Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame
Reexamination Certificate
2002-01-31
2004-03-09
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
C257S667000, C257S670000, C257S676000, C257S787000
Reexamination Certificate
active
06703692
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to integrated circuit (IC) packages. More particularly, this invention relates to leadframes and methods for manufacturing leadframes for use in IC packages.
In normal operation, IC dies produce heat which must be efficiently dissipated to prevent damage to the dies. Heat dissipation is a particularly acute problem for high power devices.
One common technique for effectively dissipating heat from an IC package involves placing a heat sink on top of the IC package, opposite to the side of the package that is mounted to a printed circuit board. Heat sinks, however, are generally large and are therefore not optimal or practical in many instances, such as in mobile electronic devices, where the size of the IC packages must be small to fit within the confines of the device (e.g., a cell phone).
One conventional small form factor package uses the printed circuit board upon which the package is mounted, in effect, as a heat spreader. According to this design, the bottom of a die paddle, upon which the die is mounted, is exposed so that it is in direct contact with the printed circuit board. This design effectively transfers heat to the printed circuit board.
FIG. 1
is a cross-sectional side view of a known IC package in which the die paddle (upon which the non-active side of a die is disposed) is configured to have a surface that forms a portion of one of the exterior surfaces of the package. In this manner, the exposed side of the die paddle may easily be placed in contact with another surface to dissipate heat from that is thermally transferred from the die.
As shown in
FIG. 1
, an IC die
10
is disposed on a die paddle
12
, which is supported by a number of support members
18
connected to a frame
14
. Die paddle
12
, frame
14
and support members
18
together form a leadframe. A number of leads (not shown) extend from the package to provide the electrical connections from die
10
to external circuitry. The leads are electrically connected to the die by bond wires
13
, which are individually connected from one of a number of bond pads (not shown) disposed on die
10
to the leads. Once all of the bond wires
13
have been connected, molding compound
16
surrounds die
10
, die paddle
12
(except for the bottom exterior surface portion described above) and frame
14
.
In the package shown in
FIG. 1
, the bottom of die paddle
12
is aligned on a plane that is parallel to, but lower than, the plane on which the leads of frame
14
are aligned. To support die paddle
12
in its lower position, support members
18
are connected between die paddle
12
and frame
14
.
There are many known techniques for lowering die paddle
12
so that its upper surface is substantially parallel to frame
14
. One of those techniques involves bending support members
18
down so that they form an angle between the upper and lower planes. This technique is illustrated in
FIGS. 2A and 2B
(and may, for example, result in a leadframe being formed similar to that shown in FIG.
1
).
FIG. 2A
is a cross-sectional side view of die paddle
12
, frame
14
and support members
18
prior to the bending operation. Connection points
19
represent the place where die paddle
12
and support members
18
are connected together (and one of the locations at which bending will occur). As shown in
FIG. 2A
, die paddle
12
and support members
18
are initially co-planar (as is frame
14
).
FIG. 2B
is an overhead view of the leadframe assembly shown in FIG.
2
A. In addition to die paddle
12
and support members
18
,
FIG. 2B
also shows the configuration of frame
14
, which surrounds die paddle
12
and support members
18
. Moreover,
FIG. 2B
shows that, in this instance, there are four support members
18
, each of which is curved to accommodate the bending operation as follows. To vertically offset die paddle
12
into the lower plane, die paddle
12
is gently pushed down, which causes support members
18
to bend and straighten out. If die paddle
12
is lowered a maximum distance, support members
18
will be substantially straight, as is illustrated by the leadframe shown in FIG.
1
.
As illustrated in
FIG. 1
, the offset angle between one of the support members
18
and a vertical axis (as shown by the dashed line in FIG.
1
), as projected on to a vertical plane may be, for example, forty-five degrees (45°) or more. The offset angle results in a lengthwise offset “L” (which is labeled “OFFSET” in
FIG. 1
) between die paddle
12
and frame
14
. Large offset angles using known bending techniques often result in an OFFSET of five mils or more.
Large offset angles, which result in large OFFSETs, are generally undesirable because they necessarily decrease the allowable area for the die. In particular, a die paddle's length (and therefore the permissible area for the die) is limited because the ends of the die paddle are offset from the frame by a length of twice L.
In addition to limiting the die surface area, package configurations such as those shown in
FIGS. 1
,
2
A and
2
B also may tend to limit the possible numbers of electrical connections between a die and a leadframe. Specifically, referring again to
FIG. 1
, in known packages, a bond wire
13
runs from the die
10
and connects to one of the support members
18
adjacent to the die paddle
12
, on the horizontal plane along which the die paddle
12
lies. The bond wire
13
typically serves as a low impedance ground connection between the die
10
and the frame
14
. In other words, the frame
14
serves as a ground to the die
10
. Although it is often desirable to have such a ground connection, the existence of the additional bond wire
13
increases the complexity of the package, which in turn makes the package harder to design. This difficulty is exacerbated by limiting the location of the connection between a die and a support member to the lower horizontal plane.
Therefore, it would be desirable to provide leadframes in which the permissible area for the die was increased in comparison with known techniques.
It would also be desirable to provide leadframes in which the offset between the die paddle and the frame were reduced.
It would be further desirable to provide leadframes that allow greater design flexibility with regard to the position of bond wires that serve to connect a die with a ground or a signal that is passed through the body of a leadframe.
SUMMARY OF THE INVENTION
In accordance with these and other objects of the present invention, novel lead frame geometries and methods of making the same are presented in which, for example, the permissible area for the die paddle is increased. The advantages of these geometries include, but are not limited to, decreasing the offset of a die paddle.
According to an embodiment of the present invention, a leadframe has a frame, a die paddle with a receiving surface, and a number of support members that connect the frame to the die paddle. As is described more fully below, the support members extend in a direction that is substantially parallel to the side of the die paddle to which they are attached, rather than being perpendicular to that surface. Because the support members extend in a direction that is mainly parallel to the side of the die paddle to which they are attached, as opposed to extending directly towards that side of the die paddle, the amount of room available to the die paddle is increased. Thus, a leadframe constructed in accordance with the present invention may be used with larger die paddles (and accordingly, larger dies), as compared to leadframes in which the support members extend substantially perpendicular to the side of the die paddle to which they are attached.
Moreover, the leadframe further has a number of leads connected to the frame that will eventually serve to electrically connect an integrated circuit (i.e., the die) mounted on the die paddle to an external electrical device.
The frame, which lies in an upper horizontal plane, generally has the shape of a rectangle that defines an
Court Andrew Van
Fish & Neave
Huynh Andy
Linear Technology Corp.
Morris Robert
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