Abstract:
Disclosed are novel methods and apparatus for efficiently providing dynamic solder attach, in part, to decrease the affects of thermal variations. In an embodiment, a spacer provides a gap between a semiconductor package and a device, an attachment material is disposed between the device and the semiconductor package, and an environmental control device provides an appropriate environment to activate the attachment material. In another embodiment, while the attachment material is substantially activated, the spacer increases the gap between the semiconductor package and the device to elongate the attachment material in a plane substantially perpendicular to the device and the semiconductor package. In yet a different embodiment, the elongated attachment material assumes a substantially hourglass shape.

Description:
FIELD OF INVENTION  
         [0001]    The subject of this application relates generally to the field of electronic device manufacturing and, more particularly, to improving solder attach applications.  
         BACKGROUND OF INVENTION  
         [0002]    As integrated circuit fabrication technology improves, manufacturers are able to integrate additional functionality onto a single silicon substrate. As the number of these functionalities increases, however, so does the number of components on a single chip. Additional components add additional signal switching, in turn, creating more heat. The additional heat adds to the already-existing thermal expansion issues.  
           [0003]    Thermal expansion differences between a semiconductor device and a system motherboard have been a fundamental problem facing the semiconductor industry. Generally, a semiconductor package provides a device with electrical connection to the motherboard, heat dissipation, and mechanical and environmental protection. As part of the mechanical protection function, the package can provide a solution to the thermal mismatch issue between the device and the motherboard.  
           [0004]    During normal operation, the device is expected to survive a fairly wide range of temperature fluctuations. While undergoing these fluctuations, if the device expands and contracts at one rate while the package and/or board move at vastly different rates, a great deal of stress can be generated within the combined structure. These stresses can produce failures within the components themselves or at any of the interfaces between these components.  
           [0005]    An attach design that is quickly gaining acceptance by semiconductor industry is flip chip. Flip chip technology generally places chips on circuit boards face side down. The chips are then connected to circuits by small “bumps” of solder. This configuration eliminates wire bonding and allows shorter interconnections between circuits and components to provide more robust, lighter, smaller, and faster networks. An ever-increasing number of semiconductor manufacturers are adapting flip chip designs, in part, because of the increasing number of I/Os employed in devices.  
           [0006]    As the number of I/Os for each device increases, there is correspondingly less space to place the I/Os around the periphery of a device. To solve this problem, many semiconductor manufacturers are moving to full area array or partial area array I/O designs. Such designs, in turn, increase the need for the advantages provided by a flip chip process.  
           [0007]    Unfortunately, no method of device to package attach seems to be more sensitive to thermal expansion problems than flip chip. This sensitivity is, in part, based on the flip chip technology requiring a small bump size, which brings the die very close to the package. This lack of distance combined with the rigid nature of the solder results in a high stress interconnect when the CTE is not matched. In addition, as the device size grows, the problem becomes worse because as the distance from neutral point grows, the relative movement (or strain) increases.  
           [0008]    A classic question facing the package designers is: Should the package thermal expansion be matched to the device or to the motherboard. Both approaches have been employed with varying degrees of success throughout the industry. If the package is matched more closely to the device, then the attachment method between the board and the package must be compliant enough to absorb the movement. Typical solutions include sockets, pins, solder columns, and interposers. All of these can provide a compliant interface either with the materials themselves or sufficient distance (i.e. stand off) between the package and board. Each of these methods, however, has drawbacks.  
           [0009]    In the case of sockets, there exists a significant cost versus electrical performance trade off. A socket, which adds marginally to the overall costs, can significantly degrade electrical performance. In addition, these sockets usually require pins to be placed on the package, adding cost and process steps. Conversely, a socket which does not degrade electrical performance or require pins can cost as much as the package itself. In addition, these types of sockets typically require a great deal of force to be placed on the package to ensure good socket contact. This can limit the mechanical and thermal design solutions.  
           [0010]    Solder columns provide another solution by providing the proper stand off with good electrical connection, but are difficult to process and limited in supplier base. Another approach is to make use of solders with different melting, or re-flow, temperatures. Components within the solder attach method can be designed with higher melting solders. These can act as a stand off to maintain a greater distance between the package and motherboard because they would not melt and collapse during the normal board mount process. This method adds complications to the assembly process.  
           [0011]    The interposer solution is relatively untested and is inherently undesirable because it, like sockets, adds a component to the assembly process and bill of materials.  
           [0012]    If the package is matched thermally to the board, then the attachment method between device and package must absorb the inherent stresses. Presently, this method is achieved by using an epoxy, or epoxy like material, called an under-fill which is dispensed between the device and the package after the flip chip attach is completed. The under-fill acts to absorb stresses. This method, however, can be employed successfully for relatively small devices. Unfortunately, as the devices grow larger, even the under-fill cannot reduce the stresses to non-lethal levels. A great deal of process and material development will be required to achieve success with a larger die.  
         SUMMARY OF INVENTION  
         [0013]    The present invention includes novel methods and apparatus to provide dynamic solder attach, in part, to decrease the affects of thermal variations. In an embodiment, an apparatus is disclosed. The apparatus includes a semiconductor package, a device to be attached to the semiconductor package, a spacer to provide a gap between the semiconductor package and the device, an attachment material disposed between the device and the semiconductor package, and an environmental control device to provide an appropriate environment to activate the attachment material.  
           [0014]    In another embodiment, while the attachment material is substantially activated, the spacer increases the gap between the semiconductor package and the device to elongate the attachment material in a plane substantially perpendicular to the device and the semiconductor package.  
           [0015]    In yet a different embodiment, the elongated attachment material assumes a substantially hourglass shape.  
           [0016]    In various embodiments, the apparatus may further include any of the following:  
           [0017]    a stopper to limit the increase of the gap once a desired size of the increased gap is reached;  
           [0018]    a locking device to lock in the spacer once a desired size of the increased gap is reached;  
           [0019]    a brake to maintain the gap at a desired size;  
           [0020]    a computing device to actuate the brake; and/or  
           [0021]    an aligner to align the semiconductor package and the device.  
           [0022]    In a different embodiment, a novel method is disclosed. The method includes providing a spacer to control a gap between a semiconductor package and a device, providing attachment material between the device and the semiconductor package, positioning the device and the semiconductor package adjacent to each other to provide substantial contact between the device and the semiconductor package via the attachment material, providing an appropriate environment to activate the attachment material, and utilizing the spacer to increase the gap between the semiconductor package and the device while the attachment material is substantially activated.  
           [0023]    In a further embodiment, the attachment material is elongated in a plane substantially perpendicular to the device and the semiconductor package.  
           [0024]    In yet another embodiment, the elongated attachment material assumes a substantially hourglass shape.  
           [0025]    In yet a different embodiment, the attachment material conducts electricity.  
           [0026]    In a certain embodiment, the method further includes actuating a brake to maintain the gap at a desired size.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0027]    The present invention may be better understood and its numerous objects, features, and advantages made apparent to those skilled in the art by reference to the accompanying drawings in which:  
         [0028]    [0028]FIG. 1A illustrates an exemplary partial cross-sectional view of a device  100  in accordance with an embodiment of the present invention;  
         [0029]    [0029]FIG. 1B illustrates an exemplary partial cross sectional view of the device  100  of FIG. 1A after the solder balls  106  are elongated;  
         [0030]    [0030]FIG. 2A illustrates an exemplary partial cross sectional view of a device  200  in accordance with an embodiment of the present invention;  
         [0031]    [0031]FIG. 2B illustrates an exemplary partial cross sectional view of the device  200  of FIG. 2A after heat is applied to put the solder balls  106  in there reflow state;  
         [0032]    [0032]FIG. 3A illustrates an exemplary partial cross sectional view of a device  300  in accordance with an embodiment of the present invention;  
         [0033]    [0033]FIG. 3B illustrates an exemplary partial cross sectional view of the device  300  after the lifting mechanism  130  increases the distance between the motherboard  102  and the semiconductor package  104 ;  
         [0034]    [0034]FIG. 4 illustrates an exemplary partial cross sectional view of a device  400  in accordance with an embodiment of the present invention; and  
         [0035]    [0035]FIG. 5 illustrates an exemplary partial cross sectional view of a device  500  in accordance with an embodiment of the present invention. 
     
    
       [0036]    The use of the same reference symbols in different drawings indicates similar or identical items.  
       DETAILED DESCRIPTION  
       [0037]    In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.  
         [0038]    Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.  
         [0039]    [0039]FIG. 1A illustrates an exemplary partial cross-sectional view of a device  100  in accordance with an embodiment of the present invention. A motherboard  102  is attached to a semiconductor package  104  via solder balls  106 . As illustrated, a semiconductor device  108  is attached to the semiconductor package  104  via solder balls  110 . The semiconductor device  108  can be any semiconductor device including an integrated circuit, a processor, an application specific integrated chip (ASIC), and the like. It is envisioned that the semiconductor device  108  may be attached to the semiconductor package  104  utilizing a flip chip technique. A lifting mechanism  112  is attached to the semiconductor package  104 . The lifting mechanism  112  can utilize a spring  114  to increase the distance between the semiconductor package  104  and the motherboard  102  at a given point in time. It is envisioned that the lifting mechanism  112  can utilize a bimetallic spring  114 . The bimetallic spring  114  can be designed such that it would raise the semiconductor package  104  once the solder balls  106  are in their molten state. The spring can also be designed such that it would expand at a given rate depending on a given temperature and/or rate of temperature change applied to the spring.  
         [0040]    [0040]FIG. 1B illustrates an exemplary partial cross sectional view of the device  100  of FIG. 1A after the solder balls  106  are elongated. The spring  114  lifts the semiconductor package  104  as shown in FIG. 1B by expansion. It is envisioned that the solder balls  106  may assume an hourglass form as illustrated in FIG. 1B. The heat required to put the solder balls  106  in their molten state, or other wise to provide reflow, can be provided by putting the device  100  in, for example, furnace or on a belt furnace which may in some embodiments employ different zones for heating. The temperature of each zone and the speed of the belt movement can be adjusted for an optimal case. Additionally, the temperature that the device  100  is exposed to may be appropriately chosen to burn off any fluxes which may be present for cleaning organics or otherwise for improving the soldering process.  
         [0041]    Generally solder may have a propensity to stick to metallic surfaces. As such metal plated pads may be utilized on the contact points where the solder balls meet a device such as the semiconductor package  104 . These pads may also be plated with nickel and/or gold for better adhesion and to reduce corrosion. The propensity to stick to the metallic surfaces also helps in achieving the hourglass shape of the solder balls  106  illustrated in FIG. 1B. It is believed that an hourglass shape solder joint can be one of the most reliable structures during temperature cycling. Thus, such a structure may decrease the effects of thermal expansion substantially.  
         [0042]    Moreover, it is envisioned that the expanded spring of  114  of FIG. 1B can be locked in place to provide sufficient distance between the semiconductor package  104  and the motherboard  102 . The locking may also assist the rigidity of the solder balls during the cooling stage by avoiding undesirable movements in certain directions.  
         [0043]    [0043]FIG. 2A illustrates an exemplary partial cross sectional view of a device  200  in accordance with an embodiment of the present invention. As illustrated, the lifting mechanism  112  of FIG. 2A further includes a hard stop  116 . As the spring  114  increases the distance between the semiconductor package  104  and the motherboard  102 , the hard stop  116  limits the expansion of the spring  114  beyond a desirable point. This desirable point may be chosen, for example, based on the desired distance between the devices being attached. The stop point may also depend on the amount of solder being utilized and the appropriate curvature to be achieved for the hourglass shape.  
         [0044]    [0044]FIG. 2B illustrates an exemplary partial cross sectional view of the device  200  of FIG. 2A after heat is applied to put the solder balls  106  in there reflow state. In FIG. 2B, the hard stop  116  limits the movement achieved by expansion of the spring  114 . It is further envisioned that the lifting mechanism  112  of FIG. 2B also provide for locking the lifting mechanism once a desired distance is reached. As shown in FIG. 2B, this can be achieved by utilizing a mechanical locking design, such as the illustrated hard stop  116 . The locking in place of the lifting mechanism  112  will prevent any spacing decrease between the semiconductor package  104  and the motherboard  102  after the spring  114  has performed its task during reflow.  
         [0045]    It is envisioned that any lifting apparatus discussed herein may utilize numerous devices to achieve the lifting. Examples of other lifting apparatus include a spring (with any shape including cylindrical, spiral, conical, flat, u-shaped, and the like), a hydraulic mechanism, a screw, a gear, a wheel, a semi-solid (in an embodiment, epoxy like) material, which expands with temperature then solidifies to not only set the proper stand off but acts as an under-fill, any device that may be utilized to provide lifting, or any combination thereof. It is envisioned that a gear may be utilized that would engage teeth present on the objects being separated. Alternatively, a gear may be installed on the objects being separated with teeth on a bracket. With respect to wheels, they may be selected from material such that sufficient friction would be present for separating the objects. It is further envisioned that any of the lifting apparatus may be externally controlled, utilizing techniques including those discussed herein.  
         [0046]    [0046]FIG. 3A illustrates an exemplary partial cross sectional view of a device  300  in accordance with an embodiment of the present invention. The device  300  utilizes a lifting mechanism  130 . The lifting mechanism  130  includes a hard stop  132 , a brake  138 , a control connection  136 , and a lifting device  134 . The lifting device  134  may be any type of a device capable of lifting including those discussed herein. The brake  138  may be a secondary brake or a brake under external control through, for example, the control connection  136 . As a secondary brake, the brake  138  will ensure that no movement is provided until a desired time and/or distance is reached. In some embodiments, the control connection  136  may be wiring for external temperature or time control. In certain embodiments, the brake  138  may be externally actuated and/or be temperature sensitive. Also, wireless communication (utilizing electromagnetic waves such as radio waves, infrared, visible light, ultraviolet, X rays, gamma rays, and the like) may be employed to provide communication and/or control of elements within the device  300 .  
         [0047]    [0047]FIG. 3B illustrates an exemplary partial cross sectional view of the device  300  after the lifting mechanism  130  increases the distance between the motherboard  102  and the semiconductor package  104 . As illustrated in FIG. 3B, the lifting mechanism  130  has achieved a desired distance between the semiconductor package  104  and the motherboard  102  such that the solder balls  106  have achieved an hourglass shape. It is also envisioned that the brake  138  may be actuated under periodical and/or gradational control such that the distance between the package  104  and the motherboard  102  is controlled as a function of time and/or temperature. This can ensure that the solder balls  106  are given sufficient time to expand during the reflow, for example. It is also envisioned that finite element methods and/or fuzzy logic techniques can be utilized to ensure proper movement provided by any lifting apparatus. Any movement provided for herein can also be controlled and/or directed by a computing device such as a general purpose computer, a personal digital assistant (PDA), an embedded device, and the like.  
         [0048]    In an embodiment, the computing device includes a Sun Microsystems computer utilizing a SPARC microprocessor available from several vendors (including Sun Microsystems of Palo Alto, Calif.). Those with ordinary skill in the art understand, however, that any type of computer system may be utilized to embody the present invention, including those made by Hewlett Packard of Palo Alto, Calif., and IBM-compatible personal computers utilizing Intel microprocessor, which are available from several vendors (including IBM of Armonk, N.Y.). Also, instead of a single processor, two or more processors (whether on a single chip or on separate chips) can be utilized to provide speedup in operations.  
         [0049]    The computing device may also employ a network interface to provide communication capability with other computer systems on a same local network, on a different network connected via modems and the like to the present network, or to other computers across the Internet. In various embodiments, the network interface can be implemented in Ethernet, Fast Ethernet, wide-area network (WAN), leased line (such as T1, T3, optical carrier 3 (OC3), and the like), digital subscriber line (DSL and its varieties such as high bit-rate DSL (HDSL), integrated services digital network DSL (IDSL), and the like), time division multiplexing (TDM), asynchronous transfer mode (ATM), satellite, cable modem, and FireWire.  
         [0050]    Moreover, the computing device may utilize operating systems such as Solaris, Windows (and its varieties such as NT, 2000, XP, ME, and the like), HP-UX, Unix, Berkeley software distribution (BSD) Unix, Linux, Apple Unix (AUX), and the like. Also, it is envisioned that in certain embodiments, the computing device is a general purpose computer capable of running any number of applications such as those available from companies including Oracle, Siebel, Unisys, Microsoft, and the like.  
         [0051]    [0051]FIG. 4 illustrates an exemplary partial cross sectional view of a device  400  in accordance with an embodiment of the present invention. The device  400  includes a lifting mechanism  140 . The lifting mechanism  140  includes a lifting aligner  142 , a hard stop  132 , a lifter  134 , a control mechanism  136 , a stop  138 , and alignment pins  148 . It is envisioned that the lifting mechanism  140  may be any lifting apparatus discussed herein. The device  400  may also include the illustrated alignment holes  146  in the motherboard  132 . In some embodiments, the alignment may be provided by the semiconductor package and the lifting may be applied to the motherboard or any device being attached. As illustrated, the alignment pins  148  may be inserted in the alignment holes  146  of the motherboard  102 . The combination of the alignment brackets  142 , alignment holes  146 , and alignment pins  148  provide the device  400  with proper alignment between the semiconductor package  104  and the motherboard  102 . This is especially important as packages increase in size and the solder balls decrease in size. The device  400  may also include an optional expansion frame  144  which can be aligned with the alignment pins  148  and the alignment holes  146 . The expansion frame  144  shown can be mounted to the motherboard  102  through alignment pins  148  and/or alignment holes  146 . It is envisioned that utilizing alignment techniques discussed herein will stabilize the semiconductor package in the X, Y, and theta directions.  
         [0052]    [0052]FIG. 5 illustrates an exemplary partial cross sectional view of a device  500  in accordance with an embodiment of the present invention. The device  500  includes a lifting mechanism  504  which can control the distance between the semiconductor package  104  and the semiconductor device  108  during reflow. The lifting mechanism  504  includes a spring  506  a stop  508  and a locking mechanism  510 . It is envisioned that the lifting mechanism  504  may be any lifting apparatus discussed herein. As illustrated in FIG. 5, the locking mechanism  510  may be engagingly attached to the spring  506 . The device  500  can provide lifting to the semiconductor device itself during the device attach reflow process. Thus, elongated solder joints are provided during reflow to reduce stress on the structures.  
         [0053]    The foregoing description has been directed to specific embodiments. It will be apparent to those with ordinary skill in the art that modifications may be made to the described embodiments, with the attainment of all or some of the advantages. For example, the techniques discussed herein may be applied to any items being attached together. Also, the techniques discussed herein may be applied with other attachment material including glues (such as chemical, thermal, combinations thereof, and the like), welds, and the like. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the spirit and scope of the invention.