Exposed paddle leadframe for semiconductor die packaging

Details Exposed paddle leadframe for semiconductor die packaging Exposed paddle leadframe for semiconductor die packaging

2000-08-25

2003-08-26

Fahmy, Wael (Department: 2814)

Active solid-state devices (e.g., transistors, solid-state diode

Lead frame

With separate tie bar element or plural tie bars

C257S692000, C257S696000, C257S698000

Reexamination Certificate

active

06611048

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally in the field of semiconductor manufacturing. More specifically, the invention is in the field of semiconductor die packaging.
2. Background Art
Leadframe based molded packages are commonly used in semiconductor packaging.
FIG. 1A
shows a top view of leadframe
100
. Paddle
102
of leadframe
100
is attached to the remainder of leadframe
100
by four tie bars
140
,
142
,
144
, and
146
. There is one tie bar at each corner of paddle
102
. Tie bars
140
,
142
,
144
, and
146
are attached to dam bar
101
of leadframe
100
. A number of inner leads
112
are situated on all four sides of leadframe
100
. The top surface of paddle
102
is silver-plated copper while the bottom surface of paddle
102
is unplated copper. Tie bars
140
,
142
,
144
, and
146
, A and inner leads
112
are also copper with silver plating on the top side where bond wires will be attached.
Semiconductor die
114
is attached to the top surface of paddle
102
on leadframe
100
. Bond wires (not shown in any of the Figures) are bonded between semiconductor die
114
and leadframe
100
. These bond wires have a first end bonded to a respective bonding pad. An example of a bonding pad is pointed to as bonding pad
116
in
FIG. 1A. A
second end of each bond wire is bonded to a respective inner lead
112
. Bonding pads
116
can be aluminum while the bond wires can be gold or aluminum.
In
FIG. 1A
, leadframe
100
has not yet been through the molding process, thus leadframe
100
remains unencapsulated in mold compound. In a subsequent stage of the fabrication process, leadframe
100
is placed in a mold cavity in a molding machine and the entire die, bond wires, bonding pads, and paddle are encapsulated in a mold compound, leaving the outer leads of the package (not shown) exposed for soldering to the printed circuit board.
By way of background, in many RF applications where a leadframe based molded package is used, there is a need for a low inductance on the ground connection to the semiconductor die since the performance of various devices on the die can be significantly affected by inductance on the ground connection. One cause of high inductance on the ground connection is the relatively long path used to connect the ground of the semiconductor die to the ground of the printed circuit board. This long path includes, at a minimum, the length of a bond wire and a lead to provide a connection between the semiconductor ground and the printed circuit board ground. The bond wire utilized to make the ground connection could have an inductance of from one to three nano henrys. In addition, the leads on the leadframe could have a length of several millimeters and could result in an additional inductance of from three to six nano henrys. Thus, using the method discussed above results in an undesired inductance of from four to nine nano henrys on the ground connection of the semiconductor die.
In an effort to reduce the inductance values on the ground connection of the semiconductor die, an “exposed paddle” leadframe based molded package was developed. In an exposed paddle leadframe based molded package, the bottom surface of the paddle is not encapsulated in mold compound. The metal on the bottom surface of the paddle is left exposed in order to electrically connect the bottom surface of the paddle to the printed circuit board ground during a re-flow solder process. An exposed paddle leadframe is also referred to as a “deep downset” leadframe since the leadframe paddle resides in a plane below the rest of the leadframe. In other words, the paddle is set lower so that the bottom surface of the paddle would not be covered by the mold compound to be applied to the leadframe in a subsequent step. As such, the bottom surface of the paddle would remain exposed for soldering to the printed circuit board ground.
In an exposed paddle leadframe based molded package, the ground carrying bond wire is bonded to the ground pad on the semiconductor die. However, in contrast to an “unexposed paddle” leadframe based molded package, in an exposed paddle leadframe based molded package the second end of the bond wire is bonded to the grounded paddle of the leadframe instead of being bonded to a lead of the leadframe. By utilizing this method, the longer path of a lead and a bond wire for ground connection is replaced with the shorter path of a bond wire without the need to go through the lead. Moreover, the length of the bond wire itself is also shorter since the bond wire does not have to reach out to a lead, and instead connects to the grounded paddle. The inductance on these shorter bond wires can be as little as 0.3 to 0.5 nano henries. As stated above, the exposed bottom side of the paddle of the leadframe is soldered to the ground of the printed circuit board.
Thus, by using the paddle to connect directly to the ground of the printed circuit board, a very low inductance ground connection is made. A typical value for the total inductance of this connection between a ground pad of the semiconductor die and the ground of the printed circuit board is 0.5 nano henrys. Thus, the total inductance created on the ground connections of the semiconductor die has been reduced from as much as nine nano henrys to as little as 0.5 nano henrys. This lower inductance increases the performance and reliability of the semiconductor die.
Therefore, in order to take advantage of the low inductance on the ground connection resulting from an exposed paddle leadframe based molded package, efforts are made during the leadframe fabrication process to ensure that the bottom surface of the leadframe's paddle remains free of mold compound in order to make a good solder connection between the bottom surface of the paddle and the printed circuit board. Thus, attempts are made to design the leadframe in such a way that while the leadframe is in the mold cavity of the molding machine, the bottom surface of the leadframe's paddle is flush with the floor of the mold cavity to prevent mold compound from squeezing under the paddle. However, there is usually some residue of mold compound that penetrates between the bottom surface of the paddle and the floor of the mold cavity. This residue of mold compound left on the bottom surface of the paddle is referred to as “mold flash.”
If left on the bottom surface of the paddle, the paddle's bottom surface will comprise thin lines of solder conductive metal situated between non-conductive mold compound. As such, ground current flow would be restricted to thin lines of the solder conducting metal. This will result in a larger inductance in the ground connection of the semiconductor die. In addition, the flash molding may also result in a mechanically unreliable solder connection between the bottom surface of the paddle and the printed circuit board.
Therefore, exposed paddle based molded packages typically undergo another process, called “deflashing,” to remove the excess mold compound left on the bottom surface of the paddle, which adds to the cost and time expended in the fabrication process. This deflashing process can be done in a chemical bath or by physical abrasion. In the normal process, deflashing removes some flash molding. However, even after deflashing, usually some mold compound still remains on the bottom surface of the exposed paddle.
The problem of unwanted mold compound on the bottom surface of an exposed paddle is aggravated if the leadframe is deformed in such a way as to cause the bottom surface of the paddle not to be flush with the floor of the mold cavity. This deformation of the leadframe may occur, for example, during the wire bonding or die attach phases of the fabrication process or during the handling of the leadframe by the fabrication personnel. When an exposed paddle leadframe which has been deformed is placed in the mold cavity, the bottom surface of the paddle may not be flush with the floor of the mold cavity. As a result, a significant amount of mold compound may pene

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