ball deposit adhered to a substrate, the apparatus comprising a shear tool, a relatively rotatable support, means to secure a test substrate to the support, and means to detect shear force at said tool when the tool shears the ball deposit off the substrate.

In a preferred embodiment the support comprises a turntable. Preferably, the turntable is driven in a substantially friction free manner so as to minimize drag. A platen of the turntable may for example be provided with air bearings, and driven by an induction motor.

In a preferred embodiment the shear tool is moveable towards and away from the support so as to permit relative speed to increase through a number of successive revolutions of the turntable. This arrangement allows very high speed testing within a limited space, the tool being moved into the path of the ball when the desired relative speed is reached.

Preferably the shear tool is moveable radially with respect to the axis of rotation of the turntable, so as to align the shear tool in use with a succession of radially spaced balls. Brief Description of the Drawings

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic cross-section through an embodiment of the invention;

Figure 2 is a schematic plan view of an embodiment of the invention;

Figure 3 is a perspective view of an embodiment of the invention;

Figure 4a is a schematic cross section of an actuator mechanism used for moving a test tool in accordance with the invention;

Figure 4b is a perspective view of the mechanism shown in Figure 4a;

Figure 5 is a schematic illustration of the arrangement of control elements in an embodiment of the present invention; and

Figure 6 is a flow diagram illustrating the method steps of a method of shear testing in accordance with an embodiment of the present invention.

Detailed Description of Preferred Embodiments

An example on an apparatus in accordance with the invention is shown in cross section in Figure 1. The apparatus comprises a rotatable support in the form of a turntable 11 , rotatable about axis 12. The turntable 11 is driven by an armature of an electric motor 13 within a machine frame 14. A test substrate 18 is mounted on the turntable, and has an array of electrically conductive ball deposits 19 bonded to it. The substrate is positioned so that the ball deposits 19 are offset from the axis of rotation 12. This arrangement is shown in plan view in Figure 2.

The term "ball deposits" as used herein means any protruding deposits. "Ball deposits" are typically not ball shaped but substantially hemispherical when adhered to a substrate and can vary in size and shape depending on their composition and manufacturing technique.

The dimensions illustrated in Figure 1 are deliberately enlarged in order to show features of the invention clearly. A typical ball 19 has a diameter in the range 1000-75 μm, and the test tool is generally selected to be of similar width to the ball. The turntable is typically 300 mm in diameter.

A number of commercially available turntables are suitable for use in an apparatus of the type shown in Figure 1. In a preferred embodiment, an air bearing is used to support the turntable for substantially friction-free motion. Turntables of this type include the Revolution ST1 Spindle™ from Danaher Motion Dover, 200 Flanders Road, Westborough, MA 01581 , USA. Suitable air bearings are also available from Air Bearings Ltd of 1 Witney Road, Nuffield Industrial Estate, Poole, Dorset, BH17 OGH, UK. A very low friction bearing, such as an air bearing is preferred because it allows for more accurate shear force measurements to be taken. However, a cheaper, mechanical contact bearing could be used instead. Motor 13 is preferably an induction motor.

A shear tool 15 is shown in Figure 1 just above the turntable 11. The shear tool is of the type described in WO2007/093799, the contents of which are incorporated herein by reference, and includes a tip for contacting and applying a shear force to ball deposits 19 on the substrate 18. The shear tool 15 includes a piezoelectric crystal on a back face of the tip for measuring the shear force experienced by the tip as a ball deposit is sheared off the substrate. The shear tool 15 is reciprocable both in the direction of the turntable axis and in a radial or transverse direction with respect to the turntable axis, as indicated by arrows 16, 17.

An optical encoder 20 is shown in Figurei, positioned below the motor 13. The optical encoder is used to determine the angular position of the turntable. Optical angle encoders of this type are well known in the art and suitable products include the RESR™, RESM™ and RESA™ angle encoders available from Renishaw pic, New Mills, Wotton-under-Edge, Gloucestershire, GL12 8JR, United Kingdom.

Figure 2 is a schematic plan view of an embodiment of the invention and shows the shear tool 15 mounted to an arm or beam 21 extending over the turntable 11.

The substrate under test 18 can be secured in position on the turntable 11 by several different means. A vacuum chuck can be used to secure larger substrates or devices to the turntable. Alternatively, a simple vice assembly can be secured to the turntable and used to grip a substrate. Figure 2 illustrates a substrate 18 clamped to the turntable 11 by a pair of end plates 23 overlying the edges of the substrate, attached to the turntable using screws. Rotation of the turntable 11 and the attached substrate 18 about axis 12 provides a relative speed between ball deposits 19 and the shear tool 15. Preferably, the shear tool is stationary in the direction of rotation of the turntable 11 so that, at the point of impact between the shear tool 15 and a ball deposit 19, the relative speed between the shear tool 15 and the ball deposit 19 is solely the result of the rotation of the turntable 11. This allows the impact speed to be accurately controlled and determined by suitable control of the motor 13.

Figure 3 is a perspective view from above of an embodiment of the present invention of the type shown in Figures 1 and 2. A substrate under test 18 has an array of ball deposits 19 adhered to its upper surface. The substrate is mounted to the turntable 11 using a vacuum chuck or clamp (not shown). The turntable is driven by a motor coupled to its underside, as described with reference to Figure 1.

As shown in Figure 3, the shear tool 15 is mounted to a bracket 31 that is movable along the beam 21 in a substantially radial direction using a suitable drive, such as a servo motor. A particular mechanism for moving and positioning the shear tool is described in more detail with reference to Figures 4a and 4b.

The shear tool 15 is shown in a contacting position adjacent the top surface of the substrate. The shear tool 15 is mounted above the turntable on beam 21 which is supported in position, extending across the turntable 11. The shear tool can be moved both along the length of beam 21 and vertically towards and away from the substrate 18. Bracket 31 is movable relative to the beam 21 along the length of the beam. Cartridge 32 is movable vertically relative to bracket 31.

With this arrangement the tip of the shear tool 15 can be positioned at any radial position on the turntable 11 and can be positioned over a range of heights above the turntable 11. This allows substrates to be mounted at different positions and at different heights on the turntable 11 and allows ball deposits 19 in different positions to be tested. The movement of the shear tool 15 both vertically and horizontally can be controlled manually but is preferably carried out using suitable actuators, such as servo motors operating under software control.

Figure 4a is a schematic cross-section of a preferred system for moving the shear tool 15 towards and away from the turntable. Figure 4b is a partial perspective view similar to that shown in Figure 3. The system illustrated in Figure 4a comprises two vertical movement mechanisms. A first mechanism is used for initial vertical positioning of the tool 15 relative to the substrate 18 and allows for large but relatively slow movement of the tool 15. A second mechanism is used for small but relatively rapid vertical movement of the tool 15 while the turntable 11 is moving during a test procedure.

The first mechanism comprises a ball screw 40 and nut 41 , driven by a servo motor 42. The ball screw 40 is rigidly connected to the bracket 31. The bracket 31 is movable along the beam 21 to adjust the radial position of the shear tool using a similar mechanism, such as a servo motor 45. The nut 41 is coupled to the cartridge 32 which holds the shear tool 15 via the second mechanism. The servo motor 42 may not be operable to provide rapid enough vertical movement of the shear tool during a test procedure, but it allows for pre- positioning of the tool prior to a test.

The second mechanism is a solenoid actuated mechanism for rapid movement of the tool 15 between a contacting position and a non-contacting position, in which the tool tip is clear of the ball deposits 19 on the rotating substrate 18, during a test procedure. The solenoid 43 is shown fixed to nut 41. The shear tool 15 and cartridge 32 are mounted to a shaft 44 that is reciprocated by the action of the solenoid 43. The solenoid actuator mechanism may advantageously be binary, in other words it may allow for rapid movement of the shaft between only between two set positions based on a simply on/off signal. This rapid movement is required so that the tool 15 can drop into a contacting position and be raised to a non-contacting position while the substrate 18 is spinning, without undesirably contacting any ball deposits or other features on the substrate, as described below with reference to Figure 6.

It should be apparent that the mechanism described with reference to Figures 4a and 4b is just one possibility among many for achieving the required movement of the shear tool. For example, the rapid vertical actuation of the shear tool may be accomplished by a servo motor of a type that can operate fast enough. Such a servo motor may be used to achieve both the rapid and the slower vertical movements. Similarly, a solenoid may be used for both types of vertical movement and/or for the radial movement of the shear tool. Another alternative is the use of pneumatic actuators for either of the vertical movements and/or the radial movement.

Furthermore, the movement of the shear tool into a contact position can be in any direction, not only parallel to the axis of rotation. So, for example, the rapid actuation of the shear tool could be performed by a solenoid acting in a radial direction or in a direction having both radial and axial components.

At the contact position, the shear tool and support arrangement, as well as the overall structure of the machine, must provide sufficient stiffness between the turntable and tool tip to not introduce significant error to the test.

The illustrated embodiments rotate the substrate in a horizontal plane on a turntable rotating about a vertical axis. However, it is possible for the rotation to be performed in about a different axis and /or for the substrate to be held on a surface parallel to the axis of rotation rather than normal to it. It is also possible for the shear tool to rotate instead of, or as well as, the substrate support in order to generate the relative speed between the two.

Figure 5 is a schematic illustration showing the control components of an apparatus in accordance with the invention. All of the system components, i.e. sensors and actuators or motors, can be controlled from a personal computer (PC) 60 or other programmable processing device. The system includes a turntable motor 50 and rotary encoder 51 , as previously described with reference to Figure 1 , and a shear force sensor 52, radial and vertical actuators 53, 54 and a separate rapid vertical actuator 55, as previously described with reference to Figure 4. Shear force sensor 52 is coupled to the PC 60 via data capture electronics 59. The data capture electronics collate the shear force data from the shear tool together with data relating to the position and speed of the turntable and to the position of the shear tool. The turntable motor 50 and rotary encoder 51 are connected to the PC via a turntable controller 56, a dedicated piece of electronic hardware that monitors and maintains turntable speed under the control of the PC. The turntable controller 56 is directly connected to data capture electronics 59 and passes the turntable speed and angular position from the rotary encoder 51 to the data capture electronics 59 in real time. The fast vertical actuator 55 is also connected to the PC via dedicated control electronics 58 that operate to control the fast actuation of the shear tool based on the turntable speed and position data received from the turntable controller 56. The radial servo motor 53 i.e. the motor driving radial movement of the shear tool, and vertical servo motor 54, i.e. the motor driving vertical movement of the shear tool, are connected to a servo controller electronics unit 57, which is in turn connected to the PC.

It can be seen that the entire system can be configured and controlled from the PC but that the dedicated control electronics allow for real time, coordinated control of the moving elements and for collation of the data output streams from the shear tool, turntable and shear tool actuators, so that each of the data streams are temporally matched.

Figure 6 illustrates the steps in a shear test in accordance with an example of the invention. In order to conduct a shear test, the substrate to be tested is first mounted to the turntable at step 610. The test tool 15 is then positioned at the desired radial position using the radial axis servo motor at step 620. The radial positioning of the shear tool is typically done by eye. A microscope can be provided to allow an operator to clearly see the position of the tip or test head of the shear tool relative to a ball deposit to be tested. The position of the shear tool is altered by the servo motors (both radial and vertical), which can be controlled by a joystick or other suitable user interface connected to the servo controller 57. Once the desired position of the shear tool test head is established, the radial position can be locked in place. However, if multiple ball deposits are to be tested in a single test procedure, the desired contact position for each ball deposit can be recoded both for the shear tool and for the turntable. Then, during a test procedure the test tool can be automatically moved to the next test position in a sequence, as will be described below.

The vertical position of the shear tool is then set in step 630. The test tool is brought into light contact with the turntable 11 using the vertical servo motor, and when contact with the substrate is detected, is moved upwardly by the vertical servo motor a small distance to a predetermined test height. This procedure ensures that the test tool does not drag on the turntable, and thatsuccessive tests are comparable. Suitable systems for detecting contact of the tool tip with the substrate and then withdrawing the tool a predetermined distance are described in US6078387 and in WO2005/114722, the contents of which are incorporated by reference. As an alternative, the distance of the shear tool tip above the substrate can be controlled on the basis of a distance detected by a laser displacement sensor.

In step 640, the radial and vertical contacting position of the shear tool is set and recorded, in a memory in the PC for example, for each further ball deposit that is to be tested during the procedure.

At step 650, the test tool is withdrawn vertically using the fast vertical actuator, e.g. the solenoid, to a height at which the substrate can freely rotate with the turntable, and the turntable motor 13 is rotated. The turntable is accelerated until a desired speed is reached. It will be appreciated that the instantaneous speed of a ball 19 under the test tool is a function of angular velocity and radius, and the operator can select a suitable linear test speed at the tool, for example 20 m/s, using software running on the PC to control the turntable motor. The optical encoder can be used to verify the turntable speed.

Once the desired speed is reached, the shear tool is rapidly lowered into a contacting position at step 660 using the fast vertical actuator to intercept the path of relevant ball deposit, and a shear test is performed in step 670. Suitable strain gauging in the test tool permits the shear force to be detected, as described in WO2007/093799.

At step 680, the shear tool is withdrawn to a non-contacting position using the fast vertical actuator. The timing of the lowering and rising of the shear tool by the fast vertical actuator is controlled based on the angular position of the turntable as detected by the optical encoder.

Data capture electronics can record not only the shear force experienced by the shear tool over time but also the angular position of the turntable (from the rotary encoder) so that a force displacement curve can be generated. Alternatively, the maximum force detected by the shear tool during a ball deposit shear may be the only measurement taken. The user is able to configure the data capture electronics as desired.

As described above, in a preferred embodiment the system can store the contact position of the shear tool for a plurality of successive ball deposits. In this case, while the turntable is still rotating, the shear tool is moved to the next position and lowered to a contact position at the appropriate moment. This is indicated as step 690. The system controls can ensure that successive tests are carried out at the same impact speed.

Dependent on the arrangement of ball deposits on the substrate, it is possible for a series of ball deposits to be sheared in a single pass of the turntable, i.e. without the shear tool being withdrawn to a non-contacting position between shearing of successive ball deposits. In this case, the resulting force measurements may need to be analysed, typically using appropriate software, to resolve the data. Alternatively data relating to ball deposits other than the first deposit contacting the shear tool can be discarded.

In cases where the geometry or speed of the test do no permit the shear tool to be moved to the lower position, the sample may need preparation by removal of surrounding balls to provide access for the shear tool tip.

Once all of the shear tests have been completed the turntable is stopped at step 700 and the substrate can then be removed from the turntable. Although the preferred embodiment describes a high speed test device, it will be appreciated that the invention is also suitable for low speed testing without further modification. For certain low speed tests it may be sufficient for the turntable to move angularly through less than one full rotation, in which case the tool may not be required to move between contacting and non-contacting positions between tests.

Claims

1. A method of shear testing a ball deposit adhered to a substrate using a shear tool the method comprising the steps of:

mounting the substrate on a support,

rotating the support or the shear tool or both so as to bring the ball deposit into contact with the shear tool at a predetermined relative speed, and

detecting the force applied to the tool by the ball deposit as the tool shears the ball deposit off the substrate.

2. A method according to claim 1 , wherein the step of rotating comprises rotating the support about a fixed axis and wherein the step of mounting comprises mounting the substrate on the support so that the ball deposit is offset from the axis.

3. A method according to claim 2, wherein the tool is initially in a non-contacting position, and further comprising the step of moving the tool to a contacting position in a path of the ball deposit when a desired relative speed between the ball deposit and the tool has been reached.

4. A method according to claim 3, wherein the step of moving the tool to a contacting position is performed when the support or substrate is detected to be in a predetermined angular position.

5. A method according to claim 3 or 4, wherein the step of moving the tool to a contacting position comprises moving the tool parallel to the axis of rotation of the support.

6. A method according to any preceding claim, further comprising the step of positioning the shear tool in the contacting position subsequent to mounting the substrate but prior to rotating the support.

7. A method according to claim 6, further comprising the steps of:

recording a contacting position for each of a plurality of ball deposits prior to rotating the support; moving the tool into a recorded contacting position for a first ball deposit when a desired relative speed between the first ball deposit and the tool has been reached; and

subsequent to the step of detecting the force applied to the tool by the first ball deposit as the tool shears the first ball deposit off the substrate, moving the tool into a recorded contacting position for a second ball deposit when a desired relative speed between the second ball deposit and the tool has been reached.

8. An apparatus for high speed shear testing of a ball deposit adhered to a substrate, comprising:

a rotatable support,

securing means for securing a test substrate to the support,

a shear tool having a test head positionable proximate to the support, the shear tool including a sensor to detect a shear force applied to the test head; and

a rotary drive for rotating the support so that a ball deposit on a substrate mounted to the support can be brought into contact with the test head at a predetermined speed.

9. An apparatus according to claim 8, further comprising a controller connected to the rotary drive and to the shear tool.

10. An apparatus according to claim 9, further comprising a position sensor for detecting an angular position of the support connected to the controller.

11. An apparatus according any of claims 8 to 10, further comprising a first actuator, wherein the shear tool is coupled to the first actuator, and wherein the first actuator is configured to move the shear tool relative to the support.

12. An apparatus according to claim 11 , wherein the first actuator is configured to move the shear tool towards and away from the support substantially parallel to an axis of rotation of the support.

13. An apparatus according to claim 11 or 12, wherein the first actuator comprises a solenoid, a servo motor or a pneumatic drive.

14. An apparatus according to claim 11 or 12 or 13 when dependent on claim 9, wherein the controller is connected to the first actuator, and the controller is configured to coordinate the operation of the rotary drive and the first actuator.

15. An apparatus according to any one of claims 11 to 14, further comprising a fast actuator coupled to the shear tool, wherein the fast actuator is configured to move the shear tool towards and away from the support more rapidly than the first actuator.

16. An apparatus according to claim 15 when dependent on claim 9, wherein the controller is connected to the fast actuator, and the controller is configured to coordinate the operation of the rotary drive and the fast actuator.

17. An apparatus according to any of claims 8 to 16, further comprising a radial actuator to move the shear tool substantially radially with respect to an axis of rotation of the support.

18. An apparatus according to claim 17 when dependent on claim 9, wherein the controller is connected to the radial actuator, and is configured to coordinate the operation of the rotary drive and the radial actuator.

19. An apparatus according to claim 17 or 18, wherein the radial actuator comprises a solenoid, a servo motor or a pneumatic drive.

20. An apparatus according to any of claims 8 to 19, wherein the support comprises a turntable.

21. An apparatus according to claim 20, wherein the turntable is driven in a substantially friction free manner.

22. An apparatus according to claim 21 , wherein the turntable comprises an air bearing.

23. An apparatus according to any of claims 8 to 22, wherein the rotary drive is an induction motor.

24. An apparatus according to any of claims 8 to 23 wherein the securing means is a vacuum chuck, adjustable vice or one or more fixings.