Circuit encapsulation

Details Circuit encapsulation Circuit encapsulation

1999-06-22

2002-06-11

Ortiz, Angela (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

To produce composite, plural part or multilayered article

C264S272170, C264S275000

Reexamination Certificate

active

06403009

ABSTRACT:

BACKGROUND
This invention relates to circuit encapsulation.
The circuit
10
, shown in
FIGS. 1
a
and
1
b
, for example, has integrated circuit dies (i.e., semiconductor dies)
12
, and
14
, and other electrical components
15
,
16
,
17
, and
18
connected by a printed circuit board (PCB)
20
. Encapsulation of circuit
10
is done within a mold cavity
22
of a mold
24
(
FIGS. 2
a
-
2
e
) using a molding compound
35
. Connections to PCB
20
are made available externally to the molding compound by soldering conductive leads
26
a
,
26
b
,
26
c
,
26
d
,
26
e
, and
26
f
(
FIGS. 1
a
,
1
b
) of metal lead frame
26
to input/output (I/O) pads
30
of PCB
20
.
I/O pads
30
are also connected through conductive leads to the electrical components of circuit
10
. For clarity, only conductive leads
28
(
FIG. 1
a
) and
29
(
FIG. 1
b
) are shown. Metal lead
26
b
is connected to an I/O pad
30
which is connected to conductive lead
28
. A conductive pad
12
a
on semiconductor die
12
may be connected through wire
12
b
to conductive lead
28
(
FIG. 2
b
) or a conductive pad (not shown) on a bottom side of die
12
could be connected to conductive lead
28
. Metal lead
26
f
is connected to an I/O pad
30
which is connected by a plated through hole
31
(
FIG. 2C
) to conductive lead
29
. A conductive pad (not shown) on a bottom side of electrical component
17
is directly connected to conductive lead
29
.
Mold
24
has a top
24
a
and a bottom
24
b
that are closed on portions
26
g
and
26
h
(
FIGS. 2
a
,
2
d
), and leads
26
a
,
26
b
,
26
c
,
26
d
,
26
e
and
26
f
(
FIGS. 2
b
,
2
c
,
2
e
) of lead frame
26
. Once mold
24
is closed, transfer molding is done by pushing molding compound
35
, at 1000 psi, from a pot
36
(in mold bottom
24
b
), using a piston
40
, into a runner
38
(also in mold bottom
24
b
) and into mold cavity
22
to surround circuit
10
. After molding, circuit
10
, encapsulated in cured molding compound, is removed from mold
24
, and lead frame
26
is cut along dashed line
42
(
FIGS. 1
a
,
1
b
), and waste molding compound is trimmed away along dashed line
43
.
In one example, lead frame
26
has a thickness of approximately 0.008 inches with a tolerance of +/−0.00025 inches. When lead frame
26
is at a minimum thickness (i.e., 0.008−0.00025=0.00775), top
24
a
closes against bottom
24
b
, as shown in
FIGS. 2
a
,
2
b
,
2
c
(i.e., they touch). The pressure of mold
24
on lead frame
26
does not damage lead frame
26
. When lead frame
26
is at a maximum thickness (i.e., 0.008+0.00025=0.00825), top
24
a
does not close against bottom
24
b
leaving a gap
44
, as shown in
FIGS. 2
d
,
2
e
. The maximum height of gap
44
is 0.0005 inches (i.e., maximum thickness variance, 0.00025+0.00025=0.0005). Typical molding compounds will not leak, indicated by arrow
45
, through a gap
44
of 0.0005 inches or less due to the viscosity of the molding compound.
Referring to
FIGS. 3
a
,
3
b
,
3
c
, metal lead frame
26
(
FIGS. 1
a
,
1
b
) is not needed to make external connections to I/O pads
30
of a component
9
because only a portion of one side of PCB
20
is encapsulated in cured molding compound
50
leaving I/O pads
30
exposed. Electrical components are generally not mounted in the exposed areas.
The thickness of PCB
20
is approximately 0.020 inches and has a tolerance of +/−0.0025 inches. The maximum thickness variation of 0.005 inches (i.e., 0.0025+0.0025=0.005) makes it difficult to use conventional molds to fully encapsulate circuit
10
(i.e., top, bottom, and sides) with the exception of I/O pads
30
(component
9
,
FIGS. 4
,
5
a
, and
5
b
).
SUMMARY
In general, in one aspect, the invention features an apparatus for encapsulating a circuit on a circuit board. The apparatus has a first mold section configured to close on one side of the board. The first mold section has an exposed first conduit. The apparatus also has a second mold section configured to close on another side of the board. The second mold section has a second conduit for pushing molding compound into a mold cavity in at least one of the mold sections. The second conduit has a side opened to the first mold section when the first and section mold sections are closed on the board. The apparatus also has a piston slidably mounted inside the first conduit and configured to extend toward the second mold section to close the side of the second conduit.
Implementations of the invention may include one or more of the following. The board may include a portion that extends over the side of the second conduit, and the portion of the board is crushed by the piston. The piston may have a face configured to close the side of the second conduit and a rim extending from the face and configured to crush the portion of the board. The rim may be arcuate.
The second mold section may have a depression for receiving the board. The piston has a knife extending from a face, and the knife is configured to exert force on the board to seat the board against an end stop of the depression when the piston contacts the portion of the board. The depression may have another knife configured to exert force on the board to seat the board in the depression when the piston contacts the portion of the board. One or both of the knives may be asymmetric.
The second mold section may have a depression for receiving the board. The depression has a knife extending from the depression. The knife is configured to exert force on the board to seat the board in the depression when the piston contacts the portion of the board.
In another aspect, the invention features encapsulating a circuit on a circuit board. A first mold section is closed on a first mold section on one side of the board. The first mold section has an exposed first conduit. A second mold section is closed on another side of the board. The second mold section has a second conduit for pushing molding compound into a mold cavity in at least one of the mold sections. The second conduit has a side opened to the first mold section when the first and section mold sections are closed on the circuit board. A piston is extended through the first conduit to close the side of the second conduit.
Implementations of the invention may include one or more of the following. The board may partially extend over the side of the second circuit, and a portion of the board extending over the side of the second conduit is crushed by the piston. The piston may be used to exert lateral forces on the board to seat the board within the second mold section.
Advantages of the invention may include one or more of the following. Unused portions of the circuit board are minimized. A tight seal is formed between the mold sections and the circuit board. Tolerances in the thickness of the circuit board are accomodated.
A variety of other advantages and features will become apparent from the following description and from the claims.


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