Die attach pad adapted to reduce delamination stress and...

Details Die attach pad adapted to reduce delamination stress and... Die attach pad adapted to reduce delamination stress and...

1998-03-06

2001-01-02

Thomas, Tom (Department: 2811)

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

Superconductive contact or lead

On integrated circuit

C257S666000, C257S670000, C257S676000, C257S655000, C257S784000, C438S123000, C361S813000

Utility Patent

active

06169322

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the packaging of a die or dice (e.g., a semiconductor die or dice), including the methods used to form packaged die or dice, the packaged die or dice, and structures including packaged die or dice. In particular, the invention relates to die attach pads adapted to enable reduction of delamination stress, to methods for attaching a die or dice to such die attach pads, and to packaged die or dice including such die attach pads.
2. Related Art
FIG. 1A
is a cross-sectional side view (viewed in the direction of the sectional line
1
A—
1
A of
FIG. 1B
) of a conventional packaged die
100
. A surface (“die attachment surface”) of a die
101
is attached to a surface (“die attach pad attachment surface”) of a die attach pad
102
with a die attach material
103
. In the packaged die
100
, the entire die attachment surface of the die
101
is attached with the die attach material
103
to the die attach pad attachment surface of the die attach pad
102
(as illustrated more clearly in FIG.
1
B). Bond pads (designated singly or collectively by the numeral
107
; see
FIG. 1B
) are formed on a surface of the die
101
opposite the die attachment surface. Each of a set of bond wires
105
are attached to, and extend between, a bond pad
107
on the die
101
and a portion of a corresponding one of a set of leads
104
formed proximate to the die
101
. The die
101
, die attach pad
102
, die attach material
103
, bond wires
105
and an inner portion of the leads
104
are encapsulated by an encapsulant
106
.
FIG. 1B
is a plan view of the die
101
, die attach pad
102
, die attach material
103
, leads
104
and bond wires
105
of the packaged die
100
prior to encapsulation in the encapsulant
106
. As shown in
FIG. 1B
, the die attach pad
102
and the leads
104
are connected by a frame
109
to form an integral structure (“leadframe”) that facilitates the manufacture and assembly of the packaged die
100
, as known to those skilled in the art.
During manufacture of the packaged die
100
, the packaged die
100
can be subjected to heat. For example, the packaged die
100
can be attached to a printed circuit board by soldering to the printed circuit board the portions of the leads
105
that extend from the packaged die
100
. Such soldering can take place in a chamber that can be heated to temperatures of, for example, 220° C. Additionally, operation of the circuitry formed on the die
101
can also generate heat, causing the temperature of the packaged die
100
to increase.
Heating of the packaged die
100
can produce forces that cause stress in the die attach material
103
(“die attach stress”), as explained in more detail below. If the die attach stress becomes great enough, the bond between the die
101
and the die attach pad
102
can be broken. Breaking of the attachment between the die
101
and die attach pad
102
can ultimately cause or contribute to delamination (separation) of the encapsulant
106
from other parts of the packaged die, such as the die
101
or die attach pad
102
. Delamination can render a packaged die immediately non-operational (by, for example, causing one or more bond wires to become detached from either a bond pad or a lead) or can break the encapsulant seal so that contaminants can enter the packaged die, eventually causing the packaged die to become non-operational.
One type of die attach stress (sometimes referred to hereinafter as “differential CTE stress”) that can be produced by heating a packaged die arises from forces produced by differential thermal expansion of the die
101
and die attach pad
102
, which are typically made of different materials (e.g., silicon for the die
101
and copper for the die attach pad
102
) that have different coefficients of thermal expansion (CTEs). The die
101
is typically concentrically mounted with respect to the die attach pad
102
, as viewed in the direction of FIG.
1
B. At the center (designated by the numeral
108
in
FIG. 1B
) of the die
101
and die attach pad
102
there is little or no expansion of the die
101
and die attach pad
102
so that there is little or no differential CTE stress in the die attach material. As the distance from the center
108
increases, the expansion of the die
101
and die attach pad
102
—and, in particular, the difference in expansion of the die
101
and die attach pad
102
—increases. Thus, the differential CTE stress in the die attach material increases as the distance from the center
108
increases.
Another type of die attach stress (sometimes referred to hereinafter as “moisture expansion stress”) that can be produced by heating a packaged die arises from forces produced by the expansion of steam formed from water present within the die attach material
103
. The encapsulant
106
is typically made of a porous material that allows water to enter the packaged die
100
. When the packaged die
100
is heated to a sufficiently high temperature, water in the packaged die
100
becomes steam. Over time, the steam can escape from the packaged die
100
through the encapsulant
106
. However, steam formed from water collected in the die attach material
103
cannot easily escape the packaged die
100
, both because the steam must travel a relatively long path to exit the encapsulant
106
(especially when located at the center
108
) and because the materials (e.g., silicon of which the die
101
is typically comprised and copper of which the die attach pad
102
is often made) that are proximate to and/or the materials (e.g., silver) that are part of the die attach material
103
are relatively unporous as compared to the material (e.g., epoxy resin) of which the encapsulant
106
is typically comprised. The moisture expansion stress can be approximately constant at all positions relative to the center
108
or the moisture expansion stress may decrease slightly as the distance from the center
108
increases (due to the longer path that the steam must travel to exit the packaged die
100
).
FIG. 2
is a graph illustrating a typical relationship of die attach stress to position along a line (in the plane of
FIG. 1B
) extending through the center of a die mounted concentrically on a die attach pad. As can be seen, the die attach stress has a minimum (though typically non-zero, due to the moisture expansion stress) magnitude near the center of the die, indicated by the numeral
201
in FIG.
2
. The die attach stress increases as the distance from the center of the die increases, due to the differential CTE stress: in general, the die attach stress in any direction from the center of the die is at a maximum at the edge of the die, indicated by the numerals
202
a
and
202
b
in FIG.
2
.
Heating of the packaged die
100
can also produce forces that cause stress in the encapsulant
106
(“encapsulant stress”) where the encapsulant
106
is attached to other parts of the packaged die
100
. (Together, the encapsulant stress and die attach stress are referred to herein as “delamination stress.”) In particular, since the encapsulant
106
and die attach pad
102
are made of different materials (e.g., epoxy resin for the encapsulant
106
and copper for the die attach pad
102
) that have different CTEs, heating the packaged die
100
can produce differential CTE stress in the part of the encapsulant
106
attached to the die attach pad
102
. If this differential CTE stress in the encapsulant
106
becomes great enough, the bond between the encapsulant
106
and the die attach pad
102
can be broken, i.e., the encapsulant
106
can delaminate from the die attach pad
102
. Such delamination, either alone or together with delamination of the encapsulant
106
from other parts of the packaged die
100
, can cause the packaged die to become non-operational as described above.
At the center
108
of the die attach pad
102
there is little or no expansion of the encapsulant
106
and die attach pad
102
so that there is little or no differential CTE stress in the en

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