Abstract:
A leadless plastic chip carrier has a plurality of die attach pads on which a singulated semi-conductor die is mounted. At least one row of contact pads circumscribes the plurality of die attach pads and a power/ground ring is intermediate the contact pads and the die attach pads. Wire bonds connect the semiconductor die, the contact pads and the power/ground ring. An overmold covers the semi-conductor die, the die attach pads, the power/ground ring and the contact pads such that each of the die attach pads and the contact pads has one exposed surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Division of U.S. patent application Ser. No. 09/802,679 filed Mar. 9, 2001, now U.S. Pat. No. 6,635,957, which is a continuation-in-part of U.S. patent application Ser. No. 09/288,352, filed Apr. 8, 1999, now U.S. Pat. No. 6,498,099, which is a continuation-in-part of U.S. patent application Ser. No. 09/095,803, filed Jun. 10, 1998, now U.S. Pat. No. 6,229,200. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to integrated circuit packaging, and more particularly to an improved process for fabricating a leadless plastic chip carrier which includes a post mold each back step and a unique die attach pad design. 
     BACKGROUND OF THE INVENTION 
     According to well known prior art IC (integrated circuit) packaging methodologies, semiconductor dice are singulated and mounted using epoxy or other conventional means onto respective die pads (attach paddles) of a leadframe strip. Traditional QFP (Quad Flat Pack) packages incorporate inner leads which function as lands for wire bonding the semiconductor die bond pads. These inner leads typically require mold locking features to ensure proper positioning of the leadframe strip during subsequent molding to encapsulate the package. The inner leads terminate in outer leads that are bent down to contact a mother board, thereby limiting the packaging density of such prior art devices. 
     In order to overcome these and other disadvantages of the prior art, the Applicants previously developed a Leadless Plastic Chip Carrier (LPCC). According to Applicants&#39; LPCC methodology, a leadframe scrip is provided for supporting up to several hundred devices. Singulated IC dice are placed on the strip die attach pads using conventional die mount and epoxy techniques. After curing of the epoxy, the dice are gold wire bonded to peripheral internal leads. The leadframe strip is then molded in plastic or resin using a modified mold wherein the bottom cavity is a flat plate. In the resulting molded package, the die pad and leadframe inner leads are exposed. By exposing the bottom of the die attach pad, mold delamination at the bottom of the die paddle is eliminated, thereby increasing the moisture sensitivity performance. Also, thermal performance of the IC package is improved by providing a direct thermal path from the exposed die attach pad to the motherboard. By exposing the leadframe inner leads, the requirement for mold locking features is eliminated and no external lead standoff is necessary, thereby increasing device density and reducing package thickness over prior art methodologies. The exposed inner leadframe leads function as solder pads for motherboard assembly such that less gold wire bonding is required as compared to prior art methodologies, thereby improving electrical performance in terms of board level parasitics and enhancing package design flexibility over prior art packages (i.e. custom trim tools and form tools are not required). These and several other advantages of Applicants&#39; own prior art LPCC process are discussed in Applicants&#39; co-pending patent application Ser. No. 09/095,803, the contents of which are incorporated herein by reference. 
     Applicants&#39; LPCC production methodology utilizes saw singulation to isolate the perimeter I/O row as well as multi-row partial lead isolation. Specifically, the leadframe strip is mounted to a wafer saw ring using adhesive tape and saw-singulated using a conventional wafer saw. The singulation is guided by a pattern of fiducial marks on the bottom side of the leadframe strip. Also, special mold processing techniques are used to prevent the mold flow from bleeding onto the functional pad area and inhibiting electrical contact. Specifically, the exposed die pad surface is required to be deflashed after molding to remove any molding compound residue and thereby allow the exposed leads and die attach pad to serve as solder pads for attachment to the motherboard. 
     According to Applicants&#39; co-pending, U.S. patent application Ser. No. 09/288,352, the contents of which are incorporated herein by reference, an etch back process is provided for the improved manufacture of the LPCC IC package. The leadframe strip is first subjected to a partial etch on one or both of the top and bottom surfaces in order to create a pattern of contact leads (pads) and a die attach pad (paddle). A wire bonding the contacts to a singulated semiconductor die, followed by overmolding and curing of the mold, the leadframe strip is exposed to a second full etch immersion for exposing the contact pads in an array pattern (i.e. multi-row) or perimeter pattern (i.e. single row), as well as the die attach pad. In the case of a package with multi-row I/O leads, this etch back step eliminates the requirement for two additional saw singulation operations (i.e. to sever the inner leads from the outer leads), and in both the single-row and multi-row configurations, the etch back step eliminates post mold processing steps (e.g. mold deflashing) and ensures superior device yield over the processing technique set forth in Applicants&#39; prior application Ser. No. 09/095,803. Additionally, using this technique allows for higher I/O pad density and also allows for pad standoff from the package bottom which reduces stress in the solder joint during PCB temp cycling. Further, the technique allows for the use of a pre-singulation strip testing technique given that the electrical I/O pads are now isolated from each other and testing in strip can take place. This feature greatly increased the handling and throughput of the lest operation. 
     In Applicant&#39;s co-pending U.S. application for a Leadless Plastic Chip Carrier With Etch Back Pad Singulation, filed concurrently herewith, the contents of which are incorporated herein by reference, the etch-back LPCC process of Applicants&#39; copending U.S. patent application Ser. No. 09/288,352 is modified to provide additional design features. 
     The leadframe strip is selectively covered with a thin layer photo-resist mask in predetermined areas. Following the application of the mask, an etch-barrier is deposited as the first layer of the contact pads and die attach pad, followed by several layers of metals which can include for example, Ni, Cu, Ni, Au, and Ag. This method of formation of the contact pads allows plating of the pads in a columnar shape and into a “mushroom cap” or rivet-shape as it flows over the photoresist masks. The shaped contact pads are thereby locked in the mold body, providing superior board mount reliability. Similarly, the die attach pad can be formed in an interlocking shape for improved alignment with the die. The photo-resist mask is then rinsed away and the semiconductor die is mounted to the die attach pad. This is followed by gold wire bonding between the semiconductor die and the peripheral contact pads and then molding as described in Applicant&#39;s application Ser. No. 09/095,803. The leadframe is then subjected to full immersion in an alkaline etchant that exposes a lower surface of an array of the contact pads, a power ring and the die attach pad, followed by singulation of the individual unit from the full leadframe array strip. 
     During mounting of the integrated circuit package to a printed circuit board, solder alloy an the exposed contact pads and die attach pad tend to ball up due to surface tension that can cause the integrated circuit package to lift away from the circuit board, from the center. In the case where the die attach pad is much larger than the contact pads, solder fillet of the contact pad can be weakened or broken. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a new leadless plastic chip camer is provided. The leadless plastic chip carrier has a plurality of die attach pads on which a singulated semi-conductor die is mounted. At least one row of contact pads circumscribes the plurality of die attach pads and power/ground ring is intermediate the contact pads and the die attach pads. Wire bonds connect the semiconductor die, the contact pads and the power/ground ring. An overmold covers the semiconductor die, the attach pads, the power/ground ring and the contact pad such that each of the die attach pads and the contact pads has one exposed surface. According to a further aspect of the invention, each of the plurality of die attach pads are separated by a photo-imageable mask. The plurality of die attach pads reduces the maximum stress created as a result of the mismatch of thermal expansion coefficients between the die and the die attach pads when compared to the stress created when employing a single, larger die attach pad. 
     According to another aspect of the present invention, a new process for fabricating a leadless plastic chip carrier is provided. The process has the steps of: depositing a photo-imageable mask on a first surface of a leadframe strip; imaging and developing the mask to define a plurality of die attach pads, at least one row of contact pads and a power/ground ring intermediate the die attach pads and the contact pads; depositing a plurality of layers on portions of the first surface exposed by the imaging and the developing for creating the at least one row of contact pads, vile power/ground ring and the plurality of die attach pads; mounting the semi-conductor die to the die attach pads on a top surface thereof, wire bonding the semiconductor die to the contact pads; encapsulating the mask, the layers, and the die between a moulding compound and the first surface of the leadframe strip; etching back a bottom surface of the leadframe strip for exposing the contact pads and the die attach pads; and singulating the leadless plastic chip carrier from the leadframe strip. 
     According to yet another aspect of the present invention, a new process for fabricating a leadless plastic chip carrier is provided. The process having the steps of: applying a layer of photo-resist on a first surface of a leadframe strip; exposing and developing the layer of photo-resist to define a plurality of die attach pads, at least one row of contact pads and a power/ground ring intermediate the die attach pads and the contact pads between a remainder of the photo-resist; depositing a plurality of layers on portions of the first surface exposed by the exposing and the developing for creating the at least one row of contact pads, the power/ground ring and the plurality of die attach pads; stripping the remainder of the photo-resist; depositing a photo-imageable mask on the first surface and the plurality of die attach pads; imaging and developing the mask to expose a top surface of the plurality of die attach pads; mounting the semi-conductor die to the die attach pads on the top surface; wire bonding the semiconductor die to the contact pads; encapsulating the mask, the layers, and the die between a moulding compound and the first surface of the leadframe strip, etching back a bottom surface of the leadframe strip for exposing the contact pads and the die attach pads; and singulating the leadless plastic chip carrier from the leadframe strip. 
     It is an advantage of an aspect of the present invention to mitigate internal stress between the die and the die attach pad resulting from differences in the coefficient of thermal expansion. 
     It is a further advantage of an aspect of the present invention to simplify printed circuit board assembly mounting. Providing an array of die attach pads rather than a single large die attach pad reduces lining of the integrated circuit package caused by surface tension of the solder. Also, edge surface tension aids in self-alignment of th integrated circuit package on assembly with the printed circuit board. The force caused by edge surface tension is greater with an array of smaller pads rather than a single large pad. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood with reference to the drawings in which: 
         FIGS. 1A-1I  show processing steps for manufacturing a Leadless Plastic Chip Carrier (LPCC) with etch back and a plurality of die attach pads according to an embodiment of the present invention. 
         FIGS. 2A-2K  show processing steps for manufacturing a Leadless Plastic Chip Carrier (LPCC) with etch back and a plurality of die attach pads according to another embodiment of the present invention. 
         FIGS. 3A-3K  show processing steps for manufacturing a Leadless Plastic Chip Carrier (LPCC) with etch back and a plurality of die attach pads according to yet another embodiment of the present invention. 
         FIG. 4  is a bottom view of an IC package manufactured in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention relates to an improvement over Applicant&#39;s co-pending application for a Leadless Plastic Chip Carrier With Etch Back Pad Singulation (LPCCEBS), filed concurrently herewith. Where possible, the same reference numerals have been used in this application to denote identical features described in Applicant&#39;s above-noted LPCCEBS application. 
     Referring to  FIG. 1A , an elevation view is provided of a Cu (copper) panel substrate which forms the raw material of the leadframe strip indicated generally by the numeral  100 . As discussed in greater detail in Applicants&#39; co-pending application Ser. No. 09/095,803, the leadframe strip  100  is divided into a plurality of sections, each of which incorporates a plurality of leadframe units in an array (e.g. 3×3 array, 5×5 array, etc.). Only one such unit is depicted in the elevation view of  FIG. 1A , portions of adjacent units being shown by stippled lines. 
     Referring to  FIG. 1B , an upper surface of the leadframe strip  100  is coated with a layer of photo-imageable solder mask such as a photo-imageable epoxy. 
     Next, the layer of photo-imageable solder mask is imaged with a photo-tool. This is accomplished by exposure of the photo-imageable solder mask to ultraviolet light masked by the photo-tool, as shown in  FIG. 1C , and subsequent developing of the solder-mask, as shown in FIG.  1 D. The solder mask is thereby patterned to provide pits in which an upper surface of the Cu substrate is exposed. 
     As shown in  FIG. 1E , several layers of metals are deposited on the upper surface of the exposed copper substrate to form a plurality of die attach pads  202 , a plurality of contact pads  203  and a power or ground ring  204 . Different deposition options are provided. 
     According to options A-1 and A-2, a layer of flash Cu (for example, 50 microinches) is provided over the Cu substrate for creating an etch down cavity  203 B following post etching as will be discussed further below. The etch down cavity  203 B for attaching solder balls  203 A will also be discussed further below. An etch barrier layer of Au (gold) (for example, 20 microinches) is then deposited, followed by layers of Ni (nickel) (for example, 40 microinches), and Cu (3-4 mils). According to option A-1, final layers of Ni (for example, 40 microinches) and Au (for example, 20 microinches) are deposited whereas in Option A-2 a final layer of Ag (silver) is deposited (for example, 100-300 microinches). 
     In plating Options B-1 and B-2, the initial flash Cu deposition is omitted and the layer of etch barrier Au is deposited directly on the Cu substrate. The Au layer is followed by layers of Ni, Cu, Ni, and Au in Option B-1 and Ni, Cu, and Ag in option B-2. 
     In Options C-1 and C-2 the etch barrier of Au and subsequent Ni deposition are replaced by an etch barrier of Sn (for example, 100-300 microinches) deposited on the Cu substrate. The Sn layer is followed by layers of Cu, Ni and Au in option C-1 and Cu and Ag in option C-2. 
     Referring now to  FIG. 1F , a singulated semiconductor die  206  is conventionally mounted via epoxy (or other suitable means) to the plurality of die attach pads  202 , and the epoxy is cured. Note that in Applicant&#39;s Leadless Plastic Chip Carrier With Etch Back Pad Singulation, filed concurrently herewith, a single die attach pad was formed. A plurality of die attach pads  202  such as an array of smaller pads can result in reduced maximum stresses caused by the difference in thermal expansion coefficients between the die  206  and the die attach pads  202  as compared to those stresses resulting from a the single, larger die attach pad. 
     Gold wires  205  are then bonded between the semiconductor die  206  and the contact pads  203 . The leadframe  100  is then molded using a modified mold with the bottom cavity being a flat plate, and subsequently cured, as discussed in Applicants&#39; application Ser. No. 09/095,803. The leadframe  100  after the foregoing steps is shown in  FIG. 1G , which includes overmold  401  of cured plastic or epoxy (0.8 mm). 
     The leadframe  100  is then subjected to a final alkaline etching via full immersion which exposes an array of exposed contact pads  203 , power/ground ring  204  and the die attach pads  202 , shown in FIG.  1 H. According to Option A, the etch down cavity  203 B is left after etching away the flash Cu. This cavity is used for attachment of the solder balls  203 A to the contact pads  203 . 
     At this stage of manufacture, the power/ground ring  204  and the die attach pads  202  are fully isolated and exposed. 
     Singulation of the individual units from the full leadframe array strip  100  may them be performed either by saw singulation or die punching resulting in the final configuration of FIG.  11 . Since the entire LPCC contains short circuit connections prior to singulation, it is contemplated that the multiple circuits may be gang tested before singulation. 
     Referring now to  FIGS. 2A-2K , there is provided processing steps for manufacturing a LPCC with etch back and a plurality of die attach pads according to another embodiment of the present invention. 
     Referring to  FIG. 2A , similar to  FIG. 1A , an elevation view is provided of a Cu panel substrate of the leadframe strip indicated generally by the numeral  100 . Again, only one such unit is depicted in the elevation view of  FIG. 2A , portions of adjacent units being shown by stippled lines. 
     Next, rather than coating with a layer of photo-imagable solder mask, a dry film layer of photo-resist is applied to the upper surface of the lead frame strip, as shown in FIG.  2 B. The photo-resist is then patterned with ultraviolet light exposure to through a photo-tool as shown in FIG.  2 C.  FIG. 2D  shows the patterned photo-resist after developing. The photo-resist thereby provides pits in which an upper surface of the Cu substrate is exposed. 
     Referring to  FIG. 2E , several layers of metals are then deposited on the upper surface of the exposed copper substrate to form a plurality of die attach pads  202 , a plurality of contact pads  203  and a power/ground ring  204 . Different deposition options are provided, as described in derail in the above-described embodiment. The photo-resist is then stripped and the structure is cleaned resulting in a pattern of die attach pads  202 , contact pads  203 , and power/ground ring  204 , as shown in FIG.  2 F. 
     Next, the upper surface of the leadframe strip  100 , including the die attach pads  202 , contact pads  203 , and power/ground ring  204 , is coated with a layer of photo-imageable solder mask. The resulting solder mask layer can be thicker than the die attach pads  202 , contact pads  203 , and power/ground ring  204 , as shown in FIG.  2 G. 
     The layer of photo-imageable solder mask is then imaged by exposure to ultraviolet light using a photo-tool, at shown in FIG.  2 H. The solder mask is subsequently developed, resulting in die attach pad slots  202 A in the solder-mask that expose an upper surface of the die attach pads  202 , as shown in FIG.  2 I. The solder mask around the contact pads  203  can be fully exposed to ultraviolet light and developed, thus resulting in columns of contact pads  203  on the Cu substrate. 
     Referring now to  FIG. 2J , a singulated semiconductor die  206  is conventionally mounted via epoxy (or other suitable means) to the plurality of die attach pads  202 , in the die attach pad slots  202 A and the epoxy is cured. 
     As in the above-described embodiment, gold wires  205  are then bonded between the semiconductor die  206  and the contact pads  203 . The leadframe  100  is then molded using a modified mold with the bottom cavity being a flat plate, and subsequently cured. The leadframe  100  is then subjected to a final alkaline etching via full immersion which exposes an array of exposed contact pads  203 , power/ground ring  204  and die attach pads  202 . 
     Singulation of the individual units from the full leadframe array strip  100  may then be performed either by saw singulation or die punching resulting in the final configuration of Figure  2 K. 
     Referring now to  FIGS. 3A-3K , there is provided processing steps for manufacturing a LPCC with etch back and a plurality of die attach pads according to yet another embodiment of the present invention. 
     Steps 1 to 7 of  FIGS. 3A  to  3 G are substantially identical to Steps 1 To 7 of  FIGS. 2A  to  2 G as described above. 
     The layer of photo-imageable solder mask is then imaged by exposure to ultraviolet light using a photo-tool, as shown in FIG.  3 H. The solder mask is subsequently developed, resulting in die attach pad slots  202 A in the solder-mask that expose an upper surface of the die attach pads  202 , as shown in FIG.  3 I. Similarly, slots in the solder-mask expose the upper surface of the contact pads  203  and the power/ground ring  204 . It will be understood that this embodiment differs from that shown in  FIG. 2I  as th solder-mask layer extends to the edges of the panel  100  in the present embodiment. 
     Referring now to  FIG. 3J , a singulated semiconductor die  206  is conventionally mounted to the plurality of die attach pads  202 , via epoxy (or other suitable means) in the die attach pad slots  202 A and the epoxy is cured. 
     As described in the prior embodiments, gold wires  205  are then bonded between the semiconductor die  206  and the contact pads  203 . The leadframe  100  is then molded using a modified mold with the bottom cavity being a flat plate, and subsequently cured. The leadframe  100  is then subjected to a final alkaline etching via full immersion which exposes an array of exposed contact pads  203 , power/ground ring  204  and die attach pads  202 . 
     Singulation of the individual units from the full leadframe array strip  100  may then be performed either by saw singulation or die punching resulting in the final configuration of FIG.  3 K. 
       FIG. 4  is a bottom plan view of the assembled IC package according to the present invention, with an array of rectangular die attach pads  202 , contact pads  203  and power/ground ring  204 . It is contemplated that the IC package can also have an array of round die attach pads  202 , contact pads  203  and power/ground ring  204 . 
     Other embodiments of the invention are possible. For example, the thickness of each of the metal layers deposited in step  5  of  FIGS. 1E ,  2 E and  3 E are given for exemplary purposes only and can vary, as will occur to those of skill in the an. All such embodiments are believed to be within the scope of the present invention as set forth in the claims appended hereto.