Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package
Reexamination Certificate
2001-03-16
2002-05-21
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
C257S698000, C257S734000, C438S108000, C438S118000
Reexamination Certificate
active
06392291
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor manufacture, and more particularly to an improved semiconductor component, and to a method for fabricating the component.
BACKGROUND OF THE INVENTION
Semiconductor components, such as packages, dice and wafers can include terminal contacts, such as contact balls, contact bumps or contact pins. The terminal contacts are in electrical communication with integrated circuits, and other electrical elements, contained on the components. For some components, such as chip scale packages and BGA packages, the terminal contacts can be arranged in a dense grid array, such as a ball grid array (BGA), or a fine ball grid array (FBGA). The terminal contacts provide an input/output capability for a component, and permit the component to be surface mounted to a supporting substrate, such as a printed circuit board (PCB).
FIG. 1A
illustrates a prior art semiconductor component
10
having an array of terminal contacts
12
in the form of contact balls. In this example, the component
10
comprises a semiconductor package, such as a chip scale package, a BGA package, or a FBGA package, having a board-on-chip (BOC) configuration. The terminal contacts
12
are typically formed of a solder alloy such as 95%Pb/5%Sn, 60%Pb/40%Sn, 63%Sn/37%Pb, or 62%Pb/36%Sn/2%Ag. Typically, the terminal contacts
12
have the shape of a sphere, a truncated sphere or a hemispherical bump.
In addition to the terminal contacts
12
, the component
10
includes an array of bonding pads
14
formed on its backside for attaching the terminal contacts
12
to the component
10
. Typically, the bonding pads
14
comprise a solderable metal such as molybdenum, copper or gold. As shown in
FIG. 1D
, the component
10
also includes conductors
28
, wire bonding pads
44
, and wire bonds (not shown), that form separate electrical paths between the terminal contacts
12
and the semiconductor die (not shown) contained in the component
10
. In the illustrative embodiment, the component
10
also includes plating conductors
42
that facilitate plating of the bonding pads
14
and the wire bonding pads
38
. The component
10
also includes a solder mask
18
for protecting and electrically insulating the conductors
28
and the terminal contacts
12
. As shown in
FIG. 1D
, the solder mask
18
includes openings
38
aligned with the bonding pads
14
and with the wire bonding pads
44
.
One conventional method for attaching the terminal contacts
12
to the bonding pads
14
uses a solder reflow process. With solder reflow, a layer of eutectic solder is deposited on the bonding pads
14
using a deposition process such as screen printing. A platen can be used to hold the component
10
, while the eutectic solder is deposited on the bonding pads
14
. Prior to depositing the eutectic solder, a flux (not shown) can be applied to the bonding pads
14
for chemically attacking surface oxides, such that the solder can wet the surfaces to be bonded. The flux also performs a tacking function for the terminal contacts
12
prior to solder reflow. Following application of the flux and eutectic solder, the terminal contacts
12
can be placed on the bonding pads
14
in physical contact with the eutectic solder. A fixture can be used to center and maintain the terminal contacts
12
on the bonding pads
14
.
Following placement of the terminal contacts
12
on the bonding pads
14
, the component
10
can be placed in a furnace at a temperature sufficient to reflow the eutectic solder and form solder joints
16
. The solder joints
16
metallurgically bond the terminal contacts
12
to the bonding pads
14
.
FIG. 1C
clearly shows the solder joints
16
and the terminal contacts
12
bonded to the bonding pads
14
. The component
10
can then be removed from the furnace and cooled. As an alternative to a solder reflow performed in a furnace, the bonding process can be performed using a pulse-thermode, a hot-air thermode, or a laser. A solder ball bumper, for example, uses a laser to form the eutectic solder joints
16
, and bond the terminal contacts
12
to the bonding pads
14
. Alternately, the terminal contacts
12
can be bonded to the bonding pads
14
by brazing, by welding, or by application of a conductive adhesive.
As shown in
FIG. 1A
, following the bonding process, the component
10
is typically surface mounted to a supporting substrate
20
, such as a printed circuit board (PCB), a FR-4 card, or a module substrate to form an electronic assembly
22
. For attaching the component
10
to the substrate
20
, additional eutectic solder joints
24
bond the terminal contacts
12
on the component
10
to an array of contact pads
26
on the supporting substrate
20
. A solder reflow process, as previously described, can be used to form the solder joints
24
, and to bond the terminal contacts
12
to the contact pads
26
on the supporting substrate
20
.
One factor that can adversely affect the reliability of the assembly
22
during operation in different environments are fatigue failures of the terminal contacts
12
and the bonding pads
14
. Typically, these fatigue failures are induced by thermal expansion mismatches between the component
10
and the supporting substrate
20
. For example, if the component
10
comprises a first material, such as ceramic or plastic having a first CTE, and the supporting substrate
20
comprises a second material, such as FR-4 having a second CTE, cyclic loads can be placed on the terminal contacts
12
and on the bonding pads
14
as the assembly
22
is thermally cycled during operation. As shown in
FIG. 1C
, the forces acting on the terminal contacts
12
and on the bonding pads
14
include tensile forces
31
T, moment forces
31
M and shear forces
31
S.
These forces acting on the terminal contacts
12
and on the bonding pads
14
can also occur during testing of the component following the fabrication process. In particular, semiconductor manufacturers routinely test the components by placement on test boards having sockets for holding the component
10
. During these tests the component
10
can be subjected to temperature cycling. As the socket and component
10
typically have different CTEs, cyclic loads as described above, can be placed on the terminal contacts
12
and on the bonding pads
14
.
One aspect of the fatigue failures is that some of the terminal contacts
12
and bonding pads
14
are much more likely to fail because they experience the highest loads.
FIG. 1B
illustrates this phenomena. In
FIG. 1B
, the relative displacement of the terminal contacts
12
in the X direction is plotted on the left hand Y axis. Nominal shear strain experienced by the terminal contacts
12
is plotted on the right hand Y axis. Also in
FIG. 1B
, the terminal contacts
12
have been labeled A
1
-J
1
on the X axis. The inner row adjacent to the A
1
-J
1
row would be the A
2
-J
2
row.
Line
30
of
FIG. 1B
represents nominal shear strain on the terminal contacts
12
. Line
32
of
FIG. 1B
represents relative displacement in the X direction. Line
34
of
FIG. 1B
represents theoretical displacement were the terminal contacts
12
not soldered to the board
20
(FIG.
1
A). As shown in
FIG. 1B
, the terminal contacts
12
, and associated bonding pads
14
on the ends of the component
10
(e.g., A
1
, J
1
), move the most in the X direction, and also experience the highest strain. On the other hand, the terminal contacts
12
in the middle of the component (e.g., E
1
, D
1
, F
1
), and their associated bonding pads
14
, experience the least movement, and the least amount of strain.
FIGS. 1E and 1F
illustrate two possible adverse effects of fatigue failures caused by the forces acting on the terminal contacts
12
and on the bonding pads
14
. In
FIG. 1E
, the bonding pad
14
associated with the A
1
terminal contact
12
has separated from the component
10
. This situation can cause the conductor
28
which is in electrical communication with the A
1
terminal contact
12
to break, preventing
Gratton Stephen A.
Tran Mai-Huong
LandOfFree
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