Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Making plural separate devices
Reexamination Certificate
2000-01-11
2001-05-29
Picardat, Kevin M. (Department: 2822)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Making plural separate devices
C438S123000, C257S669000, C257S670000
Reexamination Certificate
active
06238953
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a lead frame, a resin-encapsulated semiconductor device and a fabrication process for the device and particularly, to a lead frame that is applied to a semiconductor element, the element and its periphery being resin-encapsulated while the rear surface of a die pad is exposed to the external environment, an resin-encapsulated semiconductor device and a fabrication process for the device.
2. Description of Related Art
There have been known a conventional resin-encapsulated semiconductor device (a resin-encapsulater semiconductor device will hereinafter be referred to as a semiconductor device) in which the rear surface of a die pad is exposed to the external environment, for example, as shown in
FIG. 4
, which is a perspective view of appearance of the semiconductor device as viewed from the front side thereof, in
FIG. 5
, which is a perspective view of appearance of the semiconductor device as viewed from the rear side thereof and in
FIG. 6
, which is a sectional view taken on line II—II of FIG.
4
.
That is, the semiconductor device
10
comprises a lead frame
11
including: a die pad
12
; support bars
13
that are protruded outwardly from sides of the die pad
12
; inner leads
14
; and outer leads
15
, wherein the constituents are separated from frame selvages each having a band like shape.
Not only is a semiconductor element (a semiconductor chip)
16
made to firmly adhere to a surface of the die pad
12
of the lead frame
11
with silver paste applied therebetween, but electrodes (not shown) of the semiconductor element
16
are respectively connected to the inner leads
14
using gold wires
17
. Further, the semiconductor element
16
is packaged with a construction in which an encapsulating section
18
made of a resin material that encapsulates the semiconductor device
16
and its periphery is formed while the outer leads
15
and the rear surface of the die pad
12
are both exposed to the external environment.
In fabrication of the semiconductor device
10
, though not shown in the figures, the lead frame is provided which comprises the die pad
12
, bar supports
13
and inner leads
14
all of which are formed in an opening inside the frame selvages having a band-like shape, and the outer leads
15
that are connected to the frame selvages. At first, the semiconductor element
16
is made to firmly adhere to a surface of the die pad of the lead frame with silver paste.
Following the adhesion of the element, the electrodes of the semiconductor element
16
and the inner leads
14
are connected by the gold wires
17
and thereafter, the semiconductor element
16
and the periphery thereof are encapsulated with a resin material while the outer leads
15
and the rear surface of the die pad
12
are exposed to the external environment, so that the device is packaged. Further, the outer leads
15
are press cut using a dedicated lead shaping metal mold so as to be separated from the frame selvage, thereby completing the semiconductor device
10
.
This fabricated semiconductor device
10
is mounted on a printed wiring board by soldering the rear surface of the die pad
12
and part of the outer leads
15
, which are exposed to the external environment, to the printed wiring board to obtain a printed circuit board. In the semiconductor device
10
mounted on the printed wiring board, since the rear surface of the die pad
12
is directly soldered on the printed wiring board, heat that the semiconductor element
16
generates can be dissipated through the die pad
12
and the printed circuit board.
In a conventional semiconductor device
10
in which the rear surface of the die pad
12
is exposed to the external environment, however, the rear surface of the die pad
12
and the rear surface of the encapsulating section
18
made of a resin material are formed to be coplanar with each other. Hence, in a resin-encapsulating step of semiconductor device
10
fabrication, a flash
19
of the resin material can have a chance to generate on the rear surface of the die pad as shown in
FIG. 5
by an encapsulation pressure.
That is, in the resin encapsulating step, the lead frame on which the semiconductor element
16
is mounted and which is still in one piece with the frame selvage is disposed in a cavity formed by a pair of an upper metal mold half and a lower metal mold half in assembly, for example, so that the rear surface of the die pad
12
is oppositely in direct plane contact with the top surface of the lower meal mold half with no resin material covering the rear surface of the die pad
12
. Further, outside the periphery of the die pad
12
in such an arrangement, a space is formed in which the resin material can be filled up to the top surface of the lower metal mold half at a position where the rear surface of the die pad is located.
On the other hand, when a lead frame is clamped by the upper and lower metal mold halves, a strain that works to deform the die pad
12
is imposed on the die pad
12
through the support bars
13
of the lead frame, thereby entailing deformation of die pad
12
. Therefore, the flash
19
of the resin material occurs on the rear surface of the die pad
12
by an encapsulating pressure when the resin material is injected into the cavity. It should be noted that while description is herein made of the example in which the rear surface of the die pad
12
is put into plane contact with the top surface of the lower metal mold half, another encapsulating metal mold may also be adopted in the encapsulation in which, contrary to this example, the rear surface of the die pad
12
is put into plane contact with the top surface of the upper encapsulating metal mold half wherein the lead frame is placed in the cavity of the mold interchanging positions of the upper and lower sides. In the latter case as well, similar to the former case, a flash
19
of the resin material generates on the rear surface of the die pad
12
by an encapsulating pressure.
If such a flash
19
of a resin material onto the rear surface of the die pad
12
occurs, an exposure area of the rear surface of the die pad
12
is reduced and thereby a level of exposure is lowered. Therefore, when the semiconductor device
10
is actually mounted on a printed wiring board, the rear surface of the die pad
12
is raised while being separated from the mounting surface of the printed wiring board in the portion corresponding to the flash
19
and an inconvenience occurs because of reduction in mechanical strength of a soldered joint, decrease in heat dissipation and others. As a result, problems arise because of reduction in reliability of an electronic component fabricated with the device and a poor production yield thereof.
Further, in a case where the semiconductor device
10
is actually mounted on a printed wiring board, there are two ways: the die pad
12
and the support bars
13
for the die pad
12
are both soldered to the printed wiring board and one of the die pad
12
and the support bars
13
is soldered thereto. However, since the rear surfaces of the die pad
12
and the support bars
13
are coplanar with each other, if the latter case is selected and, for example, only the support bars
13
are soldered, the solder on the support bars
13
flows out to the die pad
12
side with the result that an inconvenience also arises since a sufficient mechanical strength of the soldered joint cannot be attained.
SUMMARY OF THE INVENTION
Accordingly, there has been a strong demand for development in technique that enables fabrication of a semiconductor device with an improved level of exposure of the rear surface of a die pad, of which fabrication deformation of a die pad in a resin-encapsulating step is prevented from occurring, and which the device can be mounted on a printed wiring board with a sufficient soldering strength.
Therefore, in order to solve the problem, a lead frame according to the invention comprises a frame selvage provided with an opening; a die pad that is disp
Tanaka Kenzo
Yotsumoto Takahiro
Kananen Ronald P.
Picardat Kevin M.
Rader Fishman & Grauer
Sony Corporation
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