Abstract:
In a microelectronic chip package for which grounding and thermal dissipation is desired, a cover is provided having an opening which is aligned with a contact on the substrate connected to ground potential. The cover is connected to the electronic device and the ground contact. This invention provides for a method and electronic package to overcome the difficulties encountered when attempting to simultaneously attach a cover to two different surfaces with two different adhesives.

Description:
BACKGROUND OF INVENTION  
         [0001]    This invention relates to improved electronic packages and in particular, improved methods and devices for providing grounding and thermal dissipation in such packages.  
           [0002]    In the manufacture of electronic circuit assemblies, substrates or circuit boards contain electronic components, integrated circuits or chips, and other devices mounted thereon. The assemblies are then encapsulated into packages.  
           [0003]    In a typical microelectronic chip package construction, a cover, plate or lid, which is usually electrically and thermally conducting, is attached, by means of adhesive, to a chip which has been previously joined to the substrate. In this description the terms cover, plate or lid are intended to be used interchangeably to refer to the same elements. The adhesive material is often thermally conductive. The adhesive is deposited on the chip and the lid is then placed in contact with the adhesive and the adhesive is subsequently cured to provide a permanent connection of the cover plate to the chip. This arrangement results in the cover plate providing protection for the chip and also acting as a heat sink to conduct unwanted heat away from the chip. Good thermal conductivity requires a thin, uniform adhesive bond layer between the lid and the chip.  
           [0004]    For example, with reference to FIG. 1, a typical flip chip electronic ball grid array package  10  is shown. Chip  11  has a plurality of solder bumps  12  for connecting to corresponding pads (not shown) on a surface of substrate  13 . Solder balls  14  are provided on the underside of substrate  13  in order to attach the package to other circuitry on a substrate or circuit board. Chip  11  is electrically connected by means of the chip solder bumps  12  and conductive circuitry through substrate  13  to solder balls  14  in a well known manner. Chip  11  is sealed to substrate  13  by underfill  15  and possibly other encapsulating material as is known in the art. Cover plate or lid  16  is attached to the upper surface of chip  11  as shown by means of thermally conductive adhesive  17 . Typically, adhesive  17  is applied to the upper surface of chip  11 , lid  16  is placed thereon and the adhesive is subsequently cured. Lid  16  provides physical protection for chip  11  and also in conjunction with the thermally conductive adhesive  17  acts as a heat sink to dissipate unwanted heat that is generated by the electronic device or chip  11 . Good thermal conductivity between chip  11  and lid  16  requires adhesive  17  to be in the form of a relatively thin and uniform adhesive bond line between lid  16  and chip  11 .  
           [0005]    Present day electronic circuitry components require the presence of radiation shielding or electromagnetic interference (EMI) protection, either to prevent radiation of electromagnetic waves from the active circuit components or to prevent interference to the components from external sources. In view of this requirement, it is sometimes desirable to electrically connect the lid to one or more ground connections existing on the substrate. Typically, conductive regions on the substrate are linked to ground pads, an electrically conductive adhesive is then applied to the pads and the conductive lid is placed on the chip so as to contact the conductive adhesive. Typically, the thermally conductive adhesive is applied to the chip and the electrically conductive adhesive is applied to the pads and the lid is attached to both of the adhesives at the same time, followed by a step of curing both of the adhesive materials. In order to accommodate construction tolerances for the electronic package, the design space between these ground pads and the lid is much greater than the desired thickness of the lid to chip layer for the thermally conductive adhesive. These differences demand the use of different types of adhesives with different rheological properties.  
           [0006]    When attempting to simultaneously attach the lid to two different surfaces with two different adhesives, difficulties are typically encountered which require compromises in the lid to chip bond line thickness, or in other words, the thickness of the adhesive layer between the lid and the chip, lid to chip adhesive run-off, and excessive lid to ground pad adhesive spread are varied.  
           [0007]    For example, referring to FIG. 2, the relatively simple structure of FIG. 1 is reproduced showing a typical prior art arrangement for providing for both electromagnetic interference protection for chip  11  as well as minimizing electromagnetic radiation. Ground pad  18  as shown, for example, on the upper surface of substrate  13 , is electrically connected to ground potential as by circuitry through substrate  13  to one or more solder balls  14 , as is well known. Electrically conductive adhesive  19  connects ground pad  18  to lid  16 . Conventionally, electrically conductive material  19  is deposited on ground pad  18  in the same operation that the chip to lid adhesive  17  is deposited on chip  11  and then lid  16  is placed so as to contact both adhesive layers  17  and  19  at the same time followed by a step to cure both of the adhesives layers. As a result of physical tolerances required in the design of electronic packages, the space between ground pads  18  and lid  16  is much greater than the thickness of the lid to chip bond line between chip  11  and lid  16 . Thus, it is apparent that in the processes of attaching lid  16  to the two different surfaces, namely chip  11  and ground pad  18 , difficulties may be encountered such as the differences in the thickness of the lid to chip bond line between chip  11  and lid  16  or the run-off of adhesive  17  such that adhesive does not effectively remain on the surface of chip  11  or excess spreading of adhesive  19 .  
           [0008]    As an alternative to the use of conductive adhesive for the connection of the lid to the ground pad, the lid to ground pad connection may be achieved with solder. In view of the relatively large spacing as described above, the use of solder demands a relatively large amount of solder to be pre-deposited on the lid or on the ground pads themselves. However, alignment must be somehow predetermined between the solderable regions as the bottom side of the lid is not visible and this blind bottom side of the lid is to be soldered to the substrate ground pads. Any variations in deposited solder volume may result in either incomplete or absent lid to chip bond line or incomplete or absent lid to ground pad connection and thereby preventing the cover plate from properly functioning as a heat sink or to provide EMI protection.  
           [0009]    As had been previously described, the ground pad lid connection may be made out of solder and comparable difficulties are experienced with the use of solder instead of conductive adhesive, resulting in incomplete or absent connections between the lid to chip bond line, or the lid to ground pad connection.  
         SUMMARY OF INVENTION  
         [0010]    The present invention is directed to eliminating the difficulties in attaching a cover plate to a microelectronic chip package as described above as well as other shortcomings resulting from existing technology. The present invention has the object to provide methods and electronic packages which alleviate the above drawbacks.  
           [0011]    According to one aspect of the present invention there is provided an electronic package containing an electronic device and a cover plate for providing thermal dissipation and electrical shielding for said electronic device. The package comprises a substrate, at least one electronic device mounted on said substrate and said substrate has a contact thereon for connecting to ground potential. A cover is provided for said electronic device has an opening therein which is positionally aligned with the contact. A thermal connection is provided between said electronic device and said cover and an electrical connection is provided proximate to said opening in the cover and the contact on the substrate.  
           [0012]    According to another aspect of the present invention there is provided a method for assembling an electronic package which includes an electronic device and provides thermal dissipation and electrical shielding for the electronic device. The method comprises the steps of mounting an electronic device on a substrate wherein said substrate has a contact for connection to ground potential located thereon and positioning a cover over said electronic device wherein said cover has an opening therein and positionally aligning the opening of the cover with the contact on said substrate. The cover is then attached to the electronic device and electrically connected to the contact by way of the opening in the cover. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]    The above and other advantages of the present invention will be better understood with reference to the following drawings, wherein like reference numbers represent like elements of the invention embodiments:  
         [0014]    [0014]FIG. 1 is a cross-sectional view of a representation of a typical flip chip ball grid array electronic package as exists in the prior art;  
         [0015]    [0015]FIG. 2 is a similar cross-sectional view of the package of FIG. 1, showing a prior art arrangement for electrically grounding the lid to the substrate;  
         [0016]    [0016]FIG. 3 is a cross-sectional view of a flip chip package providing one embodiment for electrically grounding the lid to the substrate in accordance with the present invention;  
         [0017]    [0017]FIG. 4 is a representation of a cross-sectional view of a flip chip package showing an alternative embodiment of the invention;  
         [0018]    [0018]FIG. 5 is a diagram of a lid or cover plate for an electronic package showing a variety of configurations of the openings in the lid or cover which could be used as part of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]    With reference to FIG. 3, aspects of a preferred embodiment of the present invention will be described which eliminates the above shortcomings of the prior art as discussed with reference to FIGS. 1 and 2. A hole or opening  20  is provided in lid  16  at one or more appropriate locations corresponding to ground pads  18  on substrate  13 . Following traditional attachment of lid  16  to the top surface of chip  11 , desired material  19  to electrically connect lid  16  to ground pad  18  is deposited into the hole or opening  20  of lid  16 . This provides for an electrical ground connection between lid  16  and pad  18  of controlled shape and size. Of course, only one hole or opening  20  is shown in FIG. 3 but it is apparent that more than one hole could be employed to provide connection to more that one ground pad  18  as may be needed.  
         [0020]    By providing one or more holes  20  in lid  16 , the attachment of lid  16  to chip  11  may be achieved separately and independently from the attachment of lid  16  to ground pad  18  which then can be achieved in a controlled reproducible manner. For example, in the case of use of electrically conductive adhesive  19 , the adhesive is dispensed in each lid hole  20  where the lid hole  20  is physically located in the x-y plane within the vicinity of, or proximate to, the substrate ground pad  18  corresponding to the hole  20  during the attachment of lid  16  to chip  11 . Hole or opening  20  takes up any variation in volume of adhesive  19  that may have been applied and acts as a wicking up surface allowing a consistent shape of connection to ground pad  18 , thereby preventing conductive adhesive from spreading to undesired regions of the surface of substrate  13  or in any manner of lifting or distorting lid  16  as lid  16  has already been attached to chip  11  with the cured adhesive  17 . It stands to reason, therefore, that owing to the ability to control the spread of adhesive  19  that a smaller region of the surface of substrate  13  need be allocated for ground pad  18 , thus saving and preserving surface real estate on substrate  13  for other purposes.  
         [0021]    While the above describes actually curing the chip adhesive  17  in advance of the dispense and cure of adhesive  19  as an alternative, adhesive  17  could simply be initially partially cured in order to “gel” adhesive  17  to effectively fix both lid-chip position and lid-chip bond-line thickness, then deposit adhesive  19  in hole or opening  20 , and then subsequently simultaneously cure the two adhesives  17  and  19 .  
         [0022]    The use of opening  20  in lid  16  can also accommodate the use of a solder connection between lid  16  and ground pad  18 . This may be required because of stringent requirements for the resistivity or joint strength of the lidpad connection. Holes  20  provide a simplified and controlled means to achieve a solder connection. With this approach, typically, no solder is required to be predeposited on lid  16  or substrate ground pad  18 . In one preferred embodiment, hole  20  and ground pad  18  are designed to receive a standard solder ball or other preformed solder shape element or solder preform as are known to those familiar with this field, such that the solder element is simply dropped into opening  20  where the opening  20  has been physically located within the vicinity of the corresponding substrate ground pad  18  or positively aligned with pad  18 , during the previous attachment of lid  16  to chip  11 . Locating of lid  16  such that hole  20  is positioned over the corresponding ground pad  18  is simply achieved in that there is no blind bottom side of lid  20  to be aligned with ground pad  18  as lid  16  can be positioned so as to visually positionally align hole  20  with pad  18  from above. In this embodiment, soldering of lid or cover  16  to ground pad  18  is achieved during subsequent attachment of solder balls  14  to substrate  13  which typically is performed after lid  16  is attached to chip  11  in a ball grid array product and therefore would not require any additional solder reflow step. Alternatively, soldering of lid or cover  16  to ground pad  18  may also be achieved during subsequent attachment of solder balls  14  of substrate  13  to a circuit board which is also typically performed after lid  16  is attached to chip  11  in a ball grid array product and therefore would not require any additional solder reflow step. The solderable inside surface of hole  20  serves to wick any excess molten solder and thus accommodate any variation in spacing between substrate  13  and lid  16  in the vicinity of the ground pad  18 . The soldered inside surface of opening  20  of lid  16  constitutes a region of the subsequent solder joint, thus increasing the joint integrity between ground pad  18  and lid  16 . This may also allow for the design of physically smaller regions for ground pad  18  on the surface of substrate  13 .  
         [0023]    Flux may or may not be necessary for the attachment of lid  16  to pad  18 , depending upon the nature of the solder element and the surfaces of ground pad  18  and hole  20 . As an alternative to this embodiment, the solder element may be replaced with a controlled volume of solder paste which is dispensed into hole  20 . In addition, it is apparent that flux or paste (which contains flux) depending upon their nature and the subsequent solder ball attachment processes used, may or may not require a separate cleaning process to remove flux residues after attachment.  
         [0024]    The existence of one or more openings  20  in lid or cover  16  provides for various forms of electrical and physical connections between lid  16  and ground pad  18 . If the design of the electronic package  10  requires a more compliant connection between lid  16  and one or more ground pads  18  than what is achievable with a fixed adhesive or solder connection as previously described, a compliant member  21  as shown in FIG. 4, such as a spiral or leave connector made out of spring material, may be placed in opening  20  of lid  16  where the opening  20  has been designed to be physically located proximate to the area of the corresponding ground pad  18  on substrate  13  during the attachment of lid  16  to chip  11 . By providing such a compliant member  21  with pre-soldered regions at each extremity thereof, compliant member  21  will be readily soldered to both lid  16  and ground pad  18  as shown by soldered connections  22  during the subsequent solder flow process of attaching solder balls  14  to the substrate  13  or circuit board. The attachment of compliant member  21  in this manner maintains compliance between the two soldered connections  22 . With the approach as described for this aspect of the embodiment of the invention, minimal pre-alignment, pre-placing or pre-solder attachment is required between compliant member  21  and lid  16 . In addition, no pressure is required to be applied between lid  16  and substrate  13  to ensure the compliant member  21  appropriately contacts lid  16  and ground pad  18  during the attachment of lid  16  to chip  11  since the placement of compliant member  21  and the soldering of compliant member  21  to lid  16  and ground pad  18  is achieved subsequent to and separate from the process of attachment of cover  16  to chip  11 . Furthermore, there is no adverse requirement, for example, to blindly align compliant member  21  to ground pad  18  from the bottom surface of cover  16  as the connection is visible from the top of opening  20 .  
         [0025]    Pre-fabricated openings in cover  16  have been implied in the above description to illustrate the concepts of the present invention. With reference to FIG. 5, these openings may be of any shape or partial shape hole such as a slot  22 , round opening  23 , partial round hole  24 , angles slot  25  or a hole  26  which is countersunk that is formed or created in lid  16  prior to attachment of lid  16  to chip  11  as described above. These openings or holes may be achieved by drilling, stamping, pre-forming or any other means known in the art. Any one or more of these shapes for the openings or holes in lid  16  may be used as may be appropriate in order to implement the present invention. This flexibility permits the advantages of affording superior control in the creation of the connection between cover  16  and ground pad connection  18  which is separate from the process of attaching lid  16  to chip  11 .  
         [0026]    Some examples of material compositions for cover or lid  16  will now be provided, but these examples are not limiting as would be familiar to those of ordinary skill in manufacture of electronic packages. Cover  16  may consist of copper (Cu) and in particular, oxygen-free electronic (OFE) grade pure copper. The copper may be plated with nickel (Ni) and in addition have selectively solderable surfaces inside the openings  20  of either the copper of the original material comprising cover  16  or some other deposited material such as gold (Au), palladium (Pd) or silver (Ag). As another alternative cover  16  could be made of aluminum (Al) with all surfaces anodized except for the surfaces of openings  20  which would be coated with an electrically conductive and optionally solderable material.  
         [0027]    Any appropriate adhesives  17  and  19  that are known to those of ordinary skill in this art could be suitably used in the various described embodiments of the invention. The invention as described provides for significant latitude in the choice of adhesive  19  including wider volume range and viscosity properties. Examples of compositions for the electrically conductive adhesive  19  would consist of combinations of the following components 1, 2 and 3:  
         [0028]    1. Polymers from one or more of the following families:  
         [0029]    Epoxy  
         [0030]    Acrylate  
         [0031]    Polyester  
         [0032]    Silicone  
         [0033]    2. Electrically conductive particles consisting of one or more of the following materials:  
         [0034]    Silver  
         [0035]    Silver plated copper  
         [0036]    Silver plated aluminum  
         [0037]    Carbon  
         [0038]    3. Low molecular weight molecules as hardeners, crosslinkers and viscosity regulators such as:  
         [0039]    epoxides  
         [0040]    silanes  
         [0041]    amines  
         [0042]    organic solvent  
         [0043]    PDMS  
         [0044]    Compliant member  21  could be a spring material made of berillium copper (BeCu) which could be obtained from suppliers including Brush-Wellman Inc., Fremont, Calif. and NGK Metals Corporation, Sweetwater, Tenn.  
         [0045]    It will be understood from the foregoing description that various modifications and changes may be made to the preferred embodiment of the present invention without departing from its true intent and spirit. It is intended that this description is for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the claims which follow.