Abstract:
A lead frame having a coating of organic compounds on its lead fingers prevents tin and flux from contaminating the lead fingers after die attach. The coating is removed prior to wire bonding. The coating allows for reliable second bonds (bond between wires and lead fingers) to be formed, decreasing the likelihood of non-stick and improving wire peel strength.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the packaging of integrated circuits (ICs) and more particularly to coating the leads of a lead frame to prevent contamination of the leads during the die attach process. 
     In many types of semiconductor packages, especially those for high power devices, an integrated circuit die is attached to a die pad of a lead frame with solder paste and then pads on the die are electrically connected to lead fingers of the lead frame with wires. The solder paste is used in order to enhance reliability and thermal conductivity. However, when the die is attached to the die pad, some flux may be deposited on the lead fingers, such as during a reflow operation, causing the formation of tin clouds on the lead fingers. The flux and tin clouds can compromise the subsequent bond between the wire and the lead fingers, leading to bonds that don&#39;t stick or a bond with a low peel strength. 
     It would be advantageous to be able to attach a die to a lead frame with solder paste but not compromise the integrity of subsequent wire bonds to the lead fingers of the lead frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of preferred embodiments of the invention will be better understood when read in conjunction with the appended drawings. The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. It is to be understood that the drawings are not to scale and have been simplified for ease of understanding the invention. 
         FIG. 1  is an enlarged cross-sectional view of a lead frame with a coating disposed on lead fingers thereof in accordance with an embodiment of the present invention; 
         FIG. 2  is an enlarged cross-sectional view of the lead frame of  FIG. 1  with contamination layers thereon; 
         FIG. 3  is an enlarged cross-sectional view of the lead frame of  FIG. 2  with the coating and contamination removed from the lead fingers and a die attached to a die pad thereof; and 
         FIG. 4  is a flow chart of a process for coating a lead frame in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of a presently preferred embodiment of the invention, and is not intended to represent the only form in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention. In the drawings, like numerals are used to indicate like elements throughout. 
     In one embodiment, the present invention provides a method of preparing a lead frame prior to attaching a semiconductor die to a die flag of the lead frame. The method includes the steps of washing the lead frame in an acid bath; performing a first water rinsing operation on the acid washed lead frame; neutralizing the acid on the rinsed lead frame; performing a second water rinsing operation on the lead frame; disposing a coating solution on the lead fingers of the lead frame; performing a third rinsing operation on the coated lead frame; and drying the rinsed, coated lead frame. 
     In another embodiment, the present invention provides a method of packaging a semiconductor integrated circuit, including the steps of providing a lead frame having a coating of organic compounds at least on lead fingers of the lead frame; attaching the semiconductor integrated circuit to a die flag of the lead frame using solder; performing a solder reflow operation on the lead frame and die attached thereto, wherein during said reflow operation, tin and flux contact the coated lead fingers; and removing the coating from the lead fingers, wherein removing said coating removes the tin and flux from the lead fingers. 
     A method of attaching a semiconductor integrated circuit or integrated circuit die to a lead frame will now be described with reference to  FIGS. 1-3 . 
       FIG. 1  shows a lead frame  10  including a die flag  12  and a plurality of lead fingers  14 . The die flag  12  is generally square shaped and sized to receive a semiconductor integrated circuit die. As is understood by those of skill in the art, the lead fingers  14  can surround the die flag  12  on either one, two, three or four sides. The lead frame  10  may be constructed of a conductive metal, such as copper and plated, for example, with Nickel, Palladium and Gold, as is known in the art. The lead frame  10  may be formed by cutting or stamping a metal foil and then plating the foil. Typically, multiple lead frames are formed simultaneously, such as an array of lead frames formed from a single foil sheet. 
     In accordance with the present invention, the leads  14  of the lead frame  10  include a coating  16  on at least an upper surface thereof. The coating  16  is made of materials such that the coating  16  has good adhesion with the metal and/or plating of the lead frame, and does not permit solder flux and Tin, which may be inadvertently deposited on the lead fingers  14 , to seep through the coating  16  and contact the lead fingers  14 . Further, the coating  16  is readily dissolved with a de-coating operation, such as with a flux cleaning solution. In one embodiment, the coating  16  comprises a solution of organic compounds, including about 5.5% cationic surfactants and 93.5% non-hazardous compounds. In one embodiment, the coating is formed of non-hazardous compounds, such as Alkyl-alkene, Aryl-hydrocarbon and relative ramifications, Ester, Oxygenous compounds (organic acid, etc.) and other atomic organic compounds, and cationic surfactants, such as Poly(methyl(γ-(2-hydroxyl-3-N,N-dimethyl-N-benzyl ammonium chloride))-propyl)siloxane. 
       FIG. 2  shows a semiconductor die  18  attached to the die flag  12  with solder  20 . As is known in the art, high power circuits often are attached to a die flag using solder. Unfortunately, after a solder reflow operation of the die attach process is performed, flux and tin are deposited on the lead fingers and tin clouds are formed on the lead fingers.  FIG. 2  shows such tin clouds  22  and flux  24  on the lead fingers  14 . Note that with the coating  16 , the tin clouds  22  and flux  24  do not contact at least the upper surface of the lead fingers  14 . That is, the coating  16  does not prevent formation of the tin clouds  22 , but the coating  16  allows the tin clouds  22  to be removed during a flux cleaning operation. 
     In order to allow for good bonds to be formed between wires (not shown) and the lead fingers, such as by a wire bonding process, the tin clouds  22  and flux  24  are removed prior to wire bonding.  FIG. 3  shows the lead fingers  14  with the coating  16 , tin clouds  22  and flux  24  removed therefrom. The coating  16 , tin clouds  22  and flux  24  may be removed from the lead fingers  14  with a de-coating process. The de-coating process involves a cleaning process that uses a cleaning solution that dissolves the coating  16  so that the tin clouds  22  and flux  24  are removed. Once the coating  16 , tin clouds  22  and flux  24  are removed from the lead fingers  14 , a wire bonding operation is carried out to attach wires  26  between bonding pads on the die  18  and respective ones of the lead fingers  14 . 
     Referring now to  FIG. 4 , a method of preparing a lead frame prior to attaching a semiconductor die to a die flag of the lead frame is shown. At step  40 , a plated lead frame, such as one formed of copper and plated with Nickel, Palladium and Gold, is provided. As discussed above, an array of lead frames may be formed simultaneously from a conductive metal, such as from a copper sheet or copper foil. At step  42  the lead frame is loaded into a frame that holds the lead frame for subsequent processing. 
     First, at step  44 , the lead frame is dipped into an acid bath in order to remove the oxides The acid may be hydrochloric acid and the lead frame may be dipped bath for about 60 seconds. After the lead frame is removed from the acid bath, at step  46  a first water rinsing operation is performed, preferably with de-ionized water. At step  48 , the lead frame is dipped into ammonia in order to neutralize any remaining acid and then at step  50  a second water rinsing operation is performed on the lead frame, again preferably with de-ionized water. 
     At step  52 , the lead fingers of the lead frame are coated with a material that will prevent tin clouding and flux from contaminating the lead frame during a later die attach process. The coating material, as discussed above, comprises a solution of organic compounds, and in one embodiment, the coating comprises about 95% non-hazardous compounds and about 5% cationic surfactants. Then at step  54 , a third rinsing operation is performed on the lead frame, again, preferably with de-ionized water. 
     At step  56 , the lead frame(s) are removed from the frame and at step  58 , a drying operation is performed. The lead frames may be dried by nitrogen gas at 75 degrees. After drying, at step  60 , a visual inspection is performed to check for if there are any defects. The visual inspection step may be performed by machine or operator. Finally, at step  62 , the lead frame is packed until such time as it is used in the chip packaging process. 
     As is evident from the foregoing discussion, the present invention provides a coated lead frame and a method of preparing a coated lead frame that is especially useful for packaging high power circuits. In one embodiment, the coated lead frame is used for forming a power quad flat no-lead (PQFN) packaged device. The present invention allows for improved wire to lead finger bonds and therefore, improved overall package reliability. 
     The description of the preferred embodiments of the present invention have been presented for purposes of illustration and description, but are not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.