FIELD OF THE INVENTION

[0001 ] The present invention pertains to the field of automation technology.

More specifically, it relates to an electronic component placement machine with an air bearing axis for its pick and/or place system.

BACKGROUND OF THE ART

[0002] State of the art electronic placement machines comprise a plurality of linear motion axes, each having one single degree of freedom of motion. In order to allow for more complex movements, axes are often stacked, sometimes in combination with rotary motion axes.

[0003] One important field of electronic component placing is die bonding, that is, the bonding of semiconductor dies or chips onto a substrate, where the substrate may also be (or include) a semiconductor wafer, chip or die. An ever increasing complexity of bonded structures, especially for stacked die or flip-chip configurations, leads to a permanent demand for faster and more accurate machines. At the same time, to achieve desirable levels of bonding quality the chips tend to be pressed onto the substrate with ever increasing force to compensate for increasing chip size and/or to allow the use of novel bonding agents. For accuracy and bonding force, machine parts tend to be mechanically rigid and stable; however, this undesirably tends to lead to increased weight. To increase the number of components per time unit that can be handled, the maximum motion speed of machine parts is often increased. However, this involves higher accelerations and thus generally results in increased heating due to increased dissipation of energy within the machine. Heating, on the other hand, is generally detrimental to accuracy, as it results in deformation of machine parts.

[0004] In certain electric component placement machines, cylindrical air bearings have been employed, in particular for motion axes along which machine parts travel a relatively long distance (which is typically between a first point above a component supply and a second point above a placement location). EP 1 1 43489 A1 , which is hereby incorporated by reference in its entirety, discloses a linear guide with a cylindrical air bearing for a die bonder. Due to a comparative lack of stiffness and load capacity, conventional air bearing axes based on cylindrical air bearings as described in EP 1 1 43489 A1 may face problems when being scaled up to carry heavier, more complex, pick and place heads or when higher stiffness or peak force capabilities are desired.

[0005] In addition, conventional cylindrical air bearings may have issues with an air bearing gap between a moveable carriage and a stationary bar being affected by the temperature of the carriage and the stationary bar. EP 1 143 489 A1 seeks to overcome this by controlling a thermal state of the carriage and the stationary bar with active heating, to preserve a uniform air bearing gap. This solution may not solve the problem sufficiently well under all circumstances, and it consumes power to drive the heaters, and it may be subject to practical limitations of how much heat can be applied to the structure and how quickly and uniformly the heat can be controlled. Active heating of parts may also limit their ability to serve as a heat sink for a motor that drives the carriage. A corresponding air bearing system may be sensitive to any curvature induced on the stationary bar or on the carriage. Only a few microns of bending in either part due to non-uniform heating (such as may be experienced when a linear motor coil unit is carried on the carriage) can significantly increase the chance that air bearing surfaces make contact with each other, leading to premature failure.

SUMMARY OF THE INVENTION

[0006] According to an exemplary embodiment of the present invention, an electric component placement apparatus, in particular a die bonder, is provided. The apparatus includes a carriage configured to travel in at least one travel direction along a linear motion axis. The carriage includes a plurality of air bearing pads and a preloading mechanism for each of the air bearing pads. The apparatus also includes a guide structure defining a plurality of guiding surfaces, each guiding surface being configured for engagement with at least one of the air bearing pads. [0007] According to another exemplary embodiment of the present invention, an electric component placement apparatus, in particular a die bonder, is provided. The apparatus includes at least three air bearing pads, where each air bearing pad is guided by an associated guiding surface of the apparatus that allows for movement with respect to at least two degrees of freedom. The three pads are connected by a carriage to allow for movement of the carriage along one common degree of freedom. A preloading mechanism is provided for each of the air bearing pads.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:

FIG. 1 is a block diagram representation of a pick and place motion axis of a basic die bonder;

FIG. 2 is a perspective view of an apparatus in accordance with an exemplary embodiment of the present invention;

FIG. 3 is another perspective view of the apparatus shown in FIG. 2, with some parts removed for better visibility; and

FIG. 4 is a cross sectional block diagram of a pivotal mount of an air bearing pad and associated components in accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0009] FIG. 1 is a block diagram representation of a pick and place motion axis of a basic die bonder. Semiconductor dies 90 are picked at a pick position from a wafer 91 or the like using a pick and place head 99. The dies are then lifted above the wafer and transferred to a place position where they are placed and fixed onto a substrate 95. Due to dimensions of the wafer, substrate and associated transport mechanisms, a distance between pick and place position cannot be decreased below a certain value that may, for example, correspond roughly to a wafer diameter.

[0010] FIG. 2 is a perspective view of an exemplary linear motion axis for picking and placing of dies in a die bonder, the linear motion axis featuring five air bearing pads 1 1 , 12, 13 (not visible: 14, 15) with magnetic preload. The air bearing pads are mounted on a carriage 2 that can move in a y-direction along a guide structure 3. The guide structure 3 includes three guiding surfaces 31 , 32, 33. Each air bearing pad 1 1 , 12, 13, 14, 15 is associated with one of the guiding surfaces 31 , 32, 33.

[001 1 ] The carriage 2 illustrated in FIG. 2 is configured to carry a pick and place head (not shown in FIG. 2, but similar to the head shown in FIG. 1 ). As is understood by those skilled in the art, the air bearing pads 11 , 12, 13, 14, and 15 (in conjunction with a preload mechanism such as preload magnets) provide for motion of the carriage 2 along the y-axis labelled in FIG. 2. The preload force provided by the preload mechanism is intended to counterbalance a repulsive force created by a positive air pressure between a planar active surface of each air bearing pad and each guiding surface of guide structure 3 (see FIG. 4 for exemplary illustration of arrangement), thereby providing stiffness in both positive and negative directions normal to the planar active surface of each air bearing pad.

[001 2] FIG. 3 shows another perspective view of the linear motion axis of FIG.

2. The carriage 2 is not shown in this Figure to make fourth and fifth air bearing pads 14, 15 visible. Preloading magnets 41 , 42, 43, 44, 45 are also shown for each of the air bearing pads 1 1 , 12, 13, 14, 15. The preloading magnets 41 , 42, 43, 44, 45 provide for an attraction between the carriage 2 and the guide structure 3. When the air bearing pads 1 1 , 12, 13, 14, 15 are pressurized, a repulsive force between pads and associated guiding surfaces 31 , 32, 33 results. Attractive and repulsive forces give rise to a stable equilibrium state and thus a well defined air bearing gap. One particular advantage of magnetic preloading is that ensures a substantially constant preload force and therefore a substantially constant air bearing gap and a substantially constant stiffness. [0013] The air bearing pads in FIGS. 2 and 3 are arranged in a kinematic arrangement configured to allow motion of the carriage along one degree of freedom. As is understood by those skilled in the art, by constraining the carriage along 5 degrees of freedom (using the air bearing pads in conjunction with the preload) the carriage is kinematically arranged to move in the 6 ^th degree of freedom, that is, the y-axis. This ensures fundamental insensitivity to any deformation of guide structure 3 and/or of the carriage 2 itself. Axis straightness may change slightly, but there is virtually no increased chance of air bearing surfaces of the guiding surfaces 31 , 32, 33 and the air bearing pads 1 1 , 12, 13, 14, 15 coming into contact due to thermally-induced distortion.

[0014] Due to the use of air bearings, the design shown in FIG. 2 is advantageous for the pick and place axis of a die bonder because, compared to rolling element bearings, air bearings have lower friction, which allows better position accuracy at the end of a movement for a given servo bandwidth. Additionally, air bearings have an increased life compared to mechanical bearings. This increased life will become increasingly significant as distances travelled on die bonders increase, and as speeds increase on die bonders. With increased travel and increased speed, mechanical bearings will be limited by life, and will eventually need to be replaced, whereas air bearings have no tribological wear behaviour and therefore have an essentially infinite life.

[0015] For a given size, air bearing pads having planar active surfaces are substantially stiffer and have a higher load capacity than cylindrical air bearings. Thus, larger and more complex pick and place heads may be used or better performance may be achieved with smaller lightweight heads, particularly when high bonding forces are to be achieved.

[001 6] The combination of air bearing pads having planar active surfaces in a kinematic arrangement such as described herein has additional benefits compared to other air bearing arrangements, such as cylindrical bearings or other arrangements featuring opposed air bearing surfaces connected by a rigid structure. Such benefits include, for example, insensitivity of the air bearing characteristics to thermal growth and other small deformations of the structure. With any of the various sources of preload contemplated (e.g., magnetic, vacuum, opposed air pad mounted with a compliant structure, amongst others) the preload force remains substantially constant irrespective of even relatively large temperature increases, and is similarly unaffected by other sources of small deformation. As a result, the air bearing gap, and therefore stiffness and maximum load of the air bearings remain substantially constant. In contrast, in conventional arrangements, the air bearing gap may vary considerably (increase or decrease) as a result of temperature changes or deformation of the parts (e.g. , the carriage, the guide surfaces, etc.) . Such changes in the air bearing gap in conventional arrangements tend to have undesirable effects on the maximum load that can be carried, the stiffness of the system, amongst other problems. Thus, the present invention provides substantial benefits over these conventional arrangements.

[001 7] While the present invention is not so limited, it may be desirable that the air bearing pads 1 1 , 12, 13, 14, 15 be pivotally mounted to the carriage 2 as shown in FIG. 4. FIG. 4 illustrates air bearing pad 11 carried by a portion of carriage 2, where air bearing pad 11 is separated from guide surface 31 of guide structure 3 by a gap 50. Also shown is an exemplary air inlet supply 47 to air bearing 1 1 . A ball joint 61 is provided at one and of an adjustment screw 6. A corresponding ball socket 7 provided in the air bearing pads 1 1 , 12, 13, 14, 15 allows for rotation in all directions. Of course, pivotal structures other than that shown in FIG. 4 (and non- pivotal structures) are contemplated within the scope of the present invention.

[001 8] Although the preload mechanism for the air bearings has been illustrated and described in connection with preload magnets, it is not limited thereto. In another exemplary embodiment of the invention, preloading of one or more air bearing pads 1 1 , 12, 13, 14, 1 5 may also be achieved by using suction heads or the like connected to, integrated with or mounted in a vicinity of, the respective air bearing pad.

[001 9] In yet another exemplary embodiment of the invention, and as described above, preloading of one or more air bearing pads 1 1 , 12, 13, 14, 1 5 may be achieved by one or more preloading air bearing pads that are configured to counteract the repulsive force between pads and associated guiding surfaces 31 , 32, 33. For example, this may be achieved by providing a preloading guiding surface on the guide structure 3, where said preloading guiding surface is preferably parallel to one of the guiding surfaces 31 , 32, 33 but facing in an opposite direction. [0020] In yet another exemplary embodiment of the invention, one or more air bearing pads 1 1 , 12, 13, 14, 15 have non planar active surfaces. For example, it is understood that such planar surfaces of the air bearing pads described above may actually exhibit some curvature (e.g., spherical concavity) or other non-planarity. Further, the existence (or non-existence) of such non-planarity of the air bearing surfaces may depend on the particular time during operation of the air bearings (e.g., in a pressurized state, in an unpressurized state, etc.) Further, in certain exemplary embodiments, active surfaces might be concave so that they can be engaged by a cylindrical surface.

[0021 ] Although the invention is illustrated and described herein with reference to specific embod iments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.