Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame – With dam or vent for encapsulant
Reexamination Certificate
1999-07-29
2003-10-28
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
With dam or vent for encapsulant
C257S666000, C257S669000, C257S676000, C257S787000
Reexamination Certificate
active
06639306
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to semiconductor packages, and more particularly to a semiconductor package having a die pad for a semiconductor chip to be mounted thereon.
BACKGROUND OF THE INVENTION
Semiconductor devices are required to be miniaturized in profile in order to cope with the recent tendency of reduction in size and weight of electronic equipments. As a result thin type semiconductor devices, such as TSOP (Thin Small Outline Package), SSOP (Shrink Small Outline Package), and TQFP (Thin Quad Flat Package), are developed to conform to industrial needs.
Illustrated in
FIG. 1A
is a cross-sectional view of a conventional thin type semiconductor package in the mold injection process. The laminated structure of a die
1
and a die pad
3
on which the die
1
is mounted, is received within a mold cavity
71
of a conventional encapsulating mold
7
composed of an upper mold
4
and a lower mold
5
. Melted encapsulating resin
2
is introduced via a gate
6
of the encapsulating mold
7
into the mold cavity
71
and is thereby divided into an upper resin flow
24
passing through the space above the die
1
and a lower resin flow
25
passing through the space below the die pad
3
. When the upper resin flow
24
moves at a speed substantially equivalent to the lower resin flow
25
, the laminated structure of the die
1
and die pad
3
is kept horizontally stable in the mold cavity
71
until the encapsulating resin
2
completely fills the mold cavity
71
, as shown in FIG. 18.
However, as the size of the chip chances, the velocity of the upper resin flow
24
would differ from that of the lower resin flow
25
. This tends to incline the laminated structure of the die
1
and die pad
3
and thereby results in an adverse effect on the reliability of a semiconductor device. Referring to
FIG. 2A
, while chip
1
′ is of a size smaller than the chip
1
of regular size as shown in
FIG. 1A
, the upper resin flow
24
moves faster than the lower resin flow
25
and incurs pressure against the chip
1
′ by the upper resin flow
24
greater than that against the die pad
3
by the lower resin flow
25
. As a result, the laminated structure of the die
1
′ and die pad
3
downwardly inclines toward the inner surface
51
of the lower mold
5
, so that the die pad
3
will be exposed to the outside of the encapsulant formed by the encapsulating resin
2
at the time the transfer molding process is completed, as shown in FIG.
2
B.
Referring to
FIG. 3A
, while chip
1
″ is of a size larger an the chip
1
of regular size as shown in
FIG. 1A
, contrary to what has been described above, the upper resin flow
24
moves slower than the lower resin flow
25
and incurs a pressure against the chip
1
″ by the upper resin flow
24
smaller than that against the die pad
3
by the lower resin flow
25
. As a result, the laminated structure of the chip
1
″ and die pad
3
inclines upwardly toward the inner surface
41
of the upper mold
4
, so that the die
1
″ and/or gold wires (not shown) for electrically connecting the chip
1
″ and inner leads (not shown) are exposed to the outside of the encapsulant formed by the encapsulating resin
2
at the time the transfer molding process is completed, as shown in FIG.
3
B.
Therefore, to avoid the aforesaid exposures of the die and die pad to the encapsulant from taking place, U.S. Pat. No. 5,371,044 discloses a process for preparing a semiconductor device in which a control plate is used in the molding process to prevent the resin flows above the die and below the die pad from being different in velocity. As shown in FIG. 4, in the U.S. Pat. No. 5,371,044, a control plate
8
, which projects inwardly toward the die pad
3
and horizontally extends from the inner edge of a lead frame (not shown), is formed near the gate
6
of the encapsulating mold
7
to balance the velocity of the upper resin flow
24
and that of the lower resin flow
25
. However, the control plate
8
is only designed to decrease the velocity of the upper resin flow
24
so that it is substantially equal to that of the lower resin flow
25
where the upper resin flow
24
moves faster than the lower resin flow
25
in transfer molding. Thus, while the size of the chip
1
is increased to cause the lower resin flow
25
to move faster than the upper resin flow
24
, the control plate is useless for balancing the upper resin flow
24
and the lower resin flow
25
such that in this case the chip
1
and die pad
3
tend to upwardly incline and are exposed to the encapsulant. Therefore, the use of the control plate of the U.S. Pat. No. 5,371,044 can not effectively solve the problem caused by the velocity unbalance between the upper resin flow and the lower resin flow in the transfer molding process.
Accordingly, as shown in FIG. 5, the U.S. Pat No. 5,623,162 discloses a die pad
3
′ having a pair of wing leads
8
′ which outwardly end from a pair of opposing sides of the die pad
3
′ to the spacing between a pair of adjacent leads
89
′. The wing leads
3
′ are slightly projected from the side surface of the encapsulant
9
′, as shown in
FIG. 6
, after the transfer molding process is completed in the transfer molding process, the outer end of the wing lead
8
′ is sandwiched between the upper mold and the lower mold (not shown) such that the die pad
3
′ is held in position by means of the wing leads
8
′. As a result, the die pad
3
′ does not have upward or downward slant problem caused by the velocity unbalance between the upper resin flow and the lower resin flow as described above.
Although a die pad constructed with wing leads as disclosed in the U.S. Pat No. 5,623,162 is able to solve the positioning problem of the die pad, it is primarily designed to provide the moisture in the encapsulant with a path to expel the moisture from the encapsulant to the atmosphere while avoiding the crack of the encapsulant caused by the “popcorn” effect, but not to solve the velocity unbalance between the upper resin flow and the lower resin flow in the transfer molding process. Meanwhile, the trimming process for cutting the wing leads is different from the conventional lead cutting/trimming process. As a result, it becomes necessary to incorporate an additional wing lead trimming step to the packaging process and to use equipment different from conventional ones, whereby the manufacturing cost is undesirably increased. Further, the die pad formed with wing leads increases the material cost for the lead frame, resulting in the wing-lead design not being widely accepted in the industry.
SUMMARY OF THE INVENTION
The present invention was made in accordance with the circumstances of the prior art, and therefore the object of the present invention is to provide a semiconductor package having a die pad with downward-extended tabs which allows the die pad to be maintained in position and not to be exposed to the encapsulant in the transfer molding process without increasing the material cost and packaging cost and using additional treatment process and equipment.
The above and other objects of the present invention are achieved by a semiconductor package comprising a semiconductor chip, a die pad for bearing the semiconductor chip, a plurality of leads each having an inner lead electrically connected with electrode pads disposed on the semiconductor chip by bonding wires such as gold wires, and an encapsulant formed by a molding resin for encapsulant the semiconductor chip, the die pad, and the inner leads of the leads The die pad is formed with a plurality of tabs extending downwardly from the plane where the die pad is positioned, allowing a lower resin flow of the molding resin flowing below the die pad to move slower than an upper resin flow of the molding resin flowing above the die in the molding process. Thus, a downward pressure is produced to press the die pad with the die mounted thereon until the tab of the die pad reaches the inner source of a
Corless Peter F.
Edwards & Angell LLP
Jensen Steven M.
Loke Steven
Parekh Nitin
LandOfFree
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