Patent Application
20070273017
The bottom of a conventional QFN chip carrier is shown in
As previously described, since copper lead 6 is limited in its protrusion, the surface over which solder may wet in joining the chip carrier to the pads on a substrate, such as a PCB, is also limited. This limited protrusion not only limits wetting and the ability to make reliable contact in joining the chip carrier to the PCB, it also creates other problems with joining, some of which were previously described.
The bottom of the QFN chip carrier, in accordance with the present invention, is shown in
The rounded protrusions 29 and 33 may be formed in the copper plate of the leadframe before forming the pattern of leads and die paddle. Alternatively, the rounded protrusions may be formed after the pattern of leads and die paddle is formed. The protrusions in the leads and die paddle may also be formed at the same time that the leads and die paddle are formed. Regardless, the copper plate of the leadframe itself is used to form the rounded protrusions. One method of forming the protrusions is to employ a half-stamp or partial stamp process. The die of the stamp would comprise an array of rounded protrusions, similar to the array of protrusions to be stamped into the copper plate of the leadframe. The die of the stamp would then be pressed partially into the plate such as to form a dimple but not so as to break through the plate. Alternatively, a single rounded protrusion in a die may be employed and the protrusions stamped, one at a time, at selected points in the copper plate.
The extent to which the stamp is pressed, i.e., depth of penetration of the dimple, is somewhat a matter of design choice and thickness of the plate. Typically, it would be preferred to obtain the greatest stand-off height, i.e., protrusion height, possible without punching through the plate, consistent with the particular application. For example, rounded protrusions of up to 100 μm would be consistent with current QFN chip carrier technology. It is to be noted that the stand-off protrusions, as formed from the copper plate itself, simplifies the process of forming the protrusions, since protrusion attachment method, materials used, position and the like, of the protrusions are readily determined and formed during the leadframe forming process. It should be understood that although the use of stamping has been described for making the rounded protrusions, a molding process for creating the copper plate with integral rounded protrusions could also be employed. The mold, thus, would include dimples in the flat bottom part of the mold.
After stamping the rounded protrusions and the leads and die paddle have been formed, the copper leadframe is then plated. In most cases only the top surface of the leadframe is plated and typically, the top surface of the leadframe is plated with silver, as shown by silver layer 37, although other conductive materials may also be used. As also shown, the voids 41 created by the dimples 33 are also filled with silver. The bottom layer may be plated or otherwise formed on the copper leadframe. Typically the bottom surface of the leadframe is covered with a layer of solder 39, for example, after the chip carrier molding process. There are also other options, such as, using pre-plated leadframes wherein the leadframe is plated on both sides after stamping the rounded protrusions, and the leads and die paddle are formed. In such a case, the top surface is typically plated with silver, as shown by silver layer 37. The bottom layer may be plated or otherwise formed on the leadframe. For preplated leadframes, the bottom surface is typically plated with nickel palladium with a gold flash although other conductive layers may be used.
As can be seen, because of the protrusions, the area covered by solder is increased. It is to be noted that the protrusions in the area of die paddle 31 also act to provide stability and, because the surface is not flat, provides some degree of relief from shock-induced stress, and like stress, for chip 7. In this
regard, chip 7 is attached to die paddle 31 with an adhesive 51, such as, Ablebond 2300 paste adhesive or Ablefilm 5320CE dry film adhesive. Although
After attaching chip 7 to die paddle 31, the wire connections are made, as hereinabove described, and the assembly is encapsulated in mold 43 of insulating material. The mold may be made, for example, from Loctite FP4450, Nitto GE 1100L, or Sumitomo Bakelite EME-G700 material, for example. After encapsulation, the QFN chip carrier with rounded protrusions for enhanced stand-off is ready for connection to a substrate, such as, a PCB.
Although description has been provided of embodiments of a QFN with leads on four side of a square package, it is clear that the arrangement described in accordance with the present invention could as readily be employed in no lead packages with leads on only two sides, for example. In addition, QFNs with two or dual rows of leads on four sides could also be employed. Similarly, either square or rectangular no lead packages with and without exposed die paddles could, as well, be employed.
It will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. It is intended that this description is for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.
