Leadframe of quad flat non-leaded package

Details Leadframe of quad flat non-leaded package Leadframe of quad flat non-leaded package

2001-01-23

2002-07-23

Clark, Jasmine J B (Department: 2815)

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

Lead frame

With dam or vent for encapsulant

C257S666000, C438S123000, C438S124000

Reexamination Certificate

active

06424024

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to the leadframe of a quad flat non-leaded package. More particularly, the present invention relates to a quad flat non-leaded package having a leadframe that includes a thinner connected region.
2. Description of Related Art
Semiconductor packages can be roughly divided into pin through hole (PTH) type and surface mount type. The pin through hole (PTH) type is a type of package which leads can be inserted into a plurality of pre-drilled holes of a circuit board. Typical pin through hole (PTH) type package includes the dual in line (DIP) package and the pin grid array (PGA). The surface mount type package requires printing a solder paste over the corresponding mounting pads of a circuit board followed by a solder reflowing process. Conventional surface-mount packages include small outline package (SOP), quad flat package (QFP) and ball grid array (BGA).
Due to its limitation of pin count, the dual in line (DIP) type package is gradually phased out. In turn, the pin grid array (PGA) package is difficult to assemble and has surface area that is larger than other conventional packages, while the production cost is generally high. Among various types of packaging structure, the techniques for fabricating the quad flat package are currently mature.
The leads of a quad flat package extend from the periphery of the integrated circuit (IC) to four horizontal directions, while the pitch between the leads also shrinks from a former value of 1.27 mm to about 0.3 mm. Most of the quad flat packages are made of plastic. Currently, a quad flat package can have at most 300 external leads. At present, quad flat type package is employed to package logic IC and low-to-moderate level micro devices. The U.S. Pat. No. 5,942,794 discloses a quad flat non-leaded (QFN) package. “Non-leaded” means that the quad flat package does not have outer leads. Hence, signal transmission path is shortened and a signal decay thereby can be reduced.
The plastic molding process of a conventional quad flat non-leaded package can be roughly classified into strip mode (in strip mode, the leadframe of various packaging units are connected from one dimension) and array mode (in array mode, the leadframe of various packaging units are connected from two dimensions). No matter how various packaging units are connected together, the standard procedure for manufacturing the quad flat non-leaded package includes die bonding, wire bonding, molding and singulation.
FIG. 1
is a top view showing a leadframe of molded array type used to form an array of conventional quad flat non-leaded packages. As shown in
FIG. 1
, the leadframe
109
has a sheet structure. The leadframe
109
has a plurality of packaging units
111
. Each packaging unit
111
can be divided into two major regions: a lead portion and a flat portion. The lead portion includes only internal leads
100
. The flat portion includes a die pad
106
for holding a die
102
(the die
102
is shown in FIG.
2
). To facilitate the automation of the production, the leadframe
109
may include side rails (not shown) for linking various packaging units and pilot holes (not shown) for an in-process alignment. The leadframe
109
also has tie bars
103
for fixing the die pad
106
in position relative to the leadframe
109
. A dam bar
108
is used to connect various leads
100
with the side rails. The dam bar
108
also serves to prevent an extrusion of the plastic compound from occurring during the molding process. The dam bar
108
is also directed to linking various packaging units
111
inside the leadframe
109
. The outermost packaging unit
111
is surrounded by the dam bars
108
and side rail.
FIG. 2
is a schematic cross-sectional view showing the leadframe in
FIG. 1
after die attaching, wire-bonding, and encapsulating but before singulation. As shown in
FIG. 2
, the conventional quad flat non-leaded package comprises the die pad
106
and the plurality of inner leads
100
surrounding the die pad
106
. The backside
102
b
of the die
102
is attached to the upper surface
106
a
of the die pad
106
using epoxy
101
. Bonding pads (not shown) on the active surface
102
a
of the die
102
is electrically connected to the upper surface
100
a
of the inner leads
100
through the gold wires
104
. The die
102
, the gold wires
104
, the upper surface
106
a
of the die pad
106
, the upper surface
100
a
of the inner leads
100
are encapsulated by the molding compound
105
. Only the lower surface
106
b
of the die pad
106
and the lower surface
100
b
of the inner leads
100
are exposed. Utilizing the exposed lower surface
100
b
of the inner leads
100
, the package is connected to an external printed circuit board (not shown).
After encapsulating, the singulation process is conducted. The purpose of singulation is to separate the molded array package into individual package units
111
. A basic criterion in singulation is the clean removal of excess plastic and dam bar material to form a smooth package. However, the hardened molding compound and the leadframe material have different physical properties. This may cause some problems in the cutting process because the cutting blade may distort the leadframe when encountering a non-uniform material.
There are three major drawbacks in a conventional quad flat non-leaded package, including:
1. Penetration of moisture: as shown in
FIG. 2
, due to distortion of the inner leads
100
in a quad flat non-leaded package after blade cutting, gaps are often formed between the inner leads
100
and the molding compound
105
. Hence, moisture can easily penetrate into the package.
2. Outward-plugging of leads after singulation causes a drop in reliability: position of the cutting street
113
is indicated by dashed lines in
FIGS. 1
,
2
and
3
. The cutting street
113
having a width A is the location where a cutting blade
107
(shown in
FIG. 3
) needs to pass through.
FIG. 3
is a cross-sectional view showing a cutting blade on top of the cutting street between two neighboring packages in an array of packages. As shown in
FIG. 3
, the cutting blade
107
is posed on top of the cutting street
113
at the junction of two packaging units
111
.
In the conventional quad flat non-leaded package, the inner leads
100
have a uniform shape. Hence, some of the inner leads
100
can be easily pulled out from the molding compound
105
by the cutting blade
107
tension while cutting.
FIG. 4
is a cross-sectional view of two quad flat packages after singulation showing some partially pulled inner leads.
3. Dam bar residues on packages is caused by a blade misalignment during singulation. As shown in
FIG. 1
, the cutting street
113
has a width A, and the leadframe
109
has a thickness t
1
as shown in FIG.
2
. According to general design rules, dam bar should have a minimum width w
1
of about 0.7t
1
. Consequently, maximum misalignment tolerance of the cutting blade is A−w
1
/2=A−0.7t
1
/2=0.5A−0.35t
1
. In general, width A of the cutting street
113
is about 0.3 mm (width of the cutting blade
107
) and the leadframe
109
has a thickness t
1
of about 0.2 mm. Following standard design rule, the dam bar thus should have a width 0.7×0.2=0.14 mm. Hence, the misalignment tolerance of the cutting blade is (0.3−0.14)/2=0.08. Such a small tolerance of the cutting blade often results in the attachment of some dam bar residue on the package leading to poor package products.
FIG. 5
is a top view showing dam bar residue on singulated quad flat non-leaded packages. As shown in
FIG. 5
, a portion of the dam bar material is attached to the package after singulation. Hence, a lower production yield is obtained.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a leadframe having a thinner connected region that can lower the cutting force on a blade required to perform singulation.
Another object of the present invention is to provide a leadframe having a

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