Patent Publication Number: US-2007096290-A1

Title: Active device bases and leadframes utilizing the same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to package technology and in particular to active device bases and leadframes utilizing the same.  
      2. Description of the Related Art  
       FIG. 1  is a cross-section of a conventional package utilizing a leadframe having an active device base  10  and terminals  20  serving as a package substrate. Wires  32  electrically connect the terminals and a chip  30  attached to the active device base  10 . The chip  30  and wires  40  are encapsulated by an encapsulant  40 .  
      In  FIG. 1 , thermal stress induced by reliability tests standardized by Joint Electronic Engineering Council (JDEC) can typically damage the interface between the chip  30  and active device base  10 , resulting in delamination therebetween, resulting in rejection of the package.  
      Environmental concerns have lead countries or economies to increasingly request processes not utilizing lead. The soldering temperature of unleaded solder is at least 40° C. higher than that of tin-lead eutectic solder, resulting in critical requests for reliable green electronic products. Thus, development of a more reliable package utilizing a leadframe is desirable.  
     BRIEF SUMMARY OF THE INVENTION  
      Active device bases and leadframes utilizing the same are provided.  
      The invention provides an active device base comprising a plate, predetermined attachment area for an active device, and at least one through hole. The predetermined attachment area is disposed on a surface of the plate. The through hole is disposed in the plate, beyond the predetermined attachment area.  
      The invention further provides a leadframe comprising a plate, a predetermined attachment area for an active device, at least one through hole, a plurality of terminals, and a peripheral boundary. The predetermined attachment area is disposed on a surface of the plate. The through hole is disposed in the plate, beyond the predetermined attachment area. The terminals are beyond the plate. The peripheral boundary sandwiches the terminals with the plate, and is connected to the respective plate and the terminals.  
      Further scope of the applicability of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
      A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:  
       FIG. 1  is a cross-section of a conventional package;  
       FIG. 2  is a top view of an active device base and a leadframe of a preferred embodiment of the invention;  
       FIG. 3  is a cross-section along line AA in  FIG. 2 ; and  
       FIG. 4  is a cross-section showing advantages of the active device base and the leadframe of the invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.  
       FIG. 2  is a top view of an active device base and a leadframe of a preferred embodiment of the invention, and  FIG. 3  is a cross-section along line AA in  FIG. 2 .  
      Referring to  FIG. 2 , the inventive active base  100  comprises a plate  104 , preferably of a conductive material, to act as a grounding element for an active device (not shown) predetermined for attachment thereto. The plate  104  is more preferably metal when utilized by a leadframe. The plate  104  comprises a predetermined attachment area  110  for the active device. The plate  104  further comprises at least one through hole  120  in the plate  104 , beyond the predetermined attachment area  110  and substantially extending through the plate  104 .  
      The inventive leadframe comprises the inventive active base  100 , a plurality of terminals  130 , and a peripheral boundary  140 . The terminals  130  are disposed beyond the plate  100  and between the plate  100  and the peripheral boundary  140 , connected to the peripheral boundary  140 . Further, the peripheral boundary  140  is connected to the plate  100  via supporters  142 .  
      The through hole  120  is typically formed by punching utilizing a punch head of predetermined shape and size. In this embodiment, the through hole  120  is substantially round. In other embodiments, the through hole can be rectangular, polygonal, stellar, or other shapes, formed by punch utilizing a corresponding punch head as desired.  
      Further, an optional wire-bonding area  105  can be formed outside the predetermined attachment area  110  as desired. When an active device is attached to the predetermined attachment area  110 , the active device and the wire-bonding area  105  can be electrically connected by a conventional wire bonding process, grounding the active device. Gold wires are typically utilized to electrically connect the active device and the base  100 , and thus, an adhering layer (not shown), comprising excellent adhesion with both the plate  104  and gold, is typically plated on the plate  104  in the wire-bonding area  105 . When the plate  104  comprises copper, for example, the adhering layer can comprise a nickel/gold layer, a nickel/palladium/gold layer, or other suitable materials. At this time, the through hole  120  is arranged outside the wire-bonding area, at a side different from the predetermined attachment area  110 .  
      The advantages introduced by the through hole  120  are shown in  FIG. 4 . In  FIG. 4 , following  FIG. 3 , an encapsulant  150  is formed overlying the inventive leadframe. The encapsulant  150  enters and fills the through hole  120  during an encapsulation process, and thus, the encapsulant  150  tightly nails on the active device base  100 , increasing the adhesion therebetween, thereby preventing delamination therebetween due to exertion of thermal stress. As a result, the reliability of a product utilizing the inventive active base and leadframe can be improved.  
      After encapsulation, the bottom surfaces of the active device base  100  and the terminals  130  shown in  FIG. 2  are exposed by the encapsulation  150 . The resulting package is electrically connected to a circuit board of a product via the exposed active device base  100  and terminals  130  by conventional surface mount technology utilizing a solder material comprising tin-based alloys as an adhesive. The encapsulant  150  typically comprises a mixture of epoxy and silica fillers, and thus, the solder material can neither wet nor solder the encapsulant  150 . As a result, the formation of the through hole  120  potentially deceases the effective adhering area between the active device base  100  and the circuit board. Therefore, the opening area of the through hole  120  on the plate  104  is preferably limited to prevent substantially reducing the adhesion between the active device base  100  and the circuit board. In this embodiment, the adhesion between the active device base  100  and the circuit board is not substantially reduced when the opening area of the through hole  120  is as large as the area of the plate  104  or less.  
      Preferably, a plurality of the through holes  120  can be formed in order to further improve the product reliability introduced by the invention. The thermal stress exerting on the package induced by the thermal process, reliability test, thermal cycling, or other factors is typically isotropic. Thus, the plurality of the through holes  120  are preferably substantially symmetrically arranged in the plate  104  utilizing the geometrical center of the plate  104  as a reference point for an equilibrium of adhesion improvement to resist the thermal stress. In this embodiment, the inventive active base  100  comprises four through holes  120  as shown in  FIG. 2 . Further, the thermal stress distribution on the plate  104  typically shows positive gradients from the geometrical center to corners or edges thereof. Thus, the effect of reliability improvement rises as the through holes  120  are farther away from the geometrical center of the plate  104 .  
      In this embodiment, the plate  104  is substantially rectangular. In other embodiments, the plate  104  can be polygonal or other shapes as desired. In  FIG. 2 , the four comer areas near the diagonals  101  and  102  are the farthest away from the geometrical center of the plate  104 , and thus, the relatively highest thermal stress exerts thereon when the thermal is stress is induced. As a result, the four through holes  120  are preferably respectively disposed at the four comers of the plate  104 . Moreover, the effect of reliability. improvement rises when the diagonals  101  and  102  respectively substantially pass the through holes  120 .  
      Similarly, a sum of the opening areas of the through holes  120  on the plate  104  is preferably as large as the area of the plate  104  or less to prevent substantially reducing the adhesion between the active device base  100  and the circuit board. The individual opening area of every through hole  120  is reversely proportional to the quantity thereof. When the opening area of one of the through holes  120  is too small to hold the silica fillers of the encapsulant  150 , the encapsulant  150  cannot fill the corresponding through hole  120 , leaving voids therein. As a result the product reliability cannot be improved, but in the contrary, decreases due to the popcorn effect occurred during the thermal process such as surface mount technology, reliability tests, thermal cycling, or other thermal processes, resulting from the quick and sharp volume increases of the gas or liquid droplets accumulating in the voids. Thus, the required quantity of through holes  120  preferably depends on the process requirement and the properties of the encapsulant  150 . In this embodiment, the active device base  100  preferably comprises four through holes  120  when the plate  104  is substantially rectangular.  
      The efficacy of the inventive active device bases and leadframes at improving the adhesion between the encapsulants and the bases, provide improved device reliability.  
      While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.