Patent Publication Number: US-6655020-B2

Title: Method of packaging a high performance chip

Description:
This application is a divisional of Ser. No. 09/041,580, filed on Mar. 11, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to semiconductor chip packaging and carriers. More particularly, the present invention relates to a chip packaging with a cover plate that is held onto a stiffener by an interposed bonding material. The present invention also includes an alignment device for the cover plate and a method of packaging a chip. 
     2. Related Art 
     Heretofore, adhesives or mechanical structures have been used to attach cover plates to stiffeners on chip packaging. The use of adhesives create a number of problems. For instance, the use of adhesives provides poor tolerances because of uneven layering because of a failure of the adhesives to evenly or uniformly coat surfaces of a cover plate and stiffener. Precise tolerances are required because the pressures created by the adhesives being unevenly layered creates pressures on the chip, stiffener and cover plate that can create cracking. Further potential for cracking is developed by the need to repeatedly heat the package. For example, heating is required to connect the chip to the carrier, then again for connecting the stiffener and cover plate, and then again to connect the carrier to other circuitry like a printed circuit board. Poor tolerances also can create large gaps between the cover plate and stiffener resulting in poor thermal performance. To address the above problems, cover plates and stiffeners must be very accurately sized. However, this requirement adds complexity to the manufacturing process, i.e., the need to accurately form the parts and exactly locate them for joining. The need to compensate for poor tolerances also necessitates the addition of material and, hence, weight to the cover plate and the overall package. 
     The use of mechanical connectors encounters similar problems as to those outlined above. Another disadvantage is that mechanical connectors add more undesirable weight to the package. 
     For the above reasons, there is a need in the art to have a chip package, and process of manufacture therefor, where a cover plate that is more accurately attached to a stiffener to achieve lower weight and reduced potential for die crack initiation. Further, there is, as always, a need to increase thermal performance. 
     SUMMARY OF THE INVENTION 
     In a general aspect in accordance with the present invention is provided a chip package having a chip carrier, a stiffener attached to the carrier, and a cover plate attached to the stiffener by a reflowable bonding material. The bonding material is preferably a solder material. This aspect provides a number of advantages. For instance, the need to precisely size parts is greatly reduced in that the cover plate is attached to the stiffener in such a way that it self-centers itself within the stiffener when the reflowable bonding material is heated. As a result, the potential for chip cracking is reduced. Further, because the cover plate need not be so precisely sized, lower weight can be obtained for the cover plate and the overall package. There is also an increase in thermal performance not exhibited in the related art devices because of the better cover plate to stiffener connection. 
     The above described aspect may also advantageously include an interface material attaching the cover plate and the chip itself. This further aids in thermal conductivity and allows for setting the cover plate at an advantageous predetermined electrical potential, e.g., ground, if the interface material is electrically conductive. 
     In a second general aspect in accordance with the present invention is provided a chip carrier including: a stiffener, a cover plate to cover the stiffener, and an alignment device to aid in properly aligning the cover plate on the stiffener. This aspect aids in properly aligning the cover plate on the stiffener to prevent die (or chip) cracking and assure even dispersion of the bonding material. 
     In a third general aspect in accordance with the present invention is provided a method of packaging a chip, the packaging having a carrier, a stiffener having an opening in which the chip is positioned on the carrier, and a cover plate to cover the opening, the method comprising the steps of: providing a reflowable material between the cover plate and stiffener for attaching the cover plate to the stiffener, and attaching the cover plate to the stiffener simultaneously with an attaching of the carrier to other circuitry. This aspect provides a process of manufacturing a chip package with fewer heating cycles and, thus, the potential for reduced die (or chip) crack initiation in the package. 
     The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
     FIG. 1 shows an exploded cross-sectional side view of a chip package in accordance with a first embodiment of the present invention; 
     FIG. 2A shows a cross-sectional side view of a chip package in accordance with the first embodiment of the present invention; 
     FIG. 2B shows a cross-sectional side view of a chip package with an enlarged cover plate in accordance with the first embodiment of the present invention; 
     FIG. 3 shows an exploded cross-sectional side view of a chip package in accordance with a second embodiment of the present invention; 
     FIG. 4 shows a cross-sectional side view of a chip package in accordance with the second embodiment of the present invention; 
     FIG. 5 shows a side view of a cover plate in accordance with a third embodiment of the present invention; 
     FIG. 6 shows a plan view of a stiffener in accordance with the third embodiment of the present invention; and 
     FIG. 7 shows a plan view of a chip package in accordance with the third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although certain preferred embodiments of the present invention will be shown and described in detail, it should be understood that various changes and modifications may be made without departing from the scope of the appended claims. The scope of the present invention will in no way be limited to the number of constituting components, the materials thereof, the shapes thereof, the relative arrangement thereof, etc., and are disclosed simply as an example of the preferred embodiment. 
     Referring to the drawings, FIG. 1 shows an exploded cross-sectional side view of a chip package in accordance with a first embodiment of the present invention. The chip package  10  generally includes a cover plate  20 , a stiffener  30 , a chip  40  and a carrier  50 . The stiffener  30  includes a base section  32  with a chip opening  37  provided therein. The opening  37  is provided to house a chip  40 . The base section  32  may also include risers  34 ,  36  that aid in generally positioning the cover plate  20  within the opening  37 , the advantages of which will be discussed below. The stiffener  30  is attached to a carrier  50 , in the form of a plate  52 , and acts to rigidify the plate  52 . The chip  40  generally includes a circuitry portion  42  and a plurality of connectors  44 , e.g., bumping or controlled collapse chip connecting (C 4 ), extending therefrom for connection to the carrier  52 . The carrier  52  may also include circuitry (not shown) along a surface thereof to which the chip  40  is connected and a ball grid array  54  (BGA) for connection to other circuitry on a substrate  80 , e.g., an electronic circuit board. 
     The cover plate  20  is constructed to have a base portion  21  and may also include a raised area or pedestal  22 . The stiffener  30  and cover plate  20  are made of metals, e.g., copper, stainless steel, aluminum, etc. or alloys thereof. The base portion  21  of the cover plate is sized to cover the opening  37  so as to close the opening  37 . If the pedestal  22  is used, the pedestal  22  is sized to fit into the opening  37  to further close off the opening. However, it is important to note that the cover plate  20 , as shown in FIG. 2B, need not include the raised area  22 . Further, the cover plate may be altered in size as necessary. For instance, the cover plate  20  may cover not only the opening  37  but also the entire stiffener  30 , as shown in FIG.  2 B. In this instance, the cover plate  20  may require alteration from that of FIG.  1 . In particular, the cover plate  20  may require openings or grooves  23  to accommodate the shape of the risers  34 ,  36 , the benefits of which will be discussed infra. 
     In accordance with the present invention, and as shown in FIGS. 2A and 2B, the cover plate  20  is attached to the stiffener  30  by a thermally conductive bonding or reflowable material  70 ,  72 . The bonding material  70 ,  72  may take a variety of forms but must be able to bond the cover plate  20  to the stiffener  30  so as to provide good thermal conductivity therebetween. For instance, an adhesive or low melt solder may be used, with solder materials being most preferable. Another possibility is low melt solder paste which has been screened on to one of the components. In terms of more particular substances, an adhesive that has been used is a silicone-based material manufactured by General Electric, Co. and sold as GE 3281®. In terms of low melt solder or low melt solder paste, tin lead based materials have been used. The general feature of the .low melt solder. is that it be highly thermally conductive and readily flowable (reflowable) when heated so as to uniformly coat the surfaces between the stiffener  30  and cover plate  20  when heated. General features of other materials, such as GE 3281®, are thermally conductive and conformable when assembled. 
     The reflowable bonding material  70 ,  72  may be initially located along the un-raised portion  23  of the cover plate, if a raised area  22  is provided, or along an edge  33  of the opening  37 . However, in order to attach the cover plate  20  to the stiffener  30 , the material  70 ,  72  must be heated to melt or cure the reflowable bonding material. This step is completed after the heating step to connect the chip  40  to the carrier  52 , and during or before connecting the carrier  52  to other circuitry, e.g., to a substrate  80  of, for example, an electronic circuit board. The chip  40  is connected to the carrier  52  by the plurality of connectors  44 . The temperature for the process is preferably in the range of 180 to 240 degrees Centigrade. 
     A further advantage of the present invention is that as the reflowable bonding material  70 ,  72  melts and flows it allows the cover plate  20  to self-center within the opening  37 . As a result, the bonding material  70 ,  72  is more evenly layered around the cover plate and, hence, pressures which can create cracking in the chip  40  are reduced. 
     As an alternative, the cover plate  20 , and in particular the raised area  22  if provided, may also be attached to the top of the chip  40  by an interface material  74 . This allows for more heat conductivity away from the chip  40  and enhanced thermal performance overall. Further, if connection of the cover plate  20  to the chip  40  is chosen, the interface material  74 , and if desired reflowable bonding materials  70 ,  72 , may also be electrically conductive. The provision of electrically conductive material  74  (and, if desired, material  70 ,  72 ) allows the cover plate to be set to an advantageous predetermined electrical potential with regard to the chip  40 . For instance, the cover plate may be grounded thus providing an additional safeguard for the chip  40  from such things as static shock. In terms of the type of interface material used, the material may take a variety of forms. For instance, the interface material  74  may be low melt solder paste, an adhesive such as the silicone-based material discussed above, and thermal grease, e.g., zinc oxide based material. If the interface material is to be electrically conductive, then low melt solder paste, such as a tin lead based material has been found to be preferable. 
     Referring to FIGS. 3 and 4, a second embodiment of the present invention in which the risers  34 ,  36  are removed. In this embodiment, when the materials  170 ,  172  and  174  are heated to melt, the cover plate  120  more freely self-centers in the opening  137 . As a result, the raised area  122  is centered within opening  137  such that an even layer of material exists between an inside portion  125  of the raised area  122  and an inner edge  139  of the opening  137 . It is important to note as with the first embodiment, that the particular size of the cover plate  120  may be altered to cover more or less of the stiffener  130  as necessary. 
     FIGS. 5-7 show a third embodiment of the present invention. In this embodiment the stiffener and cover plate combination are provided with an alignment device  290  that aids in properly aligning the cover plate  220  on the stiffener  232 . The cover plate  220  may include the raised area  222  including bonding material  274  thereon to attach to the chip. The stiffener includes an opening  237  for housing the chip  240 . 
     The alignment device  290  generally includes a plurality of posts  238  extending from the stiffener and a plurality of matching recesses or openings  224  in the cover plate  220 . The cover plate  220  is attached to the stiffener  232  by a plurality of reflowable bonding material rings or doughnuts  276 . The rings  276  may be provided either on the stiffener  232 , or the cover plate  220  if enough surface tension exists to hold the rings  276  to the cover plate  220 . In either position, the rings will eventually encircle or surround the posts  238  for proper attachment. An advantage of the rings  276  is that they may be preformed and then located around the posts  238  as necessary. 
     When the package is heated, the rings  276  melt and attach the cover plate  220  to the stiffener  232 . Once again, the rings  276  and interface material  274  may be made from the materials as outlined above except that it may no longer be efficient to screen on low melt solder paste around the posts  238  for connection of the cover plate  220  and stiffener  232 . Further, as noted above, the materials  270 ,  272 ,  274  may be electrically conductive. 
     The alignment device  290  allows for more precise positioning of the cover plate  220  relative to the stiffener  232  which aids in creating better tolerances, the reduction of uneven layering and crack prevention. It is important to note, however, that particular geometry of posts  238  and openings  224  may be altered as the alignment device may take a variety of forms. For instance, the posts  238  and openings  224  may be located at the corners of the cover plate  220 . Further, there need not be four posts  238  and openings  224 . As shown in FIG. 2B, an alignment device may not require any openings that extend through the cover plate  20 . Further, the cover plate need not require any openings, as shown in FIG. 1, as the alignment device may be provided by the provision of risers  34 ,  36  which direct the cover plate  20  to the desired position. 
     In terms of the method in accordance with the present invention, the chip  40  to carrier  50  connection step is preferably performed prior to connection of the cover plate  20  to the stiffener  30  or the carrier  50  to other circuitry. Once completed, in accordance with the present invention, the stiffener  30  and cover plate  20  connecting step is preferably completed simultaneously with the connection of the carrier  50  to other circuitry, e.g., on a substrate  80  of electronic circuit board. Further, if the cover plate  20  is to be connected to the chip  40 , then this step may also be provided simultaneously with the carrier  50  and other circuitry connecting step. The ability to make these connections simultaneously reduces the number of heating steps required to form a chip package and, hence, the potential for die (or chip) crack initiation. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims. For example, the cover plate could also be attached after chip bonding and encapsulation and prior to attachment of the carrier to the electronic circuit board.