Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Die bond
Reexamination Certificate
2000-12-06
2002-01-22
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Die bond
C257S777000, C257S686000, C257S782000, C438S109000, C438S118000
Reexamination Certificate
active
06340846
ABSTRACT:
BACKGROUND
1. Technical Field:
This invention pertains to semiconductor packaging in general, and in particular, to a method and apparatus for making semiconductor packages with tacked dies and reinforced wire bonds.
2. Related Art:
Practitioners have responded to an increasing demand for semiconductor packages that are smaller, yet more functional, by stacking two or more semiconductor dies, or “chips,” in one package.
FIGS. 1 and 2
show a prior art ball grid array (“BGA”) semiconductor package
10
having a pair of stacked dies
14
and
16
. First die
14
is attached to the top surface of an insulative substrate
12
with an adhesive
13
. Second die
16
is stacked on the top surface of first die
14
, and is attached thereto by an adhesive layer
15
. Both dies
14
and
16
have peripheral bond pads
34
that are electrically connected by conductive wires
38
to respective pads
26
in a metal layer
22
on the top surface of substrate
12
. Adhesive layer
15
is positioned entirely within the perimeter of die bond pads
34
, and provides clearance for the wires
38
that are connected to first die
14
. The pads
26
in metal layer
22
are electrically connected to traces of a metal layer
24
on the bottom surface of substrate
12
by metallized vias
23
through substrate
12
. Solder balls
25
are fused to lands
27
formed in metal layer
24
. A molded plastic body
60
(shown by dotted outline in
FIG. 2
) encapsulates dies
14
,
16
, wires
38
, and the top surface of substrate
12
.
While this package has enhanced component density, it also has certain drawbacks. For example, during wire bonding, second die
16
may crack due to downward force applied by a wire bond head
62
to the cantilevered peripheral portion of die
16
. In addition, a wedge of the molding compound may penetrate the space
19
between stacked dies
14
,
16
. Since the mold compound typically has a different coefficient of thermal expansion than adhesive layer
15
, the two dies may crack apart. Another problem is “wire sweep,” i.e., the tendency of the relatively long bond wires
38
of second die
16
to be bent or broken by the rush of molten plastic during molding. Accordingly, there is a need for an improved package design and manufacturing method.
BRIEF SUMMARY
This invention provides a simple, inexpensive method for making a semiconductor package with stacked dies that eliminates fracturing of the dies during the wire bonding process or as a result of incompatible thermal expansions, and that also eliminates the problem of broken wire bonds as a result of wire sweep. The method permits the use of ultra-thin dies having either the same or different sizes.
In one embodiment, the method includes the provision of a substrate, which may be either a conventional laminate or a lead-frame-type of substrate, and a pair of semiconductor dies. Each die has opposite top and bottom surfaces and a plurality of wire bonding pads located around the periphery of the top surface thereof. The bottom surface of the first die is attached to a top surface of the substrate, and the wire bonding pads on the first die are connected to wire bonding areas on the top surface of the substrate with a first plurality of conductive wires.
A measured quantity of an uncured, electrically non-conductive, viscous fluid adhesive is dispensed onto the top surface of the first die. The bottom surface of the second die is then pressed down onto the adhesive to squeeze, or distribute, it laterally between the two dies and toward their respective peripheries until: 1) the bottom surface of the second die is spaced apart from the top surface of the first die by a layer of the adhesive thick enough to prevent shorting contact between the second die and the conductive wires bonded to the top surface of the first die; 2) the entire bottom surface of the second die, including any portion overhanging the first die and the substrate, is supported from below by the layer of adhesive; and, 3) the wire bonding pads on the top surface of the first die and the inner ends of the conductive wires bonded thereto are encapsulated by the adhesive. The adhesive layer is then cured, and the wire bonding pads on the top surface of the second die are wire bonded to wire bonding areas on the top surface of the substrate with a second plurality of conductive wires.
After the second die is wire bonded to the substrate, a bead of adhesive is dispensed around the entire periphery of the stack of dies such that the exposed portions of the wires connecting both dies to the substrate are completely encapsulated within the bead of adhesive, which is then cured to harden it. The bead of adhesive can be the same material as the layer of die attach adhesive applied between the dies, or a material with the same or a very similar coefficient of thermal expansion. The bead of adhesive contacts and adheres to the entire perimeter of the layer of hardened die attach adhesive.
The hardened layer of adhesive below the entire bottom surface of the second die supports the die from below and prevents the second die from being fractured by forces acting thereon during wire bonding. In addition, because the adhesive layer fills the peripheral space between the opposing surfaces of the two dies, the layer prevents the molding compound or other encapsulant formed over the stacked dies from flowing into the space and forming a potentially destructive “thermal wedge” between the dies. Further, since the adhesive layer also covers the wire bonding pads on the top surface of the first die, as well as the inner ends of the conductive wires that are bonded thereto, it helps maintain the integrity of the electrical connection between the wires and the first die during the subsequent handling and manufacturing processes. The hardened bead of adhesive around the periphery of the dies encloses the portions of the conductive wires not encapsulated by the die attach adhesive layer, including the ends of the wires bonded to the pads on the second die, and the middle portions and outer ends of the of the wires attached to both dies, thereby reinforcing the wires and preventing bending or breakage caused by wire sweep during package encapsulation.
A better understanding of the above and other features and advantages of the invention may be had from a consideration of the detailed description below of some exemplary embodiments thereof, particularly if such consideration is made in conjunction with the appended drawings.
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DiCaprio Vincent
Glenn Thomas P.
Juskey Frank J.
LoBianco Anthony J.
Shermer Stephen G.
Amkor Technology Inc.
Clark Sheila V.
Lawrence Don C.
Skjerven Morrill & MacPherson LLP
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