Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame – With separate tie bar element or plural tie bars
Reexamination Certificate
2001-03-20
2003-05-20
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
With separate tie bar element or plural tie bars
C257S667000
Reexamination Certificate
active
06566740
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to lead frames used to construct semiconductor devices. The invention is also directed to a method of forming a semiconductor device incorporating a lead frame.
2. Background Art
Recent demand for miniaturized, dense, resin-sealed semiconductor devices has lead to the development of semiconductor devices as shown at
10
in
FIGS. 8 and 9
. With this type of device, leads
12
are exposed at transverse back and side surfaces
14
,
16
, but do not project from either surface
14
,
16
. Such semiconductor devices
10
are commonly referred to in the industry as SON (Small Outline Non-leaded Packages) and QFN (Quad Flat Non-leaded Packages).
The semiconductor device
10
consists of a unit lead frame
17
with a rectangular support
18
having a surface
20
to which a semiconductor chip
22
is bonded. Four leads
12
, which are part of the unit lead frame
17
, are spaced around, and project from, each of four sides on a peripheral edge
24
on the support
18
, and are electrically connected to the semiconductor chip
22
through conductive wires
26
. The support
18
, semiconductor chip
22
, and wires
26
are sealed by a solidified resin material
28
to form an overall squared configuration for the semiconductor device
10
.
The unit lead frame
17
is formed as part of a lead frame
30
(FIG.
10
), which is in the form of a layer, which may be a belt, a strip, or a sheet of material. The lead frame
30
can be made from conductive material, such as copper or iron alloy, and may be processed, as by etching or the like, to produce the configuration shown in FIG.
10
. The unit frame
17
, shown in
FIGS. 8 and 9
, is shown in the hatched region in FIG.
10
.
The lead frame
30
consists of several, and in this case nine, unit frames
17
,
17
a
,
17
b
,
17
c
,
17
d
,
17
e
,
17
f
,
17
g
,
17
h
, having the same configuration as the unit lead frame
17
, arranged in a matrix form and interconnected to each other and a guide rail
32
through a tie bar network
34
.
Once the lead frame
30
is formed, the semiconductor chips
22
are secured on the support surfaces
20
, as by using a binder or binding tape. An electrode
36
on each semiconductor chip
22
is electrically connected through a wire
26
to one of the leads
12
. This subassembly is then resin sealed within the area bounded by the square border, as indicated by the line
38
, which area encompasses a portion of the guide rail
32
.
Once the resin is cured, a saw is used to cut lengthwise, in the direction of the arrow
40
, along and through tie bars
42
,
44
between the unit frames
17
,
17
a
,
17
b
and
17
c
,
17
d
,
17
e
and
17
c
,
17
d
,
17
e
and
17
f
,
17
g
,
17
h
. Two additional lengthwise cuts of width W are made as indicated at the lines
46
,
48
where unit frames
17
,
17
a
,
17
b
and unit frames
17
f
,
17
g
,
17
h
, respectively, are joined to the guide rail
32
. Orthogonal cuts are made through tie bars
50
,
52
and at lines
54
,
56
to separate the individual semiconductor devices
10
.
The process described above is desirable in that it permits simultaneous resin sealing of a plurality of semiconductor devices
10
. With a common shape, a wide variety of products can be made.
However, there are a number of problems that result from the manufacturing process for the semiconductor device
10
described above with respect to
FIGS. 8-10
. One problem is a result of resin leakage which occurs at the guide rail
32
. This problem can be explained with respect to exemplary unit frame
17
, as shown also in FIG.
11
. The resin
28
tends to migrate past the edges
58
,
60
of the guide rail
32
. By reason of the width of the guide rail
32
, resin tends to accumulate on the back surface
14
over the leads
12
, so as to form “resin flashes”
62
, which are contiguous with the guide rail
32
. If these resin flashes are not removed, they potentially prevent establishment of a proper electrical connection with a product to which the semiconductor device
10
is connected. As a result, the manufacturing process may require at last one additional step to remove the resin flashes
62
.
Another problem relates to the resistance that is encountered as certain cuts, using a separating saw, or the like, are made. The tie bars
42
,
44
,
50
,
52
can be cut with little resistance by the saw. The tie bars
42
,
44
,
50
,
52
may have a width W
1
(shown for exemplary tie bar
42
in
FIG. 10
) that is less than the width W of a cutting blade on the saw. However, the cuts at
46
,
48
,
54
,
56
must be made through the solid guide rail
32
so that a cut equal to the width W of the saw blade is made in the guide rail
32
. With the width W of the saw greater than the width W
1
of the tie bars
42
,
44
,
50
,
52
, relatively little cutting resistance may be encountered, whereas in cutting the full width of the guide rail
32
, there may potentially be a significantly higher resistance to cutting. This condition may lead to a peeling off of the leads
12
and/or the resin material
28
from the guide rail
32
and/or deformation of the guide rail
32
during the cutting process.
SUMMARY OF THE INVENTION
In one form, the invention is directed to a lead frame for a semiconductor device. The lead frame has a layer defining a first unit lead frame including a first support for a semiconductor chip and a plurality of leads spaced around the first support. The first support has a peripheral edge. The layer further defines a guide rail extending along at least a portion of the peripheral edge and connected to at least one of the leads. At least one notch is formed in the layer between the at least one lead and a part of the guide rail so as to define a first tie bar.
The lead frame may further include a second unit lead frame defined by the layer and connected to the first unit lead frame by at least a second tie bar. The second unit lead frame has a second support for a semiconductor chip and a plurality of leads spaced around the second support. The second support has a peripheral edge.
The second tie bar may connect between leads on the first and second unit lead frames.
In one form, this first support has a polygonal shape with a peripheral edge. The peripheral edge of the first support has first and second transverse, substantially straight edge portions. A guide rail extends along the first and second edge portions. At least one notch is located between the first edge portion and a part of the guide rail. There is a second notch in the layer between the second edge portion and another part of the guide rail.
The first and second notches may extend fully through the layer.
In one form, the first unit lead frame consists of a plurality of leads extending along the first edge portion a first distance. In one form, the at least one notch extends along the first edge a distance equal to at least the first distance.
The first unit lead frame may include a plurality of leads extending along the first edge portion, with the first tie bar connected between the plurality of leads.
In one form, there is a third tie bar that connects between the first tie bar and the part of the guide rail.
The invention is also directed to a lead frame for a plurality of semiconductor devices, which lead frame has a layer defining a plurality of unit lead frames each consisting of a support for a semiconductor chip and a plurality of leads spaced around the support. The support has a peripheral edge. The layer defines a guide rail. The layer further defines a first tie bar connecting to a lead on a first unit lead frame in the plurality of unit lead frames and a second tie bar connecting between the first tie bar and a part of the guide rail.
The peripheral edge of the support on the first unit lead frame may be polygonal with first and second transverse, substantially straight, edge portions. The first tie bar may extend substantially parallel to the first edge portion, with the second tie bar extending transversely
Sugimoto Jun
Yasunaga Shoshi
Clark Sheila V.
Mitsui High-tec Inc.
Wood Phillips Katz Clark & Mortimer
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