Abstract:
Provided is a lead frame package with stand-off legs to prevent the die attach pad (DAP), which is part of the package substrate, to tilt or shift from its original position during the molding process. Also provided are methods for assembling such lead frame packages into various integrated circuit (IC) packages. Compared to conventional lead frame packages without stand-off legs, the lead frame packages of the present invention have less aesthetic and functional defects, thereby leading to an increase in product reliability and yield.

Description:
BACKGROUND 
     This invention relates generally to integrated circuit (IC) packages, and specifically to leadframe packages and corresponding methods for their assembly. 
     The forming of IC packages usually require molding equipment that encapsulates an IC die in a mold compound (for example, epoxy resin) to protect it from environmental factors such as dust, heat, moisture, static electricity, and mechanical shocks. The leadframe is the foundation of the molded IC package. The two most common leadframe metals employed by the industry are nickel-iron alloy and copper alloy. 
     A leadframe package may use a wire bond package substrate for mounting an IC die. In a wire bond package substrate, wire bond connections provide the electrical paths for power and signal distribution from the package substrate to an IC die. Lead fingers connect the IC package to other IC packages or the printed circuit board. A wire bond package substrate typically comes with a die-attach pad (DAP) for mounting the die. 
     Typically, a mold set used in a molding equipment to assemble IC packages has two halves—a top half and a bottom half. These opposing halves open to receive the package substrate, and close during the molding cycle. Opposing halves that have been mated together during the molding cycle form mold cavities. Gates are small openings into these cavities through which the mold compound is injected. 
     Unpredictable variations in the molding process, composition of the mold compound, and the molding equipment itself sometimes cause the mold compound to flow at different rates into the top and bottom cavities of a mold set. The uneven flow of mold compound often cause the DAP to tilt or shift from its original position, resulting in an exposed DAP, exposed wires, stressed wires, package breaks, cracks, or device failures. The appearance of exposed DAP or wires on the surface of the package is also aesthetically undesirable. Functional failures or cosmetic defects tend to become more significant during the formation of slim packages such as Quad Flat Pack (QFP), Thin QFP (TQFP), and Low-profile QFP (LQFP) packages or during the formation of packages with stacked or multiple dice. In slim and stacked-die packages, the cavities for the flow of mold compound are smaller; thus, these packages are more susceptible to failures or defects caused by imbalances in the mold flow. 
     One type of leadframe package is shown in  FIG. 1 , which shows a cross-sectional view of a typical leadframe package  100 . A leadframe package substrate  102  includes a DAP  104  and corresponding lead fingers  106  disposed about DAP  104 . During assembly of leadframe package  100 , die  108  is mounted onto DAP  104  with an adhesive (e.g., die attach epoxy). Die  108  is then electrically connected to lead fingers  106  using wire bond connections  112 . Generally, DAP  104  is sized to support die  108 . In one embodiment, DAP  104  is sized to include a surface area larger than that of die  108 . As such, an exposed portion of DAP  104  is available around the attached die  108 . Molding cap  110 , which is made out of molding compound, is formed over die  108 , DAP  104 , wire bond connections  112 , and at least a portion of lead fingers  106 . The other portion of lead fingers  106  is left extending outside of molding cap  110  to establish communication with other components, for example, a printed circuit board, when the IC package is mounted onto the printed circuit board. 
     The injection of mold compound into an IC package during the molding process can lead to several undesirable effects in conventional IC packages. For example, an imbalanced mold flow can cause one side of the DAP to tilt downwards and the other side of the DAP to tilt upwards. The tilting of the DAP increases the height of one side of the DAP relative to the floor of the mold cavity, resulting in disproportionate vertical heights for the tilted sides of the DAP relative to the floor. An imbalanced mold flow may occur, for example, when the mold compound flows faster into one side of the mold cavity compared to the other side. 
     During the molding process, the flow of mold compound into the mold cavity depends on several process parameters such as the preheat time of the molding equipment, mold temperature, mold transfer pressure, and the speed of mold flow. Even a slight variation in the process parameters may cause an imbalanced mold flow, which in turn may cause the DAP to tilt or move. As such, to prevent the DAP from tilting or moving, the window for variation in the process parameters must be kept small and meticulously adhered to. 
       FIG. 2  shows leadframe package  200  where DAP  204  is tilted to one side of the leadframe package. As shown in  FIG. 2 , the tilting of DAP  204  causes die  208  that is mounted onto DAP  204  to tilt as well. Furthermore, wire bond connections  212  are stressed and might protrude out of molding cap  210  or break apart if the degree of tilting increases. Stressed wire bond connections  212  may result in unreliable or loss of connectivity between the die and the lead fingers. 
       FIG. 3  shows leadframe package  300  where DAP  304  is exposed through floor  314  of the mold cavity and wire bond connection  312  is exposed through molding cap  310  of the mold cavity. As shown in  FIG. 3 , the tilting of DAP  304  causes wire bond connection  312  to extend upwards and protrude out of the upper surface or ceiling of the molding cap  310 . The tilting also causes one side of DAP  304  to extend downwards and protrude out of the lower surface or floor  314  of the mold cavity. When viewed as an assembled product, part of wire bond connection  312  will be visible at the top surface of the leadframe package, while part of DAP  304  will be visible on the bottom surface of the package. A package such as that shown in  FIG. 3  will not only be rejected due to cosmetic flaws, but may also cause reliability and quality issues in the functionality of the package. 
     The tilting or shifting of DAP in a package substrate causes physical defects in IC packages, leading to the scrapping of the affected packages and subsequently an increase in the costs of manufacturing. Some tilting may cause physical defects that are visible to the naked eye, while some tilting may cause physical defects that are hard to detect visually or through electrical tests. Nonetheless, the performance of the IC package can be substantially affected or altered by the physical defects caused by a tilting DAP even though they are not visually detectable. 
     The use of multiple or stacked dice increases the occurrence of DAP tilting. For example, as more than one die is stacked above the DAP, the space that is available for the mold compound to flow during the molding process decreases. With the decrease in space, the speed at which the mold compound flows also decreases, thereby increasing the risk of imbalanced mold flow. Thus, the probability of the DAP tilting or shifting from its original position also increases. Thinner IC packages are also susceptible to DAP tilting because the reduced vertical height also reduces the speed of mold compound flow. Therefore, the severity or frequency of DAP tilting increases in stacked-die and thin IC packages. Stacked-die and thin IC package components are typically more expensive than standard components, making the cost of physical defects in these packages even more prohibitive than in standard packages. 
     SUMMARY 
     Embodiments of the present invention provide a leadframe package with stand-off legs to prevent the DAP from tilting or shifting and thus, result in significantly less IC packages being damaged during the molding process. The embodiments of the present invention also provide methods for assembling such leadframe packages. 
     In one embodiment, an IC package includes a package substrate with a DAP and a plurality of lead fingers arranged around the DAP. A die is mounted on the DAP and a plurality of stand-off legs are connected to the DAP. The IC package also includes a mold compound covering the die, at least a portion of the package substrate, and at least a portion of the plurality of stand-off legs. 
     In one embodiment, an IC package includes a package substrate with a DAP and a plurality lead fingers arranged around the DAP. The DAP also includes a plurality of stand-off legs arranged at its periphery. Each stand-off leg is connected at one end to a bottom surface of the DAP and at the other end to the floor of the mold cavity. The height of each stand-off leg is substantially equal to the height between the DAP and the floor of the mold cavity. Each stand-off leg is placed on the floor such that the top surface of the DAP remains substantially parallel to the floor. A die is mounted on the DAP and a plurality of wire bond connections electrically connect the die to the plurality of lead fingers around the DAP. There is also a mold compound covering the die, the plurality of wire bond connections, and at least a portion of the package substrate. 
     In one embodiment, a method of assembling an IC package includes the steps of (1) forming a package substrate, which includes a DAP and a plurality of lead fingers arranged around the DAP and extensions projecting out from the periphery of the DAP; (2) down-setting the DAP relative to the plurality of lead fingers; (3) bending the extensions of the DAP to form the stand-off legs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth in the appended claims. However, for purpose of explanation, several aspects of particular embodiments of the invention are described by reference to the following figures. 
         FIG. 1  shows a cross-sectional view of a typical leadframe package. 
         FIG. 2  shows a cross-sectional view of a leadframe package where the DAP is tilted to one side of the leadframe package. 
         FIG. 3  shows a cross-sectional view of a leadframe package where the DAP is exposed through the floor of the mold cavity and a wire bond connection is exposed through the top surface of the mold cavity. 
         FIG. 4  shows a side view of an assembled leadframe package according to one embodiment of the present invention. 
         FIG. 5  shows a perspective isometric view of a package substrate according to one embodiment of the present invention. 
         FIG. 6  shows a top view of the package substrate of  FIG. 5 . 
         FIG. 7  shows a perspective isometric view of a package substrate according to another embodiment of the present invention. 
         FIG. 8  shows a top view of the package substrate of  FIG. 7 . 
         FIG. 9  shows a flow diagram of forming a leadframe package according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of particular applications and their requirements. Various modifications to the exemplary embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
       FIG. 4  shows a side view of assembled leadframe package  400  according to one embodiment of the present invention. Leadframe package  400  includes several components: die  408 , package substrate  402 , wire bond connections  412 , and mold cavity  416 . In one embodiment, package substrate  402  includes DAP  404 , stand-off legs  430 , and lead fingers  406 . All the components are configured for assembling together. For example, during the package assembly process, die  408  sits on DAP  404 , which in turn sits on stand-off legs  430 , which is on floor  420  of mold cavity  416 . Package  400  includes wire bond connections  412  for connecting die  408  to lead fingers  412 . DAP  404  can be of any shape or size, but will generally be bigger than die  408  and have exposed portions around the die. The mold compound, when injected into mold cavity  416 , will completely cover die  408  and partially cover package substrate  402 , including stand-off legs  430 . In one embodiment, the mold compound does not cover the bottom surface of stand-off legs  430 . In another embodiment, the mold compound entirely covers stand-off legs  430 , including their bottom surfaces. 
     Stand-off legs  430  are coupled at one end to bottom surface  418  of DAP  404  and at the other end to floor  420  of mold cavity  416 . Stand-off legs  430  are arranged around the periphery of DAP  404 . As such, stand-off legs  430  secure DAP  404  to floor  420  of mold cavity  416  so that the top surface of DAP  404  remains substantially parallel to floor  420 . With stand-off legs  430 , when the mold compound is injected into mold cavity  416 , the flow of the mold compound will not cause DAP  404  to tilt or shift. In other words, DAP  404  will remain substantially parallel to the floor even after the mold compound is injected into the mold cavity. This ensures DAP  404  does not tilt or shift. A tilted or shifted DAP might affect the reliability and quality of the leadframe package. 
       FIG. 5  shows a perspective isometric view of package substrate  500  according to one embodiment of the present invention. In one embodiment, package substrate  500  includes DAP  504 , stand-off legs  530 , tie bars  540 , and lead fingers  506 . In one embodiment, stand-off legs  530  and tie bars  540  are directly connected to DAP  504 . In one specific embodiment, stand-off legs  530  are directly connected to DAP  504  and contiguous with the periphery of DAP  504 . In one embodiment, there are at least three stand-off legs  530  supporting DAP  504 . In  FIG. 5 , the number of stand-off legs shown is four, although more may be used, if necessary. A larger DAP usually has more stand-off legs. In  FIG. 5 , each stand-off leg  530  is bent at a substantially perpendicular angle relative to the bottom surface of DAP  504 . 
     To fabricate the package substrate, any conductive material (e.g., copper alloy) may be used. As shown in  FIG. 5 , package substrate  502  has DAP  504  arranged such that it is surrounded by lead fingers  506  and attached to tie bars  540 . DAP  504  may also be arranged so that it is lower than lead fingers  506  as shown in  FIG. 5 . This arrangement is commonly referred to as a “down-set” DAP. 
     According to one embodiment, the minimum vertical height of each stand-off leg is 100 micrometers while the maximum vertical height of each stand-off leg is 1700 micrometers. In one embodiment, the vertical height of each stand-off leg is not more than the vertical height between the bottom surface of the DAP and the floor of the mold cavity. 
     According to one embodiment, each stand-off leg is a rectangular column. The columnar or three-dimensional shape of the stand-off legs provides a contact area between the DAP and stand-off legs to support the weight of the DAP. Although rectangular columns are proposed in the embodiment, it should be appreciated by one skilled in the art that other three-dimensional or columnar shapes can also be used as stand-off legs as long as the DAP can be supported without tilting or shifting. 
       FIG. 6  shows atop view of package substrate  500  of  FIG. 5 . Package substrate  500  includes DAP  504 , stand-off legs  530 , tie bars  540 , and lead fingers  506 . Stand-off legs  530  and tie bars  540  are directly connected to DAP  504 , whereas lead fingers  506  are connected to DAP  504  with wire bond connections. The number of stand-off legs shown in  FIG. 6  is four. However, a person skilled in the art will appreciate that the number of stand-off legs used may vary with the size and shape of the DAP. A bigger DAP usually has more stand-off legs, whereas some DAPs can be supported with fewer legs. 
       FIG. 7  shows a perspective isometric view of package substrate  700  according to another embodiment of the present invention. In one embodiment, package substrate  700  includes DAP  704 , stand-off legs  730 , tic bars  740 , and lead fingers  706 . In one embodiment, stand-off legs  730  and tie bars  740  are directly connected to DAP  704 . In one specific embodiment, stand-off legs  730  are directly connected to DAP  704  and contiguous with the periphery of DAP  704 . In one embodiment, there are at least three stand-off legs  730  supporting DAP  704 . In  FIG. 7 , the number of stand-off legs shown is eight, although more may be used, if necessary. A larger DAP usually has more stand-off legs. In  FIG. 7 , each stand-off leg  730  is bent at a substantially perpendicular angle relative to the bottom surface of DAP  704 . 
       FIG. 8  shows a top view of package substrate  700  of  FIG. 7 . Package substrate  700  includes DAP  704 , stand-off legs  730 , tic bars  740 , and lead fingers  706 . Stand-off legs  730  and tie bars  740  are directly connected to DAP  704 , whereas lead fingers  706  are connected to DAP  704  with wire bond connections. The number of stand-off legs shown in  FIG. 8  is eight. However, a person skilled in the art will appreciate that the number of stand-off legs used may vary with the size and shape of the DAP. A bigger DAP usually has more stand-off legs, whereas some DAPs can be supported with fewer legs. 
     The IC packages described in the present invention may be assembled using any number of methods.  FIG. 9  shows a flow diagram  900  of forming a leadframe package according to one embodiment of the present invention. The flow diagram  900  starts at  902 . At  904 , the etching and stamping of the package substrate is performed. Any package substrate (e.g.,  500  and  700 ) can be used for this purpose. Any conventional etching or stamping process may be used. For example, in the etching process, photolithography is used to transfer the image of the package substrate design onto a metal sheet. At  906 , the package substrate is plated to inhibit corrosion. The metal used for plating is usually gold, even though other types of non-corrosive metal can also be used. The package substrate includes the DAP, stand-off legs, tie bars, and lead fingers. In one embodiment, the stand-off legs are contiguous extensions at the periphery of the DAP. For example, for a package substrate with four stand-off legs, there will be four extensions at the periphery of the DAP. Next, at  908 , the package substrate is formed. The forming of the package substrate includes bending each of the contiguous extensions at the periphery of the DAP at a substantially perpendicular angle to the edge of the DAP to form stand-off legs. The stand-off legs enable the DAP to stand on and remain substantially parallel to the floor of the mold cavity when the mold compound is injected into the mold cavity. Next, mounting a die to the package substrate is performed at  910 . The die may be mounted to the DAP with die attach epoxy. Further, at  912 , wire bonds are connected from the die to the package substrate via lead fingers. Any conventional wire bonding process may be used. At  914 , the package substrate is placed in a mold cavity whereby a mold compound is injected. The mold compound will cover the die and at least a portion of the package substrate. Finally, process  900  ends at  916 . As will be appreciated by persons of ordinary skill in the art, some of the process operations of process  900  may be reordered or eliminated. 
     One advantage of having stand-off legs is that the position of the DAP is maintained relative to the floor of the mold cavity during the assembly of the leadframe package. Therefore, when mold compound is injected into the mold cavity, the DAP will not tilt or move. A tilted DAP may distort the position of the wire bond connections in the leadframe package or cause the wire bond connections to protrude from the top surface of the mold cavity. Such abnormalities may result in unreliable or unpredictable function of the package. Another advantage of having stand-off legs is in improved manufacturability for the leadframe package. For example, a faster flow of mold compound can be injected into the mold cavity without affecting the position of the DAP, thereby leading to a reduction in manufacturing time. Yet another advantage is in yield increase for the leadframe package as there will be fewer defects in the assembled products. Overall, embodiments of the invention can shorten production turnaround time, increase production yield, and improve product reliability when implemented. During the molding process, embodiments of the invention can also provide for a wider variation in the process parameters such as the mold temperature, mold transfer pressure, and speed of mold flow. For example, the speed of mold flow can be increased without causing the DAP to tilt or move. 
     While the present invention has been particularly described with respect to the illustrated embodiments, it will be appreciated that various alterations, modifications and adaptations may be made based on the present disclosure, and are intended to be within the scope of the present invention. While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.