Abstract:
A system and method are provided for enabling the production of semiconductors requiring very high precision thermo-compression bonding, the system comprising a thermo-compression bonding system having force and/or distance measuring sensors configured to sense thermal expansion of system components and a controller configured to counteract such expansion by exercising appropriate control over such system. This system and method may be used for both constant force profile applications as well as those requiring variable force during a bonding operation.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to semiconductor fabrication, and, more particularly, to a system and method for thermo-compression bonding of die to substrate. 
       BACKGROUND OF THE INVENTION 
       [0002]    Thermo-compression bonding is a common method of attaching a die to a substrate. A typical thermo-compression bonding system uses two opposing parallel planar surfaces, a die head and a substrate head, to apply heat and pressure to a die and substrate to be bonded. The side of the die to be bonded to the substrate will typically have many small, tear-drop shaped, “bumps” of solder, which are applied in a prior processing step, while the substrate will have “bump pads” applied to the side to be bonded to the die, which are also applied during a prior processing step. Just prior to bonding, the solder bumps and bump pads are carefully aligned. These bump pads engage with the solder bumps during a bonding operation to form an electrical and physical connection between the die and the substrate. Those skilled in the art will recognize that the use of copper pillars (or columns) and micro-bumps with solder caps, typically made of SnAg, although other alloys are possible, rather than pure solder bumps are also known, with the general concept remaining largely the same as with solder bumps. 
         [0003]    In a typical thermo-compression bonding operation, the die head is rapidly heated to between 300-450° C. while simultaneously applying compressive forces to the die and substrate package, causing the pre-applied solder bumps to flow and connect the die to the substrate. At the moment of deformation, there is typically intended to be a partial and controlled collapse of the solder bumps. 
         [0004]    After the solder bumps have connected the die to substrate, the die head is then cooled to freeze the connections in place. The substrate head also undergoes similar, albeit less extreme, thermal changes (the substrate head may reach temperatures of approximately 150° C. during a thermo-compression bonding operation, as higher temperatures could cause degradation of the substrate, which is generally made of organic materials). 
         [0005]    While these techniques have served the industry well for a number of years, existing thermo-compression bonding systems and methods are unable to achieve satisfactory bonding performance and yield when used for bonding the latest generations of dies to substrates. These new generations of dies and substrates utilize an increased number of connections, solder bumps or otherwise, between the die and substrate, typically without a corresponding increase in the size of the die or substrate. In order to allow dies of similar size to accommodate these additional connections, each connection has become smaller. These semiconductor packages are sometimes referred to as “high bump count” packages. 
         [0006]    The primary issue is that current systems and methods cannot reliably achieve the level of precision required to ensure that the individual solder bumps do not come into contact with adjacent solder bumps during a bonding operation. Keeping closely spaced solder bumps from contacting adjacent solder bumps is crucial to a thermo-compression bonding operation, as even a single poor connection out of many thousands may cause the entire package to fail to function. This failure is caused by the uncontrolled collapse of these bumps, and it may also cause issues with the later manufacturing process of under filling, where a resin is introduced between the die and substrate, since bridged adjacent solder bumps will limit the ability of the resin to flow between the die and substrate. 
         [0007]    This problem is, in part, due to the high temperatures involved in thermo-compression bonding as well as the necessity of rapidly heating and cooling the die and substrate contacting surfaces to facilitate rapid production of semiconductor packages. This rapid heating causes the die and substrate heads to expand. Since the system is not kept at a constant temperature, it does not reach an equilibrium state and thermal expansion and contraction occurs during each bonding operation. 
         [0008]    This thermal expansion of the die and substrate heads in thermo-compression bonding systems has been known for some time, but has been regarded as too insignificant to affect the thermo-compression bonding operation. The very latest dies, however, have reached a point where this thermal expansion does not allow for the precision necessary to complete the bonding operation. The expansion is causing the solder bumps to be placed under excessive force during the bonding operation, which results in their uncontrolled collapse upon melting, causing adjacent solder bumps to fuse to one another. 
         [0009]    With die feature size growing ever smaller and the latest high-bump count dies pushing the limits of current thermo-compression bonding technologies, systems and methods which can achieve higher precision bonding have become necessary. 
       SUMMARY OF THE INVENTION 
       [0010]    One embodiment of the present invention discloses a system for thermo-compression bonding of high-bump count semiconductors, the system having a die head comprising a planar surface and a substrate head comprising a planar surface, wherein the planar surface of the substrate head is oriented to oppose the planar surface of the die head; wherein the planar surfaces of the die head and the substrate head are substantially parallel and capable of motion relative to one another in at least the axis perpendicular to their respective planar surfaces, such that they may be brought into compressing contact with one another; at least one force sensor configured to measure compressive forces between the substantially parallel planar surfaces of the die head and the substrate head during a bonding operation; a controller in communication with the force sensor and configured to continuously control the relative position of the substantially parallel planar surfaces of the die head and the substrate head during a bonding operation to maintain a desired compressive force, whereby thermal expansion of the die and substrate heads is continuously compensated for during a thermo-compression bonding operation. 
         [0011]    Another embodiment of the present invention provides such a system for thermo-compression bonding of high-bump count semiconductors further comprising at least one distance measuring device in communication with the controller, wherein the at least one distance measuring device is used to measure the actual thermal expansion of the die and substrate heads perpendicularly to the parallel planar surfaces of the die and substrate heads, thereby supplementing the at least one force sensor and allowing for even finer compensation for thermal expansion of the die and substrate heads during a bonding operation. 
         [0012]    A further embodiment of the present invention provides such a system for thermo-compression bonding of high-bump count semiconductors wherein the distance measuring device is selected from the group consisting of optical sensors, ultrasonic sensors, wire draw encoders, magnetic positioning sensors, optical linear measurement sensors, linear encoders, rotary encoders, capacitive encoders, inductive encoders, eddy current encoders and optical image sensors. 
         [0013]    One embodiment of the present invention discloses a system for thermo-compression bonding of high-bump count semiconductors, the system having a die head comprising a planar surface and a substrate head comprising a planar surface, wherein the planar surface of the substrate head is oriented to oppose the planar surface of the die head; wherein the planar surfaces of the die head and the substrate head are substantially parallel and capable of motion relative to one another in at least the axis perpendicular to their respective planar surfaces, such that they may be brought into compressing contact with one another; at least one distance measuring device in communication with the controller configured to measure the thermal expansion of the die and substrate heads perpendicular to the parallel planar surfaces of the die and substrate heads during a bonding operation; and a controller in communication with the force sensor and configured to continuously control the relative position of the substantially parallel planar surfaces of the die head and the substrate head during a bonding operation to maintain a desired compressive force, whereby thermal expansion of the die and substrate heads is continuously compensated for during a thermo-compression bonding operation. 
         [0014]    Another embodiment of the present invention provides such a system for thermo-compression bonding of high-bump count semiconductors wherein the distance measuring device is selected from the group consisting of optical sensors, ultrasonic sensors, wire draw encoders, magnetic positioning sensors, optical linear measurement sensors, linear encoders, rotary encoders, capacitive encoders, inductive encoders, eddy current encoders and optical image sensors. 
         [0015]    One embodiment of the present invention provides a method for compensating for thermal expansion and contraction of die and substrate heads during a thermo-compression bonding operation comprising: providing a thermo-compression bonding system comprising: a die head comprising a planar surface; a substrate head comprising a planar surface, wherein the planar surface of the substrate head is oriented to oppose the planar surface of the die head; wherein the planar surfaces of the die head and the substrate head are substantially parallel and capable of motion relative to one another in at least the axis perpendicular to the planar surfaces, such that they may be brought into compressing contact with one another; at least one force sensor configured to measure compressive forces between the substantially parallel planar surfaces of the die head and the substrate head during a bonding operation; a controller in communication with the force sensor and configured to continuously control the relative position of the substantially parallel planar surfaces of the die head and the substrate head during a bonding operation to maintain a desired compressive force, whereby thermal expansion of the die and substrate heads is continuously compensated for during a thermo-compression bonding operation. placing a die to be bonded to a substrate on the planar surface of the die head, wherein a face of the die to be bonded to the substrate is positioned facing away from the planar surface of the die head; placing the substrate to be bonded to the die on the planar surface of the substrate head, wherein a face of the substrate to be bonded to the die is positioned facing away from the planar surface of the substrate head; moving the die head towards the substrate head until a desired compressive force is reached; heating the die and substrate heads to a desired temperature; continuously monitoring the compressive force perpendicular to the parallel planar faces of the die and substrate heads; adjusting the relative positions of the parallel planar faces of the die and substrate heads to maintain a substantially constant compressive force as the die and substrate heads expand and contract due to thermal stresses during a bonding operation; cooling the die and substrate heads; and removing the bonded die and substrate. 
         [0016]    Another embodiment of the present invention provides such a method of compensating for thermal expansion and contraction of die and substrate heads during a thermo-compression bonding operation wherein the thermo-compression bonding system further comprises a distance measuring device in communication with the controller, wherein the step of continuously monitoring further comprises continuously monitoring the amount of thermal expansion of the die and substrate heads perpendicular to their planar faces via the distance measuring device and wherein the step of adjusting the relative positions of the parallel planar faces of the die and substrate heads comprises adjusting the relative positions of the parallel planar faces of the die and substrate heads to control for thermal expansion of the parallel planar faces of the die and substrate heads by adjusting their relative position as the die and substrate heads expand and contract due to thermal stresses, as indicated by the force sensor and the distance measuring device. 
         [0017]    A further embodiment of the present invention provides such a method of compensating for thermal expansion and contraction of die and substrate heads during a thermo-compression bonding operation wherein the thermo-compression bonding system further comprises a distance measuring device in communication with the controller, in place of the force sensor, wherein the step of continuously monitoring further comprises continuously monitoring the amount of thermal expansion of the die and substrate heads perpendicular to their planar faces via the distance measuring device and wherein the step of adjusting the relative positions of the parallel planar faces of the die and substrate heads comprises adjusting the relative positions of the parallel planar faces of the die and substrate heads to control for thermal expansion of the parallel planar faces of the die and substrate heads by adjusting their relative position as the die and substrate heads expand and contract due to thermal stresses, as indicated by the distance measuring device. 
         [0018]    Yet another embodiment of the present invention provides such a method for compensating for thermal expansion and contraction of die and substrate heads during a thermo-compression bonding operation wherein adjusting the relative positions of the parallel planar faces of the die and substrate heads maintains predefined compressive force profile, as opposed to a substantially constant force, during bonding as the die and substrate heads expand and contract due to thermal stresses. 
         [0019]    The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a top, left side perspective view of a thermo-compression bonding system configured in accordance with one embodiment of the present invention; 
           [0021]      FIG. 2  is a front, right side perspective view of a gantry, a force applicator, a die bond head system and a substrate bond station, including various subsystems, in accordance with one embodiment of the present invention; 
           [0022]      FIG. 3  is a front elevation view of a die head and a substrate head, the substrate head having a substrate positioned for bonding and the die head having a die positioned for bonding, the die head and substrate head being in uncompressed positions; 
           [0023]      FIG. 4  is a front elevation view of a die head and a substrate head compressing a die and substrate; and 
           [0024]      FIG. 5  is a front elevation view of a die head and a substrate head which has allowed for uncontrolled solder bump collapse, resulting in adjacent solder bumps making contact, causing the failure of the bonded die and substrate. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Thermo-compression bonding is one method used to bond a die  302 , also known as a chip, to a substrate  304 . It is sometimes also referred to as diffusion bonding, pressure joining, thermo-compression welding or solid state welding. This process takes advantage of surface diffusion, grain boundary diffusion and bulk diffusion to physically and electrically connect a die  302  to a substrate  304 , typically via solder bumps  306 . Those skilled in the art, however, will appreciate that copper pillars, which are sometimes referred to as copper columns, micro-bumps with solder caps typically made of SnAg (although other alloys are possible) and other technologies can be used in place of solder bumps  306  in conjunction with the systems and methods described herein to achieve precision bonding of a die  302  to a substrate  304 . When discussing solder bumps  306  in the remainder of this disclosure, it should be understood that what is being described is equally applicable to the aforementioned, and other, alternatives to solder bumps  306 . 
         [0026]    Solder bumps  306 , or their alternatives, are applied during a previous processing step to the side of a die  302  to be bonded to a substrate  304 . During thermo-compression bonding, these solder bumps  306  are first precisely aligned with corresponding bump pads  308  or other equivalent features of the substrate  304 . Following alignment, heat and pressure are applied to complete the connection between the die  302  and substrate  304 . In a thermo-compression bonding operation, a die head  110  and a substrate head  108 , which are opposing parallel planar surfaces, are used to apply this heat and pressure to the die  302  and substrate  304 . 
         [0027]      FIG. 1  depicts a thermo-compression bonding system  100  including a controller  102  in controlling communication with a substrate head  108  a die head  110 , and an indicator  114  and in further communication with at least one force sensor (not shown) and at least one encoder (not shown), positioned to allow the thermal expansion of the die head  110  and/or substrate head  108  to be measured. 
         [0028]      FIG. 2  depicts the thermo-compression bonding system  100  of  FIG. 1 , with body panels  104  and other ancillary equipment removed. From this view, a gantry  200  connected to the die bond head system  202 , including the die head thermal control system body  210 , which encompasses the die head  110 , is visible. Also shown is a substrate bond station  206 , including a substrate head thermal control system body  212 , which encompasses the substrate head  108 , attached to a supporting floor  118 . Another element of this system is the force applicator  204 , which is fixed to a support member  208 . It will be apparent to those of ordinary skill in the art that many variations to this general setup are possible. For instance, rather than applying a compressive force through a force applicator  204 , the gantry  200  itself or other force applying means could supply the compressive force. In other cases, the die head  110  and substrate head  108  may be kept stationary while a die  302  and substrate  304  are brought to them via a conveyor belt, robotic grasping equipment or other means. 
         [0029]    Now referring to  FIGS. 1 and 2 , the arrangement of the aforementioned elements allows for the die head  110 , controlled by the controller  102 , via the gantry  200 , to be positioned directly over the substrate head  108  in preparation for a thermo-compression bonding operation. The controller  102  also controls heating and cooling systems contained within or adjacent to both the substrate head  108  and the die head  110 . Furthermore, the controller  102  is in communication, in this embodiment, with a force applicator  204 , which is fixed to a support member  208 . The force applicator  204  is used to apply compressive forces to the die head  110 , while the substrate head  108  is supported by the supporting floor  118 , allowing the applied force to act upon the solder bumps  306  between the die  302  and substrate  304 . During compression of the die  302  and substrate  304 , heat is applied via a die head thermal control system body  210  and a substrate head thermal control system body  212 . It will be apparent to one of ordinary skill in the art that there are a number of ways to position a die  302  and substrate  304  for thermo-compression bonding and to apply heat and force to these components after positioning. 
         [0030]    The controller  102  is configured to receive data regarding the compressive force applied to a die  302  and a substrate  304  positioned between the die head  110  and substrate head  108  from at least one force sensor (not shown), which may be positioned on the die head  110 , the substrate head  108 , or may indirectly measure the forces in a number of alternative ways. Some exemplary force sensors are strain gauge load cells, such as semiconductor gauges, thin film gauges and foil gauges, piezoelectric crystal force sensors, hydraulic force sensors, pneumatic force sensors, LVDT sensors, capacitive sensors, tuning fork sensors, vibrating wire sensors, magnetorestrictive sensors, gyroscopic sensors and force balance sensors, although alternative means of measuring force will be apparent to one of ordinary skill in the art. 
         [0031]    The controller  102  is also configured to modulate the compressive forces applied to the die  302  and substrate  304  during thermo-compression bonding, taking into account measurements from at least one of the force sensors and/or encoders. In a typical bonding process, the force desired to be applied may be constant or vary during the bonding operation. The measurements taken from the force sensor(s) and/or encoder(s) are used to alter the desired force profile to compensate for thermal expansion and contraction of portions of the thermo-compression bonding system  100 , particularly that of the die head  110 , as the expansion of this part of the thermo-compression bonding system  100  tends to be most pronounced, due to the high heat and significant temperature variations encountered during a typical thermo-compression bonding operation. 
         [0032]    The controller  102  may also be configured to communicate process stage, alarm conditions, warnings and other pertinent information via the indicator  114 , typically by varying the indicator  114  color, intensity or causing it to blink, typically in a series of short and long blinks to convey specific information. The thermo-compression bonding system also includes body panels  104 , material passages  106 , isolating supports  112 , a die head  110  movement mechanism (a gantry  200  is shown, but a number of alternative systems for positioning a die  302  and substrate  304  between the die head  110  and the substrate head  108  will be apparent to one of ordinary skill in the art). 
         [0033]    Now referring to  FIG. 3 , a front elevation view of the die head  110  and the substrate head  108  is shown. In this view, a substrate  304  with bump pads  308  is shown positioned on the substrate head  108  and a die  302  with solder bumps  306  is shown positioned on the die head  110  in preparation for a thermo-compression bonding operation. The die head  110  and substrate head  108  are shown in an uncompressed position. Also, the solder bumps  306  shown are round and relatively large in relation to the die  110 , however they are not drawn to scale and are used only to illustrate the general concept. In a typical thermo-compression bonding operation, these solder bumps  306  may be any variety of shapes and are typically smaller than 100 μm and number in the thousands per die  302 . There are a number of ways that would be apparent to one of ordinary skill in the art for precisely holding a die  302  and substrate  304  to the die head  110  and substrate head  108 , respectively, such as by vacuum. Alternatively, dies  302  and substrates  304  may be brought to a stationary bonding station having a die head  110  and a substrate head  108  without altering the inventive concept disclosed herein. 
         [0034]      FIG. 4  shows the die head  110  and substrate head  108  of  FIG. 3 , in a compressed state during a normal thermo-compression bonding operation. As shown, heat and compressive forces are being applied to the die  302  and solder bumps  306  via the die head  110 , while the substrate  304  and bump pads  308  are held stationary by the substrate head  108 . 
         [0035]      FIG. 5  shows the die head  110  and substrate head  108  of  FIG. 3 , immediately after uncontrolled solder bump  306  collapse has occurred. Although each solder bump  306  is expected to flow and deform as a natural and necessary consequence of the bonding operation, excess deformation and flow, as shown here, will cause adjacent solder bumps  306  to contact one another, preventing normal operation of the die  302  and substrate  304  while simultaneously increasing the difficulty of and sometimes even preventing the subsequent processing step of under-filing, or filling of the void between die  302  and substrate  304  with a resin. This uncontrolled collapse of solder bumps  306  is caused by the expansion of portions of the thermo-compression bonding system  100 , such as the die head  110 , exerting additional pressure on the solder bumps  306  due to thermal expansion, followed by the expected flow of solder bumps  306  after a critical temperature is met. Since there is an excess of force, the solder bumps  306  will collapse to a greater degree than is desired, causing failure of the bonding operation. 
         [0036]    To prevent the uncontrolled collapse of solder bumps  306 , monitoring of the thermal expansion, as measured via an encoder or inferred from force sensor readings, as compared to an expected or known applied force during a process stage and alteration of at least the applied compressive force or the position of either the die head  110  or substrate head  108  is necessary to achieve consistent bonding due to the precision demanded to effectively bond the latest generation of chips using thermo-compression bonding techniques. 
         [0037]    In embodiments, the thermo-compression bonding system  100  includes a die head  110  comprising a planar surface and a substrate head  108  comprising a planar surface, wherein the planar surface of the substrate head  108  is oriented to oppose the planar surface of the die head  110 ; wherein the planar surfaces of the die head  110  and the substrate head  108  are substantially parallel and capable of motion relative to one another in at least the axis perpendicular to the planar surfaces, such that they may be brought into compressing contact with one another; at least one force sensor configured to measure compressive forces between the substantially parallel planar surfaces of the die head  110  and the substrate head  108  during a bonding operation; a controller  102  in communication with the force sensor and configured to continuously control the relative position of the substantially parallel planar surfaces of the die head  110  and the substrate head  108  during a bonding operation to maintain a desired compressive force, whereby thermal expansion of the die head  110  and substrate head  108  is continuously compensated for during a thermo-compression bonding operation. 
         [0038]    In other embodiments, a distance measuring device in communication with the controller  102  may be used to measure the actual thermal expansion of the die head  110  and substrate head  108  perpendicular to the parallel planar surfaces of the die head  110  and substrate head  108 , thereby supplementing the at least one force sensor and allowing for even finer compensation for thermal expansion of the die head  110  and substrate head  108  during a bonding operation. In additional embodiments, at least one distance sensor may be employed without the aid of force sensors to similar effect as the previously described combination. The distance measuring devices used in such embodiments may be optical sensors, ultrasonic sensors, wire draw encoders, magnetic positioning sensors, optical linear measurement sensors, linear encoders, rotary encoders, capacitive encoders, inductive encoders, eddy current encoders, optical image sensors and a variety of similar sensors with which one skilled in the art will be familiar. 
         [0039]    One embodiment of the present invention provides a method for compensating for thermal expansion and contraction of a die head  110  and a substrate head  108  during a thermo-compression bonding operation comprising: providing a thermo-compression bonding system  100  comprising: a die head  110  comprising a planar surface; a substrate head  108  comprising a planar surface, wherein the planar surface of the substrate head  108  is oriented to oppose the planar surface of the die head  110 ; wherein the planar surfaces of the die head  110  and the substrate head  108  are substantially parallel and capable of motion relative to one another in at least the axis perpendicular to the planar surfaces, such that they may be brought into compressing contact with one another; at least one force sensor configured to measure compressive forces between the substantially parallel planar surfaces of the die head  110  and the substrate head  108  during a bonding operation; a controller  102  in communication with the force sensor and configured to continuously control the relative position of the substantially parallel planar surfaces of the die head  110  and the substrate head  108  during a bonding operation to maintain a desired compressive force, whereby thermal expansion of the die head  110  and substrate head  108  is continuously compensated for during a thermo-compression bonding operation. placing a die  302  to be bonded to a substrate  304  on the planar surface of the die head  110 , wherein a face of the die  302  to be bonded to the substrate  304  is positioned facing away from the planar surface of the die head  110 ; placing the substrate  304  to be bonded to the die  302  on the planar surface of the substrate head  108 , wherein a face of the substrate  304  to be bonded to the die  302  is positioned facing away from the planar surface of the substrate head  108 ; moving the die head  110  towards the substrate head  108  until a desired compressive force is reached; heating the die head  110  and substrate head  108  to a desired temperature; continuously monitoring the compressive force perpendicular to the parallel planar faces of the die head  110  and substrate head  108 ; adjusting the relative positions of the parallel planar faces of the die head  110  and substrate head  108  to maintain a substantially constant compressive force as the die head  110  and substrate head  108  expand and contract due to thermal stresses during a bonding operation; and removing the bonded die  302  and substrate  304 . 
         [0040]    In additional embodiments, a distance measuring device in communication with the controller  102  is included, wherein the step of continuously monitoring further comprises continuously monitoring the amount of thermal expansion of the die head  110  and substrate head  108  perpendicular to their planar faces via the distance measuring device and wherein the step of adjusting the relative positions of the parallel planar faces of the die head  110  and substrate head  108  comprises adjusting the relative positions of the parallel planar faces of the die head  110  and substrate head  108  to control for thermal expansion of the parallel planar faces of the die head  110  and substrate head  108  by adjusting their relative position as the die head  110  and substrate head  108  expand and contract due to thermal stresses, as indicated by the force sensor and the distance measuring device. 
         [0041]    A further embodiment of the present invention provides such a method of compensating for thermal expansion and contraction of the die head  110  and substrate head  108  during a thermo-compression bonding operation wherein the thermo-compression bonding system  100  comprises a distance measuring device in communication with the controller  102 , in place of the force sensor, wherein the step of continuously monitoring further comprises continuously monitoring the amount of thermal expansion of the die head  110  and substrate head  108  perpendicular to their planar faces via the distance measuring device and wherein the step of adjusting the relative positions of the parallel planar faces of the die head  110  and substrate head  108  comprises adjusting the relative positions of the parallel planar faces of the die head  110  and substrate head  108  to control for thermal expansion of the parallel planar faces of the die head  110  and substrate head  108  by adjusting their relative position as the die head  110  and substrate head  108  expand and contract due to thermal stresses, as indicated by the distance measuring device. 
         [0042]    Yet another embodiment of the present invention provides such a method for compensating for thermal expansion and contraction of the die head  110  and substrate head  108  during a thermo-compression bonding operation wherein adjusting the relative positions of the parallel planar faces of the die head  110  and substrate head  108  maintains a predefined compressive force profile, as opposed to a substantially constant force, during bonding as the die head  110  and substrate head  108  expand and contract due to thermal stresses. 
         [0043]    The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. Each and every page of this submission, and all contents thereon, however characterized, identified, or numbered, is considered a substantive part of this application for all purposes, irrespective of form or placement within the application. This specification is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure.